Outline

Posted February 19th, 2010 by Malconium
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The information in this blog is best if you read the posts in order since newer posts sometimes refer to information in an older post. To help you do that this page lists the posts in chronological order so that you can start wherever you wish.

•1 - Just Getting Started...

This is a brand-new Blog site that will focus on Do-It-Yourself(DIY) Prefabrication(Prefab) construction techniques. The techniques that will be presented here can be used to build various kinds of structures including simple backyard buildings as well as more complicated structures. The materials presented here will in some sort of logical order with one post building on the content of the previous posts. For that reason it is a good idea to read them in order from the oldest to the newest.

•2 - Why pre-fab? Some perspective and some advantages...

On the West Coast of the United States the term prefabrication relative to housing tends to evoke a negative image. Many people are reminded of the so-called “ticky-tack” housing that was so widely constructed during the post WWII building boom. Others think about modular homes (formerly called mobile homes) and see a mental picture of cheap trailer parks. There are many housing communities whose construction covenants and restrictions prohibit prefabricated structures in their development and insist that all new houses be “stick built”. Unfortunately there have been abuses of prefabrication techniques in years past. Part of the intent of this blog, however, is to help set the record straight regarding prefabrication.

There is nothing about “stick built” on-site construction that inherently guarantees any particular level of quality. Furthermore there is nothing about using prefabrication techniques that automatically guarantees that the result will be low quality. Good construction quality in the final analysis is the result of a number of factors including quality of design, selection of materials and quality of execution. Prefabrication in general is nothing more that a method or a tool that can be applied well or poorly depending on the project objectives and the people involved. When well-applied prefabrication can actually have some significant advantages over traditional stick built techniques.

Prefabrication techniques offer many advantages including the following:

1.) Part of the construction process can take place in a factory setting where the impact of weather on the construction process is minimized. In fact the use of prefabrication techniques can make construction in parts of the world possible. In Sweden for example the part of the year where outdoor construction is possible is limited to a few months. During the rest of the year complete houses are construction in modular form in factories. The modules can be prefabricated to a high level of completion including paint, cabinets, floor covering and appliances. The finished product can be delivered and set up sometimes in as little as one day. At the end of the day the homeowner is presented with the keys to their new house, which is ready to live in immediately.

2.) Materials can be more efficiently handled in a factory setting. Job sight theft of materials can be largely a thing of the past. Materials can be protected from the weather. Making it easier to use smaller pieces of material that might otherwise get thrown out at the job sight can also minimize material waste.

3.) Product accuracy can be improved in a factory setting by the use of special equipment and/or jigs and fixtures.

4.) Labor costs can be reduced using prefabrication techniques. On-site stick building typically requires skilled carpenters while factory assembly techniques can employ less skillful factory workers without sacrificing overall accuracy.

For some interesting historical perspective about prefabricated houses check out the following link: How Prefab Houses Work

•3 - Lots of things are prefabricated...

It is my opinion that prefabrication techniques are widely applied and largely taken for granted in virtually every industry except the residential construction sector. Most people would not even think twice about having a car built for them from scratch in their own driveway. It is just way too expensive for the common man. So why is it that we so readily accept the notion that our houses should be built from scratch on our lot? In actuality there is a lot of prefabrication going on in the construction trades for most parts of residential construction except for the shell of the house itself.

Not all that long ago in American history a carpenter might be expected to build doors, windows and cabinets at the construction site. Now we buy prefabricated windows, pre-hung doors and prebuilt cabinets. Even very high-end cabinets are usually made off-site and custom fitted at the job. It is true that some masonry work is still built at the job site but it is possible to buy such things as prefabricated fireplaces that can result in much lower costs for such features in a new house. It is even possible to buy prefabricated foundation systems in some parts of the country that can be very quickly assembled on site. Our hard wood flooring is typically not only pre-shaped but prefinished these days. Even drywall is an example of a type of product that involves factory built steps that result in more efficient job site assembly.

•4 - Key concepts of pre-fabrication...

I have had some opportunity to study wood frame prefabrication in detail as a result of my work in that industry. I suggest that there are three major components of a successful prefabrication methodology as follows:

1.)    Careful design for prefabrication

2.)    Thoughtful material handling

3.)    Reusable jigs and fixtures

Designing for prefabrication is different than for site construction. One major difference is that the designer cannot make as many design assumptions that rely on the skill of a job site carpenter to interpret what is required. Rather the designer must take into account virtually all the details that are associated with the design. When I was in charge of the design department at a wood frame panelizing company I often had to figure out details that were left to the imagination of the carpenter by the original designer. I occasionally encountered designs that could not be built as drawn. One of the more classic examples where this can cause problems is where the original designer decides to round off all of the dimensions on the drawing to the nearest 2 inches just to make the drawing look neater. There was one job where there were two bathrooms and a couple of hallways in line with each other. The bathrooms each had a tub/shower unit. As you might or might not be aware the framing for a tub needs to be exactly the right size for the tub to fit. Also hallways cannot be smaller than 36” in width. In this particular design the combined dimensions of two tubs and two hallways resulted in an actual dimension that was larger than could fit in the space that was allotted by the designer. To solve the problem I had to shave a little bit off of the overall size of the kitchen.

Thoughtful material handling starts with careful design and needs to service the needs of the actual assembly process. There does still need to be a general materials list that can be used to estimate job costs and to order the raw materials that are to be cut to size. But the level of detail does not stop there like it might for a stick built project. For most efficient material handling it is typically desirable to generate detailed material cut lists for all of the pieces that are to be assembled. These lists can be organized in various ways for better material handling. For example it might make sense for the sawyer to cut all of the pieces in descending order from the longest to the shortest. These pieces could be put in bins by size and the assembly workers can retrieve them from the bins as needed.

As I have mentioned earlier fabrication accuracy in a prefabrication methodology does not need to rely so much on the skill level of the people that are involved. Instead the accuracy needs to be established by the methods used to do the assembly. This is typically accomplished by using a combination of special equipment and by the use of jigs and fixtures specially constructed for a given task. There are companies that specialize in providing equipment for the prefabrication of house parts. Take a look at the following link for an example of the kinds of special equipment in use by prefabrication companies:

Triad/Ruvo

I intend to expand on some of the information above in future posts…

•5 - Why DIY pre-fabrication?

So why would it make sense for you to consider prefabrication techniques for building your own backyard shed, tiny house or larger project? After all it would not be feasible to spend the kind of money that commercial prefabrication operations spend for their equipment just on your one project. And what about the space that is required to prefabricate things? Doesn’t it take a lot of square footage to set up a prefabrication operation?

One major focus of this blog is to show you that you do not need to spend the big bucks that commercial ventures spend and you do not need all that much floor space to take advantage of at least some prefabrication techniques for your DIY project. Also you can strike a balance between what you prefabricate yourself and what you might buy from companies that do prefabrication. You are already thinking about that when you think about buying prefabricated windows. You could instead decide to fabricate your own windows.

Prefabrication is not for everyone. Of course DIY construction is also not something that everyone is qualified to tackle. I am going to be assuming that if you are reading this blog that you are at least giving serious consideration to building something yourself. If you are I think that some of the techniques that I will address over time will be worth your consideration.

Some of the key reasons that I think it makes sense to consider prefabrication techniques for your DIY project are the following:

1.)    The more detailed level of planning that is required prior to doing prefabrication may very well help you to avoid some construction mistakes. This of course can save you both time and money.

2.)    Adopting prefabrication techniques typically results in dividing a project into discrete parts each of which have specific clear objectives and measurable results. From the point of view of good project planning this is a good thing. From the perspective of maintaining focus and motivation this is also a very good thing.

3.)    Dividing a project up into discrete bite size projects also has an advantage for projects that you cannot spend your full time working on. It makes it somewhat easier to plan how you will spend your time when it is available. It is also easier to make sure in advance that you have the necessary materials on hand when you do have the time.

4.)    One other advantage of dividing a project into carefully planned achievable smaller steps is that the overall scope of the project will not seem so overwhelming. You can feel good about accomplishing each bite size piece of the project.

5.)    You will probably find that you can save money on materials by better planning. I have often been able to save money on lumber for my projects by thinking about how I am going to cut up the pieces that I buy. For example if your project needs some 5’ and some 11’ 2×4 pieces you could buy 6’ and 12’ pieces and end up with a lot of 1’ scrapes. Planning ahead you might notice that you could get a 5’ and an 11’ foot piece out of one 16’ piece with no waste.

6.)    There are some obvious advantages to prefabrication if you are building something that will be installed in a location that is remote to where you live. Prefabrication allows you to take advantage of the small snippets of time that you have available to keep your project moving forward. The final assembly can often be done very quickly. It could also be a fun thing to schedule a final assembly party where you call on friends and relatives to have a sort of barn raising kind of event.

•6 - Modularity and how it applies to pre-fabrication...

Modules play a large role in most any design project but are especially prominent or desirable in prefabrication techniques or methodologies. As I think of it a module is basically a sub-assembly of a larger assembly. While a module could be an entirely unique sub-assembly it is more common and more efficient to design modules that can be used more than one time in a given design. There are two key reasons that come to mind as to why this is true.

1.)    The amount of time that is required to complete a design is directly related to how many unique things the designer actually designs. Designing subassemblies that can be used in multiple places can radically reduce the total number of unique things that the designer works on with the result that design time is saved. My career in the semiconductor industry where I have been involved in the physical design of integrated circuits (or microchips) has really driven this fact home. In that industry it would be impossible to design at the level of complexity that we do without taking advantage of lots of predesigned sub assemblies. While it is possible to design a house without modules it does not really make good practical sense to do so. In fact designers do use repeatable modules almost without thinking about it. It would, for example, be unusual for every door and window in a house design that is at least nominally complicated to be entirely unique.

2.)    Fabrication time for the parts of a given house design is at least to some degree directly related to how many unique modules are in the design. This is because a certain amount of set up time is typically required to build something different than what was built before. There is a bit of an exception to this if you have access to sophisticated computer controlled assembly equipment that is able to quickly re-tool for each new object. Interestingly in the integrated circuit design flow while the use of modules has a very significant impact on design time it has very little impact on fabrication time but that it because all of the modules are essentially fabricated at the same time. That of course is not the case with DIY construction. I contend that being able to make reusable jigs and fixtures to assist with building multiple copies of each module can result in some very significant fabrication timesaving.

One level of modularity that we may take for granted is the use of standard dimension materials and even the use of standard dimensions. It is, for example, standard procedure to use pre-cut 2×4 or 2×6 lumber to frame a standard wall. It is also standard procedure to space members at 16” or 24” center to center. In general though I am referring to subassemblies that are a little more complicated than individual sticks of wood. It is important to point out however that there is a hierarchy of assemblies in most construction projects.

What I mean by the term “hierarchy” is that there are assemblies within assemblies that may themselves contain assemblies. In prefabrication it is useful to take advantage of multiple levels of hierarchy when possible. One simple example of this occurs in the construction of a typical wood wall panel. The sawyer starts with a lumber cut list of the different pieces of 2×4 in the wall. I contend that each size of 2×4 could be considered a subassembly or module. These modules could be put in bins by size. Some walls will have special features within them that require subassemblies of pieces of lumber. For example if the wall has a window there might be subassemblies for the wood members at each side of the window. It would also be possible to make a more complicated subassembly that included all the framing members for defining a given size window. There could be other subassemblies for door openings, wall corners and wall junction points. Taken to its logical extreme all the pieces parts for a given wall panel could be treated as a hierarchy of sub-assemblies and could be prepared in advance.

•7 - Design hierarchy based on function and longevity

For a while now I have been aware of a conceptual model for the design of buildings that takes maximum advantage of the modular concepts that I mentioned earlier in some pretty interesting ways. As I understand it the concepts originated with the Massachusetts Institute of Technology. They have been further refined by the involvement of Bensonwood Homes based in New Hampshire. Anyone that has been interested in timber frame construction has probably run across the books written by Ted Benson. Bensonwood Homes is Ted’s company. The name that Bensonwood Homes has coined for the concept is Open-Built®. The concept as it has been evolving disentangles the various parts of a traditional building such as wires, ducting, piping, framing, sheathing and other items and considers each as a separate system. Bensonwood claims that this makes it easier to prefabricate these systems offsite. They also feel that it makes it easier to modify and maintain over time because it is easier to work on a given system without having to disturb other parts of the structure. Longevity considerations come into play to. Parts that are more likely to need replacement are made more accessible than other parts that have a longer life span.

For some really interesting reading on the Open-Built® concepts as pioneered by Bensonwood Homes please click on the following link:

Bensonwood Homes

•8 - Global design considerations

Lets focus a bit on the design process as it relates to your DIY prefabrication project. Most of the examples that I will use are likely to be from wood frame construction simply because most of my construction experience has been with wood. I hope that I can convey the concepts in more general ways though so that you can see how they could apply to other construction approaches as well. I hope too that I do not over simplify. It has been my experience that some of the things I take for granted are not taken for granted by other people. With that in mind I will try to be a complete as reasonably possible. So what are some of the high-level or global things to think about that apply to most any design? Here is a list of some things to think about.

1.)    Determining the maximum module size is probably one of the more key decisions that you will need to make. I have designed a fairly large number of prefabricated wood frame structures that had to fit into standard shipping containers for shipment overseas to Japan. This provides some challenges in that it is simply not possible to fit in a wall panel that is 8’ tall unless its width is small enough to fit into the container. The inside of a standard container is something like 90” wide and 92” tall if memory serves.

2.)    The method of placement and assembly at the construction site can dictate aspects of your module design too. A pre-framed wall panel can be fairly heavy per lineal foot depending on the types of material used and the level of completeness of the wall panel. Use of cement based products such as Hardie Board brand siding for example can add a lot to the overall weight. Not too long ago my youngest son and I built an 8’ x 14’ storage shed in my yard. We used 4’x8’ Hardie Board sheet siding which was only 3/8” thick and framed up each wall in one piece on the floor of the shed. We were just barely able to tip the 14’ wall panels up into position. It would not have been practical for the two of us to move that size panel very far. I have also designed panels for jobs where we could anticipate a crane being on location. Some of the panels I have designed were in the range of 24 long with windows pre-installed. I once collaborated with a couple of friends to build a Spec house where we used a combination of prefabricated wood frame wall panels along with SIPS for the roof and floor. I got to operate the rented crane that we used to lift the panels into place. As I recall I designed the wall system to have a maximum panel size of about 16’. If your project is really remote you might need to design panels that are much smaller. The prefabrication panels that Michael Janzen published in the design book that is referenced at the top right of my blog are designed for transportation in a pickup truck.

3.)    Determining how you will fasten the pieces together during the final assembly stage is an important consideration. This is especially true if the wall panels will arrive at the job site fully enclosed. This might require special attention in the prefabrication stage to make sure that the necessary preparations have been made to allow assembly. Again I refer to Michael Janzen’s plans were he uses a plywood spline between panels. This approach requires that the necessary slot be included in the panels when they are prefabricated. There also are various types of special fasteners that are designed for attaching panels together that would need to be built into the panels during construction.

4.)    The degree of completeness you intend for the prefabricated modules can have a huge influence on the design. As mentioned above the more complete a panel is the heavier it will be. There is also a potential issue relative to inspection of the structure if that is required. Prefabrication companies that make modules that are completely enclosed have to make special arrangements with the appropriate government agencies to pre-inspect their products before they get to the job site. This is one reason that wood frame panelizing companies most typically leave the inner skins off of their panels.

5.)    Permanence and/or the potential need to relocate a structure should be considered. One extreme example of this is found in the modular home industry where the entire home might need to have wheels mounted on it so that it can be moved. This dictates certain things about the maximum size of the individual module – namely that it has to fit on the road. This also dictates certain things about the base frame, which is typically built out of steel. Other material choices are made with the thought of moving the structure in mind. For example overall weight and material flexibility is considered. You might want to use construction adhesive in the assembly process if you do not anticipate relocation.

•9 - Material handling and parts lists

Once design issues have been taken care of a good material handling strategy is key to making a commercial prefabrication operation work smoothly. While it is less important for a small project it can still be helpful. Material handling as I am thinking of it now includes all aspects of the acquisition and movement of materials through your prefabrication operation – large or small. It should go without saying but it is crucial for efficient construction to have the materials you need where you need them and when you need them. How many times have you tackled some relatively small project that should be simple and quick but that ends up taking way more time that you thought it would because you had to make unexpected trips to the store for something you did not plan for.

I have already mentioned that careful planning for prefabrication might help you avoid mistakes in how your project goes together. Careful planning also enhances being able to create accurate parts lists. For even simple home remodeling projects I try to think out the steps that I am going to have to take before I go shopping. I try to anticipate unexpected things and will often buy a few different parts that I may not need with the understanding that taking a few unused parts back to the store for a refund may be a lot more time efficient than having to make unexpected trips right in the middle of what you are working on. I give extra attention to this when I anticipate having to turn off the water or the electricity to do what needs to be done.

Besides carefully dealing with the acquisition of material thinking about the movement of the material through your workplace is key. This is especially true if you modify parts that you have acquired or if you have to fabricate subassemblies. One of the main aspects of parts modification that you will want to carefully consider for a DIY prefab construction project is your cut list. There are likely to be a lot of pieces of material that need to be cut to different sizes. As I mentioned earlier you also need to decide whether you are going to store cut pieces by size or by specific panel. If you are working by yourself it is a bit easier to keep track of things but if your project will be worked on as your spare time permits a little organization can go a long way.

Setting up a tool to make a particular cut might not take all that long but if you have to do it too many times your efficiency will be reduced. Planning ahead might suggest that you make all of a particular type of cut at the same time.

One other issue that I think is well worth mentioning has to do with where your larger subassemblies are built. Large prefabrication operations tend to favor an assembly line approach where materials might be cut in one location and then various parts of the assembly are done in different stations along an assembly line. The panel company that I worked for had a large framing table where workers assembled the wall studs and nailed them together. The panel was then slid along a conveyor to another station where the plywood was attached. There was a nailing station and another station where the window openings were routed out. Wall studs were cut at a station near the framing table and plywood sheets were cut at a different station near the sheathing table.

An alternative approach is to have a single assembly table where all the work on a given panel is done. For a large operation you would just need to have more assembly tables. You would also have to have workers that were more broadly capable since each person would be performing more kinds of tasks. I contend that this approach can allow a panel operation to start on a smaller scale and grow as necessary. I also think that the overall equipment costs could very well be lower for a given capacity of panel manufacturing. There would at least be fewer conveyor rollers in the shop.

For DIY prefabrication you will probably want to lean in favor of the single assembly station both for reasons of space limitations and because you might be the only person doing the work.

•10 - Repeatability – reusable jigs and fixtures

I have given some attention already to the concept that overall accuracy can be improved by the use of jigs and fixtures. I have also said some things about the desirability of breaking a design down into as few unique modules as possible. One other thing that I do not think I have mentioned yet is that the size and complexity of your jigs and fixtures is directly related to the maximum size of the modules that you have designed.

The framing table that we had in the panel company where I worked was large enough to make a wall panel that was at least 24’ wide. I think we had the capacity to make the panels up to 10’ tall but I am not sure now. That table was impressive but pretty expensive too. It had an array of LEDs along each side of the table that could be lit up under computer control to show where the framing members should be placed. We had software for designing the panels that created the necessary input for driving the LED display. I understand that there are panel machines now days that have laser projectors above them that can project the pattern of what is to be built onto the table surface.

My first exposure to panelized construction was with a small company that I visited in San Jose, California that had an approach where their panels were always 4’ wide. They were based on standard plywood sheet sizes. The operation was running in a shop that was maybe the size of a three-car garage. As I recall they had maybe 3 or 4 steel tables that they had made themselves that had various brackets to help them place materials. Some door and window openings would not fit entirely in a given 4’ wide panel so for these panels they would leave out the header that went across the top of the opening. They would of course pre-cut it but it would be placed in position when the panels were joined together at the job site.

Another issue related to the creation of jigs has to do with how you place framing members. One approach is to have brackets that hold pieces in place while another approach is to have some sort of visual indicators on your table or jig. Which approach makes sense to take depends to some degree on whether or not what you are making at that station is very specific or more general. For example if you were to discover that you had to make a lot of parts that were exactly the same you would want to lean in favor of a jig with fixed brackets into which you can quickly drop parts. A table for making wall panels needs to be a bit more generic if the walls vary in size and features. For example fixed brackets for placing wall studs might very well get in the way if a window needs to be added to a wall panel. There is an additional challenge for jigs that need to be more generic. Somehow you need to decide where to put materials each time you use the fixtures.

A framing carpenter will typically layout out a wall by drawing marks on both the top and bottom plates of the wall at the same time. They usually use symbols to indicate what is to be placed at each location. Here are a few variations on what might be drawn on an 8’ 2×4 wall top and bottom plate. The “X” indicates a full height stud, the “T” indicates a trimmer stud (supports a header beam for a window), and “C” indicates a cripple stud, which is the short member beneath a window opening.

Perhaps there is a way to build yourself a simple layout table that combines fixed brackets with visual indicators. I will attempt to describe such an approach in my next post.

•11 - General-purpose layout table

You will of course need to make a number of design decisions before you can finalize what you need in the way of jigs and fixtures. Also I think it is important to point out that you might be able to do just fine without any jigs or fixtures if your project is small enough. That is after all the basic way that stick built construction is done. The accuracy of the various parts is the responsibility of the carpenters. There are various techniques that framers use to guarantee accuracy on the job site. Most of the techniques are based on careful measurements. That is one reason why building a house on site might be more expensive unless perhaps you happen to be that skilled carpenter.

Since no particular design decisions have been made yet for this part of my blog I will need to concentrate on somewhat more general suggestions regarding how you might construct a layout table if you do indeed decide that one would be useful. Something I think I forgot to mention earlier in the section on design constraints was that some of your design decisions may need to be based on how much work space you do have available if you are prefabricating things offsite.

One key thing to think about when building a layout table is how high you are going to build it. Traditional framing is done directly on the floor of the house that is being built. There is no particular reason your layout table couldn’t be on the floor if that suited your needs. You could even put markings directly on your floor if you wanted to that would help with panel layout. One nice thing about building a new house that has a wood subfloor is that you can draw things on the floor. I have found that to be useful from time to time. It can be especially useful for determining angles of rafter cuts for example.

Traditional framing on the floor is also useful if you will be nailing the members together with a hammer. It can be useful to be able to hold members in place by standing on them while you drive nails. Also swinging a hammer from above your work is a fairly easy way to work. Nail guns that are used for framing are usually designed with an angle to them that makes if fairly easy to nail framing members together from above. They also are very easy to use if what you are nailing is right in front of you. So if you are going to be using a nail gun or if you will be using screws to assemble your panels you might want to consider having your table high enough so that you can easily nail through the sides of the frame members without having to bend over so much.

If you do raise the table surface off of the floor there are some other things to think about. For one thing it may not be convenient anymore to stand on frame members to help hold them in place while you attach them. If you will be using a hammer to assemble your panels you may need to have a way of helping hold members in place while you drive your nails. One technique for doing that is to create raised edges on your table that give you something to hold your materials against while you nail them. For example in a typical 2×4 wall most of your nails will be through the top and bottom plate into the vertical studs. You might want to have a cross member of some sort at each end of the table that you can push your frame up against. Of course it will need to be designed in such a way at to not get in your way when you are nailing from that end of the wall. Perhaps these brackets could be hinged so that you can move them out of the way when necessary.

Nail guns and screw guns are nice in that they do not disturb the placement of framing members as much when you are attaching them together. It is much easier to hold the members in place while you attach them than it is for nailing with a hammer. Brackets or ledges that hold members together might still be desirable however.

You are likely to be attaching some form of sheathing (most likely something like plywood or OSB) to your framework before you consider your panel completely prefabricated. So one thing to think about is how you will attach the sheathing in the middle of your panels. How long are your arms? Can you reach into the middle of your layout table to attach sheathing? If your panels are never any wider than 4’ you should be able to. What about if they are 8’ wide or wider? I suggest that you might want to make the height of your table low enough to make it easier to reach into the middle. You might even want to consider making it low enough that you can easily crawl out into the middle of the panel to attach sheathing.

•12 - More layout table details

So what should you build your layout table out of? Part of the answer to that question depends on how permanent you want it to be. Another part depends on what types of materials you are comfortable working with. You do want the table to be sturdy. You also want the table to be square and flat so that the panels you build on the table are square and flat. If you were starting up a small factory to turn out a lot of panels you might want to make your table out of metal. For a smaller scale operation wood might be fine. Using plywood (or maybe industrial grade particle board) as a tabletop is a good idea for several reasons.

1.)    Plywood sheets are typically fairly square. You might want to confirm that though.

2.)    Plywood sheets come in incremental sizes that might be close to what you would want your panel sizes to be. It’s not at all a bad idea to pick modular sizes for your panels that are based on a sheet of plywood or OSB. It does make some sense to pick a panel size that is a multiple of 4’ since it would require less cutting of your sheathing material during assembly.

3.)    It is easy to attach brackets at virtually any place you want them on the plywood surface.

If budget permits you might consider using a combination of wood and metal. One approach that I find appealing is to use steel angle iron of at least 1-1/2” on each leg and at least 1/8” thick to create a frame around the edges and along the joints of the table if there is more than one sheet of plywood. The steel might be straighter than a wood framework would be resulting in a flatter table. It would be very easy to attach the steel to the bottom of your plywood with screws through the steel with the result that there wouldn’t be any screws visible from on top. I would suggest that your plywood be at least ¾” thick if you are doing this. It would also be easy to attach legs under the table by screwing through the vertical flange of the angle iron into the legs.

I do think that you could make an entirely acceptable table entirely out of wood though providing you exercise reasonable caution in picking frame members that are as straight as possible.

I suggest that you let the edge of your tabletop extend past the framework for at least 1-1/2”. This makes it very easy to attach a clamp to the edge of your table at any point along its edge.

Here is basic drawing of a 4’ wide table made from ¾” plywood, 2” angle iron and 4×4 legs. The table shown here would be about 16” tall.

•13 - Markings on your layout table

I have not said very much yet about brackets and markings that might make sense to have on your layout table. One thing that I think could make a lot of sense is to make the top surface of your table out of something that is very easy to draw on. How about adding an overlay of some type of surface that you can write on with dry-erase marking pens for example? You could use shiny white plastic laminate or perhaps a layer of smooth white melamine paneling. You could put some markings on your tabletop that were more permanent if you wanted to by using paint or indelible marking pen. You might also consider using colored plastic tape of the type that you can often find in the electrical department of a home improvement store. Different colors might be useful for different things on your panels.

The following diagram shows a table for building wall panels that are a maximum of 4’ wide. The tabletop is made out of a 4’x8’ sheet of plywood. The colored bands might be painted on or could be tape. They indicate where typical framing members would be placed. In this case there is an assumption that there would be some plywood overhang of the frame to connect to a second top plate (on the right) and to overlap floor panels (on the left). The green stripes show the location of the top and bottom plates. The red lines indicate where standard studs would be placed if they were at 16” on center. The blue band in the middle of the panel shows where the middle stud would be if the studs were at 24” on center spacing instead of at 16” on center.

•14 - Brackets on your layout table

As I have already mentioned you might want to have brackets that could help hold framing members in place while nailing them. It might also be useful to have some aspect of your framing table that helps you place the plywood sheathing accurately. In the case of the table in the diagram in my last post the amount of overhang of the plywood at the top and bottom of the wall frame needs to be checked before fastening it in place.

One simple way of providing a movable bracket to help locate your framing members would be to attach brackets that can hinge or otherwise move out of the way when not needed. The following diagram shows one way that this might be accomplished for the left end of the panel table that I have shown in the post about layout table markings. In this diagram the green lines represent the wall panel that is being built on the table. The blue shape represents ½ diameter wood doweling that is used to attach the removable piece of 2×6 lumber. The dowels would be glued into the 2×6 member and would be a slip fit into the top of the table. It would be a good idea to taper or round off the bottom ends of the dowels so that they can more easily slip into the holes in the tabletop. Red is used to highlight handles on the removable bracket to make it easier to place in position when needed. Notice that in this case the wall panel under construction uses 2×4-framing members. It probably would be possible to nail or screw through the top edge of the wall plate without removing the bracket which would help locate everything so that it stays in place after removing the bracket to add the final nails or screws.

One easy modification that might makes sense to make to the bracket would be to add a piece of steel angle iron to each corner of the bracket that would indicated exactly where the corners of the plywood sheathing should come. The following drawing shows one corner of the table with part of the table top shown in orange, the wall frame in green, the removable bracket in red and the added angle bracket in steel gray.

•15 - Storage

If you are undertaking a DIY prefab project that is going to be built in you spare time and then assembled all at once you will have to deal with storing the various component parts as you build them. This of course suggests that the availability and type of storage space will need to be considered as part of your design process. The good news is that there is a tremendous amount of air contained in your finished building. This fundamentally means that your panels will take a lot less space to store them than they will require when they are finally assembled.

Another good piece of news is that you do not necessarily need to store your prefabricated parts indoors. You do of course need to think about weather exposure and the possibility of pilferage or vandalism. The larger the pieces are that you prefabricate though the harder it would be for someone to walk off with them. Depending on your design approach, the availability of storage space and the means for transportation that you pick you may also be able to store your panels is some sort of bundles or groups that you can load with a fork lift when you are ready to transport them.

One good thought to keep in mind when you are storing prefabricated parts is that you will need to access them later. I mention this because it is important to think about the relative order of access when the time comes to assemble your building. It can be a real pain if the part that you want first happens to be on the bottom of the pile. You should also be thinking about how many times the parts will need to be moved before assembly. For example if you are going to move your panels to a remote site before assembly you might want to store them in the reverse order of need so that when you load them on the truck you can more easily arrange them so that the first panel you need ends up being on the top of the pile.

Perhaps it should go without saying but think about some way of labeling your panels that will help with the assembly process. It is a very good idea to make sure that you can read the labels when the panels are in storage or when they are stacked on the truck. If you discover that the next panel that you need does not happen to be the one on the top of the pile you need to be able to quickly figure out just where it is. Also if you project will be implemented over time labels make it easier to remember exactly what is what. Use some type of label that is suitably indelible too. Years ago I heard of a situation where someone bought a Scottish castle and had it carefully disassembled and labeled. Unfortunately the labels were not waterproof. In the hold of the ship that was transporting the stones to the United States the labels washed off. They left Scotland with a castle and ended up in the United States with an expensive pile of rocks.

•16 - Shipping and handling

I have already said something about the stacking order of your building panels in my previous post. Let me reiterate the importance of carefully considering this when you are ready to transport your panels to your building site. You will lose some of the advantages of prefabrication if you do not think this out carefully.

What I suggest that you do is think about the last place your panels will be sitting just before you install them. Work backwards from that all the way to your manufacturing and storage approach. For example if you will be moving your panels to the construction site on a truck or trailer and unloading all them before you install them then the relative order of the load on the truck needs to be different than it would if you were going to immediately install each panel as it is taken off of the truck. Again let me stress just how important it is that the next panel you want should be on the top of the pile rather than buried somewhere further down.

You may be able to transport or store your panels standing up on edge rather than stacked flat on top of each other. This can be a very significant advantage if you are able to use a crane to lift your panels into position. It becomes less important that your panels be in the correct order if you can pick a panel out of the middle of the stack. When installing wall panels with a crane it is also easier to lift them by the top edge rather than having them flat on a pile. Of course it should go without saying that the panels should be placed with the top edge up.

Having panel labels or numbers easily visible while they are stacked up can be useful. This is less important if you have stacked your panels in the exact correct order but it is still nice to know that the panel that you are about to move to its final position is exactly the one that you think it is. Of course if your panels get out of order for some reason it is much easier to find the panel you want if you can just look at the edge of the pile rather than having to move all the panels one at a time to find the one you want.

•17 - Assembly – panel expansion

If you have given enough care to planning your project you should find that the pieces would all go together nicely once you are ready to assemble them on-site. There is one thing that occurs to me to mention about prefabrication of panels that does impact final assembly that you might want to think about when you are doing your design. It turns out that the more panels you assemble side by side the greater potential there is for dimensional errors. What I am referring to here is that panels are not always exactly the size that you think they are. Wood products might swell, bow or twist a bit between when you fabricate them and when you are ready to assemble them. When you are putting the panels together you might not be pulling them together as tightly as they need to be either. This issue may or may not be a problem depending on the nature of your project.

For one thing the smaller your building is the less likely it is that there will be enough of a difference to bother you. Also there might be some automatically compensating factors about the way that your building goes together. For example if your entire building is made out of panels from floor through the roof then the entire overall size of your building might just be a little bigger than you thought it would be. The assembly of your floor panels might grow a little such that you do not notice that your wall panels grow a little when you assemble them. Also the size variation tends to occur at panel junctions. This fundamentally means that fewer panels means less size variation.

There are two basic ways to deal with this issue. One is to design your building so that you can tolerate it being slightly bigger. The other approach is to intentionally fabricate some part of your design slightly smaller in anticipation of the growth. Remember it is a lot easier to add a thin spacer or shim between panels than it is to cut a panel smaller on the job site. One good place to consider doing this for wall panels is at the corner where walls will connect. One panel will typically have sheathing material that overlaps the end of the other wall. Consider making the wall framing on that panel just a little smaller. It would be a very good place to add a small space if necessary and the sheathing material makes up for their being a small gap between the panels. Refer to the drawing below which shows a top view of a wall corner for where you might do this.

•18 - Panel junctions and corners

There are plenty of books available on the topic of residential wood frame construction. I do not want to even try to give a detailed description here of all the possible aspects of wood framing. Rather I would like to talk about what might make sense for your framing at the places where panels connect to each other. There are of course a number of types of panels that might be included in any given project. I want to focus on wall panels for this post and perhaps reserve some discussion about floor and roof panels for another time.

Wall panels have a variety of requirements. Of course there are structural issues but in particular I am referring to issues that impact where you might want to have joints between panels, which then imply panel connections. This can be complicated by the fact that you want to have openings for windows and doors as well as places in the middle of a panel where an interior wall might need to connect. There also is a requirement that sheathing material have something solid to attach to at the necessary locations.

When you are framing a wall in a normal stick frame situation the framing members (studs) that are placed at the joints of the sheathing material are usually just centered on the joint such that both sheets of sheathing can attach to a single member as in the first part of the below diagram. It is possible to make prefabricated panels with a joint of this type where the stud is attached to one panel with enough of it sticking out so that the next panel can be attached to it in the field. It is often more convenient to have two studs at the intersection of the wall panels as in the second part of the diagram below. This does require an additional stud but it makes the joint much stronger and the attachment of the sheathing materials more secure.

If your panels have a single stud attached to only one of the wall panels at the intersection point then you will need to attach the sheathing and at least the top plate of the other wall panel to the stud. With two studs at the intersection you will only need to attach the studs to each other. If your panels do not have the inner sheathing installed when you are assembling them in the field then you can easily nail, screw or bolt the panels to each other through the two studs at the panel intersection. This would of course make it more difficult to take your structure apart later if once you install sheathing on the inside.

This is one of the challenges that Michael Janzen faced in the design of his prefab panels in the eBook to which I link here on my blog. Michael wanted to be able to pre-install plywood skins on both the inside and outside of his panels. He also wanted it to be possible to take the structure apart without having to remove the inner plywood skins. To resolve this issue he has detailed an approach where he makes provision for adding a plywood spline that connects the two studs together underneath the inner plywood skin. This is shown in the following detail where the solid red portion is the spline. Screws can be driven through both the inner skin and the spline from inside of the structure.

•19 - Panel junctions and corners – continued

Once your structure gets big enough to have more than one room you may encounter the need to have interior walls. These interior walls will need to be connected to other walls. Typical framing techniques might provide a connection point in a wall for another wall to attach to using an approach something like what is shown in the following two diagrams. Notice that there are 3 studs required in each approach. The only difference here is that the inner skin of the wall continues through the joint in the second example. This might be the case if the interior wall skins are going to be pre-attached to your wall panels.

There are some advantages to using the above type of wall junction. For one thing it does establish a precise point of connection for the intersecting wall – at least for the version where the inner skin is not covering up the junction. The assembly of 3 studs can be treated as a sub-module and can be pre-assembled prior to incorporation in the wall panel. One disadvantage is that it is difficult to insulate. Another disadvantage is that it will often be in conflict with other framing members such as those needed around doors and windows. This typically ends up requiring variations on the basic arrangement.

An alternative approach, which I favor, is considered to be an example of “advanced” framing in some building codebooks. It is considered advanced primarily because it is much easier to install insulation. In this approach a sort of ladder framework is constructed where there are several horizontal 2×4 members connecting between the two studs that are nearest to the location of the junction point. The two next diagrams show in plan view how this would work. As in the previous two examples one of the diagrams shows the sheathing material stopping at the point of intersection and the other shows the sheathing material continuing through the junction.

The horizontal framing members can typically be spaced at about 2’ on center from top to bottom of the wall. This approach can often result in some savings of material depending on where the wall intersection occurs. In the diagrams above for example the horizontal members are placed in a location that is part way between two studs. These studs are at 16” on center. Using the other type of wall junction you would need to add three studs at the junction point. In this approach about ½ of a stud is needed for the horizontal members. That is a savings of 2.5 studs just for this one wall junction. I think that you can see that it would also be easier to install insulation behind the ladder framework that this type of assembly creates. Here is a drawing of what the ladder assembly would look like:

More details coming…

•20 - Panel junctions and corners – more

Lets talk a bit about wall corners. The style of corner intersection that I favor the most is one where you add a pair of 2x members in an L shape. This gives a solid attachment point for the intersecting wall as well as an adequate backing for interior wall sheathing. Consider the following diagram that shows the outside corner at the lower left of the drawing.

As I have mentioned before the wood members in the corner can be prefabricated into a small sub-assembly or module.

What if your floor plan requires an inside corner on the outside wall? Well you could use the same arrangement as in the above diagram but with the outside sheathing on the other side of the panels. You would have to carefully cut back the outer sheathing at the right point for the lower right wall panel to intersect. An easier way to do this type of intersection would to let the outside sheathing of the left hand wall go all the way to the end of the wall panel as show in the following diagram.

If you take this approach then you might not actually need to use an L shaped assembly in the corner of the left hand wall. Instead it might be just fine to use the configuration shown in the following diagram. Whether or not this will work well for you depends somewhat on how you are fastening the panels together. If you are leaving off the inner sheathing so that you can attach the wall panels together through the framing members then this would work fine.

•21 - Floor Systems

Prefabrication of floor systems can actually be a more complicated problem than prefabrication of walls. This is primarily because there can be such a wide range of sizes and load bearing requirements. It is not unusual for manufacturers of prefabricated wood frame houses to just pre-cut floor systems. While I contend that any amount of factory work done prior to actual construction on-site can be classified as prefabrication this is not as much prefabrication as we have been discussing so far with walls. Another reason that floor systems are sometimes just pre-cut is that floor panels typically have a lot more air in them. This is because floor-framing members are quite often larger in dimension than those in wall panels. For example typical walls might be constructed with 2×4 or 2×6-framing members while floor systems might be built with 2×8 or larger framing members. The members could also be spaced closer together.

One other aspect of floor systems that can contribute to making panelizing more difficult has to do with insulation, wiring and plumbing. Wall panels most typically have at least the outer skin attached. This leaves ready access to the inside of the panel so that wiring, plumbing and insulation can be installed from inside of the building. It is more convenient on the other hand for a floor panel to have the top skin installed. This makes access to the inside of the panel a little more difficult since it must be accessed from below. This might not be a problem if there is enough space underneath the floor. Still it can complicate the overall process of completing the floor part of the assembly. Supplying the floor system in a pre-cut only kit sometimes is worthwhile because many of these other installation problems can be avoided.

There is also potentially a very large difference between the bottom floor of a house and the other floor levels. If the bottom floor is built directly over the ground it is easier to provide support points at predictable increments that are small enough to make building pre-assembled panels feasible. You could easily build floor panels that are 4’ x 8’ in size if you could support the from beneath at least at their corners.

One other factor to consider when you are thinking about pre-cut or panelized floor systems has to do with how you will attach the panels together. Conventionally framed floor systems typically rely on the overlap of plywood sheets to fasten the various sections of the floor together. In a panelized approach some other means is required to fasten the panels to each other. This is easily enough accomplished if you have access to the panels from beneath. If, on the other hand, the panels are fully enclosed them some other technique is required. One approach is the type that Michael Janzen adopted in his pre-fab house plans (more information is available by clicking on the link on this site). In that design Michael uses a plywood spline between the panels and under the top level of the floor sheathing. This is very similar to the way that he attaches his wall panels together. Please refer to the following drawing repeated from an earlier post that shows the spline in red. Of the course the floor panels might be thicker than what is shown here.

•22 - Floors and Foundations

Obviously any building needs a foundation of some sort. Foundations for prefabricated structures have a few requirements that might not be quite as important for stick-framed structures. The one that is probably the most important is that the foundation needs to be more accurate than might be tolerated for stick-built. For prefabrication the foundation needs to be not only the right size but also needs to be as close as possible to the exact shape as the prefabricated building that is to sit on top of it. It also needs to be as flat and level as possible. Prefabricated panels are of course designed to fit together in a specific way. They are not as easily adapted on the job site to accommodate sloppy foundation work. I can remember when I was working for a wood frame prefabrication company that we would sometimes go to the construction site before the concrete was poured and check some of the key dimensions ourselves. Also it was sometimes useful to check the poured concrete after the forms were off to see if any dimensional changes should be made to the set of panels before we finished manufacturing them.

If you are building a DIY project and you are the one that is building the foundation itself then you have somewhat more direct control of the overall dimensional quality of the foundation. If you are having someone else build it for you be sure to stress the importance of the dimensional accuracy. There are some design tricks that you can play though that make for a less critical dimensional relationship between the foundation and the prefabricated building.

One suggestion that I have in particular is that you consider some type of post and beam foundation system. In this type of system the floor framing members or prefabricated floor panels span across and beyond the supporting foundation beams. Consider the type of foundation shown in the following drawing.

I think you can see that the placement of the beam relative to the floor panel that is sitting on top of it is not particularly critical in the left-right direction as shown in the drawing. Of course the beam needs to be level along its length as well as level with the other beams in the foundation. It also needs to be long enough to support the floor panels out to their edges. Otherwise the location of the beam could vary considerably without hurting the overall structural integrity of the building. By the way this type of foundation system is what is used in Michael Jansen’s prefabricated small building plans.

•23 - Roof Systems

Whether or not a roof system can be panelized is very definitely a function of its complexity. Of course it is also related to the maximum size of panels that can be built given the means of transportation selected and the means of installation to be used. For example if panels will need to be manually hoisted into place then there will be fairly restrictive weight and size limitations. For a good example of some practical roof panel designs for manual installation take a look at Michael Janzen’s prefabrication plans listed at the top right of this blog. Most of his panels are 4’ wide and less than 12’ long. Even so their overall weight will require some muscle to put them in place.

The overall design of a roof system may very well determine the feasibility of panelizing vs. pre-cutting. I worked for a while at a wood frame prefab shop in the late 80’s where we shipped quite a few house packages to Japan. The average floor plan in Japan was definitely smaller than in the United States. At that time the average family of 4 lived in about 400 square feet of space. What I observed with the projects that we built was that what these houses might lack in overall size was very definitely made up for by complexity of roofline. More often than not the designs had full hip roof lines. For those that are not familiar with that terminology it is simply a roof where all of the sides of the roof come down to the top of the walls. This type of roof is definitely more complicated in that the rafters that are required to make it are all different lengths and typically have compound angles cut at their top end. Certainly this type of roof could be prefabricated into panels but we found it was generally easier to carefully pre-cut and label all of the pieces and furnish a detailed drawing of their placement. Usually the plywood was not precut but it could have been.

•24 - Dormers, Bay Windows and Cupolas

As I have said before the degree of pre-assembly of subcomponent parts depends on many things not the least of which is the overall size of the component in question. Architectural details such as dormers, pop out bay windows or cupolas can be pre-assembled if they will be small enough to ship and install once assembled. The company that I worked for would sometimes pre-assemble smaller garden window frameworks such as might be found over a kitchen sink. For larger pop out bay windows we would more typically pre-cut the various parts for assembly on the job site just because of their overall size and awkward shape for shipping purposes. I remember once designing and building a small dormer in such a way that it folded up for shipping.

A good example of the prefabrication of some architectural details such as cupolas and dormers can be found at the following web link:

http://pennypincherbarns.com/Accessories.aspx

•25 - Stairs

One significant advantage of building a carefully dimensioned prefabricated structure is that you can accurately predict the dimensions of various subcomponent parts. This of course is one of the key benefits of prefabrication in that more things can be prefabricated. On type of subcomponent that can be prefabricated that I have not yet said anything about are stair parts. I have personally designed a number of stair systems that were at least partially preassembled in the factory. The remaining parts that were not pre-assembled were at least typically pre-cut. Stairs that are not simple straight runs are usually composed of either triangular steps or landing platforms. Sometimes I was able to prefabricated landing levels so that they stacked on top of each other when they were installed on site. I can also remember one curved staircase where we prefabricated the curved handrail system in the factory. We first prefabricated and temporarily assembled the stairs in the middle of the factory floor. We then had an expert in stair railing come in and pre-build the railing in place as though the stairs were in their final location. The rail parts were curved, glued and clamped to the top of the stair treads with special brackets designed for the purpose. There were a few joints in the rail assembly that were only bolted together with concealed fasteners to make sure that all of the parts aligned properly with each other. In the final assembly glue would typically be applied to these joints as well when they were bolted together in their final location. One all of the glue had dried the railing was sanded and then taken a part for shipment. I think that this particular house was sent to Japan.

•27 - Plumbing Trees

So if you know where all of the walls are going to be and where the framing members in the walls will be why not do some prefabrication of plumping parts? You can at the very least make up a fairly accurate parts list to have on hand when it is time to install the plumbing. Some aspects of a typical rough plumbing installation could easily be predicted accurately enough to prefabricate if you wish. One disadvantage is that preassembled plumbing trees could be a little awkward to pack and ship to your job site. On the other hand you could easily consider pre-installing some of the plumbing components into your building panels where they might be taking up otherwise wasted shipping space. Pre-drilling holes in places where you know you will need them can result in some job-site timesavings. You might even be able to do enough pre-drilling that you do not need to do any on the job site.

•28 - Wiring Harnesses

Factories that build automobiles and airplanes typically completely prefabricate their wiring into wiring harnesses. To do this accurately requires the building of jigs and fixtures that allow the wires to be placed in the correct relationships to each other. Wire connectors can even be pre-installed at the right locations on the wires. While this level of detail might not be practical for a full house I suggest that at least some pre-cutting and prefabrication of wiring related products could be useful for you DIY project. At the very least if you carefully think out where your wiring will need to be placed you can get a fairly accurate count of what parts are needed. You should also be able to pretty accurately predict where all of your outlets and switches need to be. How about pre-installing at least some of them in your wall panels? I have read about panelized housing approaches where electrical boxes and wiring is largely installed in the panels before shipment. It is very typical for wiring to daisy chain from one box to the next. In these prefabricated panels the piece of wire that connected across panel boundaries was pre-attached to a box in one of the panels. Enough wire to connect to the next box in the adjoining panel was simply coiled up at the edge of the wall panel awaiting connection the rest of the way to the other box. If you do not want to pre-attach your electrical boxes you could perhaps at least mark where they should go on each panel as you build it.

Structural Insulated Panels (SIPS) typically have a solid core of insulation foam between skins of OSB or plywood. Adding wiring after the fact would be pretty hard. SIPS that use preformed expanded polystyrene foam (EPS) typically have wiring channels precut inside of them during the manufacturing process. The design process needs to specify where these wiring channels need to be located. SIPS that are made using polyurethane foam insulation that is foamed in place during the manufacturing process require a different approach. For this type of panel plastic electrical conduit and electrical boxes are pre-placed in the panel before the foam is injected. Obviously this approach also requires predetermination of the locations where the wiring should be located.

•29 - A Practical Example

Over this last weekend and a little into this week my youngest son and I built a back yard wood shed. While we did not strictly speaking need to apply very much in the way of prefab techniques I decided that I wanted to use it as a small practical example of how prefab techniques could be applied to a simple project even if the prefabrication was not being done in a location remote to the construction site. The building is 12′ x 12′ in size. It has a simple roof line with a roof pitch of about 4/12. If you have been following my other posts here in this blog you will recognize that I used a ladder style of wall panel design rather than the more conventional version where all of the studs are vertical. I used Hardiboard 4’x8′ sheet siding. All of the wall panels use a full sheet. The floor system sits on concrete piers. We dug relatively shallow holes for each of the piers, poured in a bag of dry ready-mixed concrete and set the pier on top. In my experience the dry mix will eventually absorb enough moisture from the ground to become solid. In the mean time it provides a sufficient base for the building even if it is still in dry form.

I wanted the floor system to be relatively low profile so as to keep the overall height of the building low. Rather than placing beams on the pier pads and then laying joists across them I took a somewhat more complicated approach where I used 4×6 beams, attached 2×4 ledgers to the bottom of each beam and then added 2×4 joists between them. The drawing below shows the arrangement. I could have used only 3 beams and used 2×6 joists. I would not have had enough height on the beams to have used the ledger approach but metal joist hangers would have been fine. The following photograph shows a part of the actual floor frame.


Each of the wall panels were sized such that the bottom of the 4’x8′ Hardiboard sheet would overlap down over the floor frame. This meant that each of the vertical members had to be cut to size. There were 3 different types of panels. One panel was a full width panel. The other two were designed so that the 4’x8′ Hardiboard sheet would overlap another wall panel at the corner. This meant that I had both a left hand and right hand version of the panels. The main thing that I would like to point out in this post is how I prefabricated the wall panels.

I build a very simple framing table out of a sheet of 7/16″ thick OSB with a frame of 2×3 lumber around the edges. I supported the table with two standard folding plastic saw horses. I used a black Sharpie marking pen to draw lines on the table to indicate where framing members should be placed. I also drew a pattern for my rafter on the table. I screwed down a few short pieces of 2×4 block at strategic locations to help make it easier to quickly position the framing members when building a panel. It was then a pretty simple matter of placing all the framing pieces, nailing them together and then adding the siding sheet on top. In this case I was fortunate to be able to borrow a framing nail gun from my son-in-law that was very helpful for nailing the 2×4 lumber together. I also rented a siding nail gun for nailing the Hardiboard siding in place. If I did not have access to a framing nailer I probably would have used decking screws to assemble the frame. I find that it is easier to hold things in place if you can quickly attach members with a nail gun or a screw gun. Manually nailing the members would certainly be possible but harder to do on a table at the height of mine. If I were to manually nail the panels I would probably have set the table much lower – maybe directly on the floor. You will have to look closely in the photos below to see some of my lines. Hopefully the idea will be obvious enough. The second photo shows the frame members on the table just after they have been nailed together. Notice the nail gun.

•30 - A Practical Example - Continued

As explained in post number 29 I decided to prefabricate the wall panels of a 12′ x 12′ storage building that I built recently. This post continues where the previous post left off. As I said before I did not strictly speaking need to use the prefab techniques that I used since I was not going to be transporting the panels to a different place for assembly. These panels were in fact installed around the edges of the floor immediately after each one was built. The prefabrication table was sitting right in the middle of the new building. I think you can see what I mean by referring to the following photo.

When the 4′ wide wall panels are attached to each other a double stud (vertical member) is created. In my design of this shed my rafters fall at 4′ on center so this means that they will always sit on top of a double stud. This also helps explain why I do not need a double top plate (the horizontal member at the top of the wall). By the way you will notice in the above photo that there is no top plate on the top of the walls. This is because I added a 12′ long top plate at the top of the wall panels when they were all in place. This next photo show pretty clearly how each of the homemade trusses sits on top of the single top plate directly above a pair of studs. It also shows how we attached 2×4 purlins (the horizontal cross members laying on top of the trusses). Flat 2×4 purlins are capable of supporting the type of roof sheathing that I chose to use which needs to have purlins rather than rafters that are closer together.

The type of roofing I used was a product that I found at Lowes called Ondura roofing. It is a composite material in a corrugated form. The sheets are 48″ x 79″. I designed my rafters very carefully so that I could use full sheets at 79″ long. This meant that my roof overhang had to be a little smaller than I might have liked but it works out OK. This next photo shows a view of the completed roof.

•31 - A Practical Example - The Finished Product

My little 12′ x 12′ woodshed is pretty much done now. I had a friend of mine who needed a little work come over and paint it for me. I also built the sliding barn style door and got it mounted to the track. I still have a couple of minor details to finish like installing some kind of latch on the door and adding the roller on the bottom edge to keep it from swinging in and out.

One neat little detail that I was able to incorporate in the door was to reuse a wood frame window that I removed from our main house a while back when we updated to double pane vinyl windows. It was originally installed tall and narrow but it was just the right size to put in the top of the door turned on its side.

See what you think…

So what did it end up costing to build this little building? I will still have to go back and total up all of the receipts but I can tell you that the total for the materials was definitely less that $1500 total. The majority of the lumber, siding and foundation materials was just a little over $800 and two pickup truck loads from Home Depot. The roofing materials came from Lowes for right around $300. The sliding door track and hardware was just over $100 from a local specialty hardware store here in Portland called W.C. Winks. I had a few odds and ends of materials on hand already but not that much overall. The window was the main special thing I had on hand. The 1×3 trim was a real deal from our local Habitat for Humanity Restore. I think I paid about $12 for most all of what I needed. I borrowed the framing nail gun from my son-in-law which was a big help. I rented the siding nail gun from Home Depot and ending up keeping it longer than I had intended and had to pay something like $97 for it. I could have bought a new one from Harbor Freight for that amount or less. I already have an air compressor and the other tools that I need. I did pay for some of the labor since my youngest son is not fully employed at the moment and my painting friend needed the work too.

How long did it take to build? I spent about 4 hours shopping for the first big pile of building materials the day before we started in earnest. I think I built the framing table top on that first partial day too. The nearest Home Depot is only about 1 mile away so making two trips was not too big of a deal. I also have a pickup truck so that helps. It would have saved some time to have ordered the stuff from a local lumber yard and have had it delivered. With my son’s help we got to the top of the floor on the first full day. We had to do a little more digging than we had anticipated for that part. Also the woodshed is maybe 75′ from where we unloaded the building materials so we had a lot of carrying to do. Day two we had built the walls all the way to the top. Day 3 we had framed the roof and installed the roofing. On day 4 I had to get back to my day job while my son installed and fully caulked all of the exterior trim. The painting took about 1 day total spread out over two partial days. I also built the door over a few hours. The total construction time to get the little building to where it is now appears to have been about 9 total man days of effort. I will have to spend an hour or two to finish up the minor details I mentioned above and to clean out the sawdust and wood scraps out of the building. The interior is completely unfinished with nothing painted so it would take more time to finish the inside more if someone wanted to do that.

•32 - A Prefabricated Boat Shed

My neighbor Art is retired and loves to build wooden boats and archery equipment in his spare time. His garage was getting a bit crowded for his hobby so he and I collaborated on the design and construction of a workshop in his side yard. Art wanted a shop that was long and narrow to accommodate the beautiful wooden canoes and kayaks he builds. So the new building needed to be 8′ wide by 20′ long. Art also wanted to have an abundance of natural light inside as well as some passive solar heat when it was available. He originally thought about having large windows on the south side of the shop but that side faces the neighbors yard. I suggested using polycarbonate panels to make at least some of the roof into a skylight instead. We eventually decided to use polycarbonate panels for the whole roof. Art also wanted to have a generous overhang all the way around the building. He wanted the ceiling height to be generous so that he could store a boat or two up near the ceiling. The design that we came up with and built is described here along with photos of the progress.

The foundation was very simple and consisted of 2 parallel 4 x 8 beams resting on adjustable concrete pier pads as shown in the following photo.

Art and I constructed a 4′ x 10′ work table out of a sheet of plywood and some 2 x 4 that we could use as a template for building both the floor panels and the wall panels. The following photos show the work table with a partially constructed floor panel in place. Notice the 2 x 4 blocks that have been screwed down to the table to facilitate quick placement of the floor framing members. In the background you can see that the first floor panel has already been positioned on the supporting beams. These panels were going to be insulated so they were built with OSB sheathing that was for the bottom side of the panel. The panels were then flipped over before attaching them to the support beams. It was easy to nail or screw through the OSB into the support beams to hold everything together. We used construction adhesive and nails to attach the OSB to the floor panel framing which was made out of 2 x 6 lumber. Art installed fiberglass insulation in the floor panel cavities and added 3/4″ plywood for the sub-floor. The 3/4″ plywood was installed length wise of the shop which helped tie the floor panels together. The weather was nice enough that Art and I could build and install the floor panels in the same session.The whole floor had to be covered up with a tarp to protect it from the rain that came in shortly after getting the plywood sub-floor in place.

The weather turned uncooperative so prefabrication of the wall panels and the roof rafters moved into Arts garage. The wall panels were each prefabricated on the same 4′ x 10′ work table that was used to frame the floor panels. Of course the locating blocks were moved a bit. Both the floor panels and the wall panels were designed with a similar ladder type of construction so the changes to the template were actually minimal. The wall panels were built taller to give more overhead space in the shop according to one of the original design objectives. The wall panels were sheathed with a rough sawn plywood and they had a tail that hung down below the bottom of the wall panel so that it would cover the edges of the floor panels and could be fastened to them. The following photo shows the assembly line in the garage with the work table and a stack of finished panels ready to install when the weather got better.

As soon as the weather got better Art and I and Art’s son-in-law were able to start installing the wall panels.

This design a bit unique in that it has a barrel vault roof line. I designed it so that the roof could be covered with 12′ sheets of corrugated polycarbonate material that spanned all the way from one side to the other of the workshop. The panels were flexible enough to bend to the barrel vault shape. Art used his band saw to cut the curved rafters out of 2 x 12 lumber. The rafters are spaced 4′ apart so that they are supported on top of the double vertical 2 x 4 studs where the wall panels come together. I designed the wall panels so that the sheathing plywood also extends above the top of the wall panel framing to close in the space between the rafters. If you look closely at the previous photo and the following one you will see that there is a notch in the plywood for placing the rafters. The plywood at the top of the wall panels on the ends of the workshop were cut to fit the curve of the rafters that are placed at the ends of the building.

The following photograph shows how we added 2 x 4 purlins length-wise of the building on top of the curved rafters. Notice too that the purlins closest to the tops of the wall panels are placed so that they can be attached to the top of the wall sheathing plywood helping to tie down the roof system to the walls. You can also see how the polycarbonate sheets have been placed across the purlins. The sheets were fastened down using special screws that have a rubber washer attached to them. Clear sealer was also used at each joint where the polycarbonate sheets overlap each other to help make sure they are water tight.

This next photo shows the outside of the building pretty much complete except for final painting – which had to wait for better weather. You can see here how the curved rafters and the polycarbonate roofing extends out to provide an overhand that is almost 2′ deep. You can also see the row of windows that have been installed on the north side of the building. These windows were custom fabricated by a local supplier to fit exactly within one of the spaces in the ladder like layout of the wall panels. The building was also dressed up a bit with 1 x 3 trim. The double front doors and the single side door were fabricated out of the same plywood as the wall panels. In fact they were made out of the piece of plywood that was cut out of the wall panel to make the door opening.

This next photo is a nice shot of part of the interior that shows the windows from the inside. It also shows the 1/4″ birch plywood interior sheathing that Art installed as well as one of his nice wooden boat projects. Art installed fiberglass insulation in the walls before installing the 1/4″ plywood.  A less than obvious detail is that I specifically designed the wall panels so that a full sheet of 1/4″ plywood would fit from the floor to the bottom of the rafters without having to be cut to length. I thought that was a nice extra touch.

The nearly completed 8′ x 20′ workshop as shown in the two following photographs has plenty of room for Arts equipment and storage. It is well lit and inviting and has extra overhead storage space that makes for more room below. Art has since completed painting the outside and has added bamboo flooring. He contemplates adding a layer of clear plastic film to the bottom side of the curved rafters or the purlins to provide extra insulation. If you look closely in the first photo you can see part of my little red barn in the background.

 

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