DIY-Prefab

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.