Best when read in chronological order – see Outline above
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.
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.
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.
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.
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.
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.
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.
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:
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.
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.