DIY-Prefab

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.