After all framing was up, I started on two rows of blocking—pieces of 2x4 material fit between each stud and run horizontally along the length of each wall. Most codes specify that all edges of each panel of sheathing should land on solid blocking. This makes the structure much stronger. This will also give me solid backing to attach my interior plywood to. Because of the height of my walls, I determined that I would need two rows of blocking. I decided to run my blocking in a straight line, toe-nailing (well, screwing) one side of each block and face-nailing the other end. For the toe-nailing, I suggest using two screws, one coming up from below and the other down from above. This is much stronger than having both nails/screws pointing the same direction. I used a couple of pre-cut 2x4s to hold up each piece of blocking at the right height while I attached it to the studs.
I could have run my first row of sheathing vertically instead of horizontally, which would have allowed me to use a single row of blocking instead. However, for aesthetic reasons I would like to have my interior plywood run horizontally, which would have required adding additional blocking anyway, making the decision a bit of a wash.
Installing sheathing is a slow and awkward process: the panels are large, heavy, tough to position, and brutally unforgiving (if your structure isn’t quite square, now is the time you’ll find out).
Code requires nails/screws spaced 6” along panel edges and 12” along studs/plates “in the field” (i.e. all other wood members covered by the sheathing panel). What they don’t say is that this leaves very little room for nailing on edges where two panels meet up in the middle of a single stud. You want to make sure that you keep your nails/screws about 3/8” away from the edge of the panel, but you also want to make sure they’re secured well into the stud. To make this a strong connection, I suggest starting your screw at 3/8” from the panel edge and angling it into the stud at about a 10-20 degree angle. When fastening the adjacent panel, angle in the opposite direction and offset the screws by about 1/4” to avoid collisions with fasteners from the first panel. On the first couple of panels I staggered the screws half way between the screws on of the adjacent panel, but soon realized that this could cause issues later when I needed to drill through studs to run wiring. If you keep the screws closer together, you won’t have as much of a mine field of fasteners to work around later.
I also took to using an 8” spacing on horizontal edges where the panels landed on blocking, since this allowed me to attach every other screw directly to a stud, where it would have much better purchase than on the blocking itself (and could be fastened further away from the panel edge).
Traditional tiny house building wisdom suggests gluing all sheathing with construction adhesive. Now that I’ve gone through this process, I am not so sure that the pros outweigh the cons. For one thing, it is S-L-O-W. It requires dry-fitting your sheathing to make sure it fits correctly, starting a few screws, removing the sheathing, applying the glue, and re-applying the sheathing. Construction adhesive is messy and smelly stuff (Ideally, you should use gloves and a respirator), and it is frustrating to apply. However, once it is cured, it certainly does have the effect of solidifying the entire structure and helping to seal any cracks where air and bugs can get through.
Unfortunately, this also pretty much rules out making an alterations or corrections, or repairs in your framing—Not good for first-time DIY builders (see my dad’s last post detailing our wall straightening woes)! And given the quality of the structural screws I am using to attach the sheathing (GRK R4 #9 2-1/2”), I do not believe that the glue is really needed for strength.
That said, I have decided to continue gluing all wall sheathing, but will forego gluing the roof sheathing. (Applying glue to the rafters before laying down each sheet at 13 feet up in the air seems like it would be both difficult and dangerous).
If you do go the adhesive route, I have two helpful tips. One is to spring for the larger size caulk gun and canisters. You will save money and time, since the smaller ones will only cover about one sheet per canister. The other tip is to be aware that the caulk gun has a shut-off tab attached to the “plunger” which you can and should use between each bead of glue you lay down. This prevents the glue from continuing to be dispensed from the gun even after you’ve stopped applying pressure to the trigger. I didn’t know about this at first, and it nearly drove me crazy as it kept dispensing glue when I didn’t want it to (and wouldn’t dispense enough when I actually needed it!). Also, be sure you break the seal thoroughly before dispensing. Either way, it is a messy business.
One more detail about the sheathing. You will often see it recommended that a gap of 1/8” be left between panels at all edges to allow for expansion of the wood. 1/8” is actually quite a large gap. Ironically and inexplicably, most sheathing comes in full size, so following this rule means shaving 1/16-1/8” off of the edges of every panel(!) This is a tremendous amount of additional labor, which is why there are some sheets that are manufactured pre-cut and “sized for spacing”. I intentionally looked for plywood that was “sized for spacing”. Unfortunately, despite the the label on the sheets that I purchased (1/2” fir ply from Emerald Forestry Products), the plywood is, in fact, the FULL 4’x8’ dimensions! (It is possible that they were cut correctly but expanded due to moisture..?) I spoke to a couple local builders who hadn’t even heard of the 1/8” gap rule, so I probably shouldn’t be too concerned. Still, I’ve tried to leave a 1/16” gap, and have had to rip the edge of a few sheets to keep them from getting off the grid.