On Saturday 7/2/16, Mikey finished cutting the last remaining piece of 5/8” plywood subfloor and fastening it in place.   There was a fairly quiet but enjoyable celebration of the accomplishment (probably some iced tea - no cake).

The plywood is arranged in six large and six smaller pieces, covering the six major rectangular spaces formed by the framing beneath.  Because a single piece of 8’ ply would not span the 99” floor completely (and 10’ long plywood does not seem to be readily available in our area), we decided to cut and stagger the pieces so that the larger pieces spanned across all but one of the joist cavities, with the last piece filling the remaining void. All joints between pieces are centered on 2x4 joists or blocking pieces, with an allowance of about 1/16” between adjacent pieces (lengthwise only) to allow for expansion. Unfortunately the fir plywood we ordered did not come “sized for spacing”, so we weren’t able to add spacing in both directions (I don’t believe this is as much of a concern with subflooring as it is with sheathing).

After each piece was cut and before it was installed, holes had to be bored in numerous locations wherever the threaded anchoring rods welded to the steel perimeter framing of the trailer protruded up through the subfloor framing.  The rods are 5/8” diameter, and after some initial experimentation with fitting the pieces (and a couple mishaps), Mikey settled on 7/8” diameter for these holes. 

Placing the cut pieces over the rods involved both of us, and some warping of the plywood made the placement a little challenging; but once the plywood was fitted over the rods, each piece dropped into place and only a very little trimming of the outside edges was needed on a few pieces to make them flush with the edges of the perimeter wood framing beneath.  After the pieces were in place and before they were glued and screwed down, we used some concrete patio blocks to flatten out the sheets. After about 24 hours the warping was completely resolved.

The plywood is fastened with #9 2 1/2” R4 GRK screws 6” apart on edges and 12” in the field. Before fastening down, we laid a bead of PL Premium adhesive to the top of all the joists, as is typically recommended for flooring.

A single thin coat of Thompson’s Water Seal was applied to the completed subfloor to keep it from absorbing any water while exposed to the elements (though we are doing our best to keep everything under tarps anyway).

Any concerns about plywood not lying flat on the subfloor framing because of rigid foam insulation being too tall turned out to be unfounded - with all screws holding the plywood in place, the resulting subfloor surface was completely flat, and - inexplicably - perfectly level in all directions! Note that if you are filling your floor cavities with rigid foam, anything thicker than 5/8” plywood is really overkill for the subfloor (even at 19 3/16” spacing), as the foam adds an immense amount of rigidity to the floor.

Next up (pun intended): Wall framing!