Frame a Floor Platform That Comes Out Square

The Problem

Every wall of the house stands on the floor platform, so an out-of-square platform sends its error up through the whole build. The squaring, the rim joist nailing, and the joist layout each have a specific order and a specific rule, and they're cheap to get right at this stage and expensive after.

The Technique

1. Square the mud sill by the diagonals. Measure corner to corner both ways. If one diagonal reads 67-3/4" and the other 67-1/4", the difference is 1/2" — shift the frame half the difference, 1/4", toward the long diagonal, and re-measure until the two numbers match.
2. Install the rim joists around the perimeter. Each rim board sits on the outermost edge of the mud sill, face-nailed through from the outside, then toenailed down into the mud sill from the inside.
3. Pull joist layout. Hook your tape on the outside face of the rim joist, mark every 16 inches, then come back 3/4" and square a line with an X on the side the joist covers. The 3/4" setback centers each 1-1/2"-thick joist on the 16-inch grid, so sheet goods break mid-joist.
4. Set the joists. Cut each joist, stand it on its line covering the X, and face-nail it through the rim joist from outside. Toenailing each joist down into the mud sill as well is the stronger job.