How to Lay the Subfloor to Wooden I-Joists

Calculate how many sheets of 3/4-inch tongue-and-groove plywood are needed to cover the floor joists. Use a tape measure to determine the square footage of the area by multiplying the length by the width -- round up to the nearest foot in each direction. Divide the total by 32 and round up to the nearest whole number to determine how many plywood sheets you need. Plywood is purchased in 4-by-8-foot sheets.

Use a circular saw to rip-cut the tongue off the first row of plywood. Measure from the rim joist -- which is the dimensional lumber that wraps around the edges of the I-joists -- to the center of the farthest I-joist that is less than eight feet from the rim joist. Snap a chalk line at that measurement so the two cut ends of the plywood will sit fully on the rim joists with the plywood running perpendicular with the I-joists.

Apply construction adhesive to the I-joists and rim joists that are going to be covered with the first row of plywood. Attach the plywood with 8d ring shank nails spaced 6 inches along the center of the rim joists and 10 inches along the center of the I-joists.

Install the first run of tongue-and-groove plywood. The tongue on the first row should be cut off so that each groove lines up evenly.

Measure from the rim joist to the center of the I-joist nearest to the center of the first piece of plywood. Cut the plywood with the cut end on the side of the rim joist. Apply construction adhesive to the I-joists and lay the plywood against the installed subfloor. Set a short block of wood on the outside edge of the plywood and tap it with a hammer until the tongue seats fully into the groove of the previously laid plywood. Nail the plywood in place and finish the run.

Install the next run of plywood with the seams on the same I-joists as the first run and alternate the seams of each run until all the subflooring is installed.