Using plywood as a subfloor has become the standard for contractors and homeowners alike. Plywood is available in 2.4 by 1.2 m ( 8 foot by 4 foot) sheets, which are easy to work with and to lay down quickly. The plywood thickness most recommended for subfloor applications is 21.8 cm (3/4 inch). The grain should run perpendicular to the floor joists. Leave the best side of the plywood facing up to allow the smoothest surface to receive the finished flooring.
Lay the first 2.4 by 1.2 m (8 by 4 foot) piece of plywood down over the joists. To prevent the subfloor from squeaking when walked on, squeeze a small bead of construction adhesive, using a caulk gun and adhesive tube, onto the joists that the plywood will contact. Remember to lay the smooth side of the plywood up, and make sure the grain runs perpendicular to the floor joists.
Nail down the plywood subfloor with a hammer and 8d nails, which are 6.2 cm (2 1/2 inches) in length. If the subfloor is being placed over a ceiling, use a drill with the appropriate bit, then secure the subfloor to the joists with decking screws. This will eliminate any plaster cracks and damage on the ceiling below from pounding required when using a hammer and nails.
Use your tape measure and mark off a 1.8 by 1.2 m (6 foot by 4 foot) piece of plywood. Use another piece of plywood as a straight edge, and draw a line with your carpenter's pencil.
Place the piece of plywood on saw horses and cut it along the marked line using a circular saw with a plywood blade.
Place the board so that the 1.2 m (4 foot) edge is aligned with the 1.2 m (4 foot) edge of the first sheet and the 1.8 m (6 foot) edge runs parallel with the 2.4 m (8 foot) edge of the first sheet. Use a credit card or an 8d nail to ensure a 3 mm (1/8 inch) gap between pieces, apply adhesive to the joists, then secure the plywood with nails or screws.
Cut a 1.2 by 1.2 m (4 foot by 4 foot) section of plywood and place it so that one side continues the straight edge formed by the 1.2 m (4 foot) sides of the first two sheets. Ensure the gap is correct with either a credit card or an 8d nail between pieces and secure it to the floor joists.
Continue laying pieces of plywood in this staggered-size method, working from side to side until the entire subfloor is complete.
For posts, studs or doorways that jut into the subfloor area, copy the shape of the intrusion on a piece of newspaper or other suitable material. Cut or tear away a basic outline in the newspaper, then transfer this outline onto a piece of plywood. Use a jigsaw to cut out the intrusion, then fit and nail or screw the plywood into place.
Always wear safety glasses when using power saws.