Tongue-and-groove plywood is the product used by contractors to install a subfloor over framing joists. A subfloor adds stability and provides a base for the finished floor on top of it. Tongue-and-groove plywood is particularly useful in applications where the subfloor is covered by resilient flooring, tile or carpet that is less than three-fourths of an inch in thickness. The tongue-and-groove design eliminates the need for the blocking between floor joists that would normally be required to provide a firm base for thinner flooring products. The plywood comes in a thickness of three-fourths of an inch, in 4-by-8 foot sheets.
Measure the area to determine the centre line of the room. Mark the centre point using your chalk line. Place the first plywood panel with the good side up and the long edge running at right angles to the joists. Align the first board along the middle of the centre joist and draw a line along the joist using your straightedge and pencil. Mark the centre points of the joists at the exterior edges of the plywood. Remove the plywood panel.
Apply construction adhesive with your caulking gun along the centre of the joists that will be underneath the plywood sheet. Place the plywood panel along the centre line with the good side up and the long edge running at right angles to the joists.
Fit the Phillips bit into your variable speed drill. Tighten the bit. Drive the deck screws six inches apart into the centre of the joists around the perimeter.
Drive the deck screws twelve inches apart along the centres of the joists that are now beneath the panel.
Place the next board along the first with the good side up and the long edge running at right angles to the floor joists. Ensure that the tongue edge of the sheet will slip into the grooved edge of the first one. Mark the outer edges with your pencil and straightedge. Apply construction adhesive along the joists as in Step 2.
Align your tongue-and-groove edges and tap them together lightly using your rubber sledge hammer. Allow an expansion joint of four-one-hundredths of an inch between the panels. Screw the panel along the joists as in Step 3.
Continue installing the plywood until it is flush with the edges of your perimeter joists. Cut the panels as necessary to eliminate overhang.
It is easier to install a full panel and cut it off at the perimeter than to measure the panel to fit a perimeter space. Plywood graded C-D indicates a good side-C and a rougher side-D. Always install your panels good side up.
Wear safety glasses when handling power tools. Hardhats and safety boots should be worn on all construction sites.