Things You'll Need
1½-inch-by-3½-inch pressure-treated lumber
1½-inch-by-5½-inch framing lumber
¾-inch-by-4-foot by 8-foot construction-grade plywood
3-inch concrete nails
2-inch decking screws
16d sinker or box nails
8d sinker or box nails
15-inch-wide R-30 foil-faced fiberglass batt insulation
Use a small sledgehammer to drive the concrete nails and keep them approximately centered in the floor plates and 3 to 4 inches from any ends to prevent the wood from splitting. Where shims are used as a support to level the floor plate piece, nail through or directly beside the shims.
The pressure-treated floor plates can be attached using a powder-actuated gun nailer and 3-inch pins in place of standard concrete nails. These gun nailers can typically be rented at construction rental outlets or home improvement stores. If this method is used, read and follow all required safety instructions accompanying the tool.
When converting a garage space into a living area, installing a framed wood floor offers an opportunity to add insulation from the cold concrete slab, access for electrical circuits to any interior walls being planned, and effectively provides a level floor platform over a standard garage floor that can be sloped 4 to 5 inches from front to back. The basic floor structure defined raises the garage floor level to match an 8-inch step from the residence floor elevation, using 1½-inch-by-5½-inch floor joists. The procedure can be adapted to any requirement by adapting the joist material size.
Measure the distance from the garage ceiling to desired final floor level at each corner of the garage and make a pencil mark on the wall. Use a chalk line to snap a straight horizontal line connecting these marks on each wall. These lines establish a floor plane that will be the basis for building the floor support structure.
Cut pieces of 1½-inch-by-3½-inch pressure-treated wood to the full inside dimension of the garage parallel to the garage door wall. This will be the first floor plate. Lay pieces flat on the concrete slab, end to end, against the inside face of the wall.
Hold the end of the floor plate flat on the floor at each end and measure from the top surface of the floor plate to the line marked on the wall in Step 1. If the two measurements are equal, secure the ends of the floor plate to the concrete slab with a single concrete nail at each end. If they are not equal, secure only the end with the shortest measurement and use shims to raise the other end to match, then secure with a concrete nail near the end and over the shims.
Nail an 8d nail on top at the ends of the floor plate cut in Step 3 and tie a piece of nylon string to each nail, as close to the top face of the plate as possible. Pull the string tight and secure so that it is held taut and straight along the full length of the plate. This establishes a line used to level the floor plate as it is attached to the floor. Use wood shims as necessary to raise the floor plate to ? inch or less below the string. Secure the floor plate to the slab, using concrete nails. Check string alignment frequently to ensure the floor plate remains level during nailing. Space the concrete nails approximately 24 inches apart or as needed to firmly secure the floor plate to the floor.
Repeat Steps 2 through 4 to install additional parallel floor plates at 48 inches on center (approximately 44½ inches apart) over the full depth of the garage, and install a full-length floor plate against the garage's rear wall. These plates must run perpendicular to the drainage slope of the underlying concrete slab floor. The joist support blocking will be attached to the top surfaces of these floor plates.
Return to the first floor plate installed in steps 2 and 3 and measure the distance at each end between the top of the floor plate and floor level line chalked in Step 1. Subtract 6¼ inches from this dimension for the floor joists width and plywood subfloor thickness. The remaining dimension defines the joist support spacer width at that floor plate level.
Rip a 1½-inch board to the spacer width determined in Step 6, using a table saw. Cut pieces to fit end to end over the full length of the first floor plate installed in Step 5 and place on edge flush with the outside edge of the floor plate. Attach the spacer pieces to the floor plate with nails approximately 18 to 24 inches apart as needed.
Repeat steps 6 and 7 for each of the floor plate lengths installed in Step 5. This creates a level support structure for the floor joists by adding incrementally diminishing support widths at each floor plate line. In this instance, there should be no spacer block required at the floor plate at the rear of the garage being the starting point of the leveling.
Lay out 16-inch joist spacing on top of the spacer blocking against the front and rear walls, using a tape measure. Make the beginning mark at 15¼ inches from the wall and 16 inches apart for the remainder of the layout. With both the first and last floor plate's layout marked, use a chalk line between the two ends to mark the joist layout on the remaining spacer blocks between.
Install the first floor joist on edge against the wall from which the layout was measured. Secure at each spacer block with nails through the side of the joist into the wall framing and/or the spacer block. Repeat for each 1½-inch-by-5½-inch floor joist across the width of the garage.
Cut 1½-inch-by-5½-inch cross blocking, also referred to as "bridging," between and perpendicular to the floor joists at 8-foot spacing. This places a row of blocking at both ends of the floor joists and above every other floor plate support location to prevent the joists from rolling or twisting.
Cut lengths of 15-inch wide batt insulation for each joist space and use a staple gun to attach the pieces to the joists sides. All open joist spaces should be insulated.
Install the ¾-inch plywood flooring by positioning the plywood length perpendicular to the joists, making certain that all end joints fall on a joist and that end joints in adjacent rows of plywood are staggered by a minimum of 32 inches apart. Secure the plywood to the top of the joists with 2-inch decking screws spaced approximately 8 to 10 inches apart. The floor structure is now complete.
Paul Massey has been writing since 2009, drawing on a 35-year career in the construction industry. His experience includes 15 years as a general building contractor specializing in architectural design, custom homes, commercial development and historic renovations.