Converting a garage to living space is a project that can not only add living space, but can improve the value of your home. The project should begin with installing a wood floor structure to eliminate the traditional slope of a concrete garage slab, as well as provide usable space to insulate the area, include access for possible plumbing or electrical circuitry, and bring the floor up to the level of the adjacent house for a smooth transition to the new living space. The procedure to build a floor raised 8 inches can be adapted to any needed floor height by simply changing the size of the lumber and support spacing.
Make a pencil mark on the wall at each corner of the garage at the desired finished floor level, measuring down from the ceiling level using a tape measure.
Connect each of the marks with a chalk line to define a horizontal line on each wall at the finished floor height. These lines will be the referenced elevation for the floor structure.
Lay lengths of 2-by-4 pressure-treated lumber end-to-end, with the 4-inch side flat on the concrete slab against the exterior wall at the low side of the floor at the garage door wall. This is the first perimeter floor plate for the floor support.
Secure both ends of the floor plate using a powder-actuated nail gun or concrete nails.
Tack an 8d nail on top and approximately 1 inch from the ends of the floor plate, then attach a piece of string between the nails, stretched tightly and positioned 1/8 inch above the top surface of the plate, from end to end, across the entire length. This string line is used to level the floor plate as it is attached to the floor.
Secure the floor plate pieces to the concrete slab, using wood shims as necessary to raise the floor plate to 1/8-inch below the string, then attach to the slab using concrete nails through the plate and shims at approximately 36 inches apart.
Repeat Steps 3 through 6 to secure floor plates parallel to the first floor plate, and spaced equally from the front of the garage to the back wall. The floor plates should be no less than 4 feet and no more than 6 feet apart. Note that garage slabs typically are contoured for drainage near the garage door opening, and the levelling shims used in the first floor plate may or may not be required for the other floor plates.
Measure the vertical distance from top surface of the first floor plate to the floor elevation line marked on the wall, then subtract the width of the floor joists and the thickness of the plywood floor sheathing from the measurement to determine the size of the plate blocking. Example: The vertical measurement is 8 1/4 inches. For 2-by-6 floor joists and 3/4-inch plywood floor sheathing, the total subtracted would be 6 1/4 inches, for a remainder of 2 inches of plate blocking.
Cut lengths of plate blocking from framing lumber to the width determined for the plate blocking, using a table saw. Cut only enough to extend over the full length of the first floor plate, then attach the plate blocking upright and centred on the floor plate, using 8d nails spaced at approximately 16 inches apart.
Repeat Steps 8 and 9 for each length of floor plates installed in Step 7. Due to the typical slope of a garage floor slab, the plate blocking will diminish in width at each floor plate location and, depending on the joist size and floor height, may or may not be required on the last floor plate at the rear wall of the garage.
Lay out 16-inch joist spacing across the first and last floor plates, beginning at the same side of each. Mark the beginning of each layout at 15 1/4 inches from the wall, then spaced 16 inches apart for the rest of the layout. Use a chalk line pulled between the two plate layouts to mark the layout on the plate blocks in between.
Install the first floor joist against the wall, on edge and perpendicular to the floor plates, beginning on the side from which the layout was measured. Make certain the joist is resting flat on top of each incremental plate block, then secure to the wall, if possible, and at each plate block point, with 16d nails at an angle through the side of the joist into the plate block.
Repeat Step 12 for each floor joist across the width of the garage. If the floor length is longer than the joist boards, lap the joists 12 inches where necessary, and nail together.
Cut bridge blocking, also called "cross blocking," between and perpendicular to the floor joists, centred on every other floor plate and at each end of the floor joists. This bridge blocking prevents the floor joists from twisting over time as the lumber moisture content fluctuates.
Install any electrical, plumbing or HVAC lines planned for the living space, if applicable. Otherwise, proceed to Step 16.
Install 15-inch-wide batt insulation in each joist space, using a staple gun, keeping the foil or kraft paper facing upward toward the new heated space.
Measure a dimension of 48 1/2 inches from each rear corner of the garage floor and mark both perimeter floor joists, then use a chalk line to connect the two marks across the tops of all the joists.
Install plywood floor sheathing perpendicular to the joists, aligning the tongue-edge of the plywood sheet with the chalk line marked in Step 17, then secure the plywood to the floor joists with 8d ring-shanked nails or 2-inch stitcher screws approximately 8 to 10 inches apart. Make certain that all the plywood end joints in adjacent rows of are offset by a minimum of 32 inches, and that all end joints split on a floor joist.
Be sure to wear safety glasses when using concrete fasteners to prevent injury.