Garage conversions are an economical way to add a room to a home because the space is already enclosed. One area in which converted garages are often lacking is the floor: garage floors are typically cold, hard, uninsulated concrete. Simply placing tile or carpet directly on the concrete improves its looks but does little to add comfort. If ceiling height allows, a floor set on joists provides a softer surface while providing room for insulation between the living space and the concrete pad.
Locate low spots in the floor with a metal straightedge and apply levelling compound for filler.
Measure ceiling height to determine the maximum allowable thickness of floor joists plus subfloor plus flooring. Example: If you want a minimum ceiling height of 205 cm (82 inches) and your current garage ceiling is 225 cm (90 inches), the total of joists, subfloor and flooring must not exceed 20 cm (8 inches). In this case 5 by 15 cm (2 by 6 inch) joists will work.
Measure the slope, if any, of the floor from the garage front to the door. If more than 2.5 cm (1 inch), shim the joists near the door, trim the joists at the opposite end, or do both.
Lay an impermeable membrane on the raw concrete as a vapour barrier. This is especially important in a wet climate.
Stand 5 cm (2 inch) wide joists on edge, 40 cm (16 inches) on centre, starting at the doors and laying the joists across the room's width. Shim or trim (with a rip saw) as necessary to create a level surface. Be certain to frame space for any door that swings in to the garage.
Place cross-blocking approximately every 1.2 linear metres (4 linear feet), and within 60 cm (2 feet) of the end walls. "Sister" any butt joints with a length of lumber of the same dimension or a metal joist tie. To allow the floor to expand, leave 9 mm to 1.2 cm (3/8 to 1/2 inch) of space between the joists and the walls on the sides and at the ends.
Place fibreglass or foam-board insulation between the joists. Staple roll insulation to the top of the joists.The thicker the insulation, the more comfortable the floor's temperature.
Lay 1.2 by 2.4 m (4 by 8 foot) sheets of OSB (oriented strand board) or 1.8 cm (3/4 inch) plywood on the floor joists and fasten securely with screws. Maintain a 9 mm to 1.2 cm (3/8 to 1/2 inch) expansion space along the walls.
Follow the manufacturer's instructions to install the wood floor. Laminate and solid wood flooring can be nailed to the subfloor and joists, or you may install a floating floor. Add baseboard and base shoe moulding to conceal the expansion space around the edges.
A very thin subfloor can be laid on flat-lying 2.5 cm by 10 cm (1 by 4 inch) or 5 cm by 10 cm (2 by 4 inch) "sleepers" on 30 cm (12 inch) centres, with rigid foam insulation between them. Flat-lying sleepers must be nailed to the concrete to prevent movement. If there is a 2.7 to 3 m (9 or 10 foot) ceiling, consider building a fully framed floor attached to the walls.