How to Build a Garage With a Flat Roof

Install a concrete slab for your garage. Follow city code. After cured, start framing the walls. Measure slab and cut treated 2-by-4 boards to make bottom, or sill plate. Anchor down sill plate to slab with bolts 12 inches from the cut. Your starting wall is the full width of the slab foundation. The side walls overlap the ends of the first wall. Assemble starting wall on the ground and move it aside.

Build the lower ledger wall, the horizontal piece of timber, at the back end of garage. The opposite wall will have the higher ledge. Build the side walls. After the two side walls are built, lift the first wall onto the anchor bolts, hold in place and thread the washers and nuts on the bolts. Continue process with the rest of walls. Nail both walls together with 16D nails at the corners and match up the edges. Stand on the ladder and add a corner brace across the two top plates.

Sheath the outside walls. Nail in plywood every six to eight inches on interior studs with 8D nails in the nailer. Measure and cut wood hangers for your roof. Mount hangers on two opposite walls, at 16 inches, or 24 inches if using 2-by-6 wooden beams. Timber beams must be inserted between the joists at least at 24 inches to avoid twists in the joists. Measure pitch with level, cover with plywood and nail down.

Use a gutter at rear overhang if you have a slight pitch. Any pitch over 10 degrees may have to dispose of rainwater. Sagged joists allow rainwater to pool at low points in roof, so Install a roof drain. Cut out a 6-by-12 inch inch drain spout hole in the roof's plywood. Around the drain hole, wrap up the metal flashing, edging and diverters. Run aluminum piping down to drain.