Flat roofs can be afflicted with leaks, bowing and rot in areas with high rainfall and when there are many trees overhead. Most garages are not conditioned, but they are used as storage space. Leaks and structural instability from water damage can ruin stored objects and the structure itself. The addition of a pitched roof to a flat-roof garage can correct many problems found with a flat-roofed structure.
Sketch ideas for the replacement roof on a sheet of paper. Try different roof types and orientations. Gable roofs have two equal-size planes and look like an upside-down 'V'. Hip roofs, similar to gable roofs, feature sloping planes on the short ends, rather than the triangular face of the gable. Gambrel roofs feature two different slopes on the two faces. Some homes have butterfly roofs, which are inverted gable roofs. The simplest roofs are shed roofs, or one-way sloped roofs. The garage should match the existing house, so determine what type of roof the house uses. This roof type is almost always the best roof for an adjoining garage.
Make a final choice about the roof type to be constructed. Every roof type has slightly different methods of construction, however most follow basic principles.
Remove the existing flat-roof finish, but keep the roof structure. Check for water damage, and replace any rotten joists or sheathing.
Hold the braced ridge boards in place at the apex of the roof, and install the rafters, using a birdsmouth cut to accept the horizontal wood members of the walls' top plates and beams. A birdsmouth cut is a notch made in the angled rafter where it meets the plate of the building.
Place the remaining rafters 40 cm (16 inches) on centre.
Sheath the roof framing with exterior-grade plywood and polythene sheeting.
Make sure the structure is sound and does not shear with lateral pressure. This can be checked by pressing sideways on the garage. If the structure sways, the garage will need to be reinforced. Reinforce by using knee bracing at the intersection of the stud framing and joists, diagonal bracing across the interior side of the framing or plywood sheathing at the corners of the structure. Plywood sheathing is the fastest and easiest method of reinforcement. However, some structures may not be able to accommodate plywood. If knee bracing or diagonal bracing is required, install bracing at a 45-degree angle using prefabricated metal hangers for easy, secure connections to the framing.
Apply roofing felt underlayment and rolled roofing or shingles. Finish the structure with flashing at any intersecting planes, as well as fascias and soffits to the exposed eaves. Any window and door openings should be flashed as well.
Always follow the installation instructions provided with building products. Check local codes and ordinances for any building requirements or proscriptions. If you do not feel comfortable constructing the roof, hire a licensed contractor to complete the task.