The key factor in converting a flat roof to a pitched roof is the reason for the change. If the goal is to improve drainage and add insulation, a low pitch, like a slope of 7.5 cm (3 inches) per 30 cm (1 foot) (3/12) will suffice. If the motive is to gain attic space or even a room, a higher pitch, like 6/12 or 7/12, will be required. Either way, the old flat roofing must be removed and the roof secured and covered with tarpaulins for the several days it will take even an experienced roofing crew to do the job.
Strip the roof down to the joists, which run across the walls. Remove any roofing paper, asphaltic material, gravel, rubberised roofing and any plywood or other wood decking. Inspect the joists and wall tops to make sure they are solid. Clean off any debris and replace or pound in any loose nails. Get the joists and wall caps to as near original condition as possible.
Measure the building with a tape measure to get the length, for the number of rafters needed; width or span, distance from wall to wall, and rafter run, half the span and the space each rafter must support. Determine a pitch and a rafter size; low-pitch roofs will require thicker rafters because roof weight is less dispersed. Check local building codes, which often specify rafter sizes, or refer to a building table for maximum runs for various boards.
Lay out one pattern rafter, using pitch and run with a framing square. Put a proper size rafter board -- 5 cm by 10 cm (2 by 4 inch) in this example -- flat with the square at one end and the point (heel) at the bottom of the board. Put the pitch mark on the narrow leg (tongue) and the 30 cm (12 inch) mark on the wide leg (blade) at the top of the board. That will form an angle at the tongue called the top or plumb cut. Mark that angle with a pencil.
Determine the length of rafter needed with "length of common rafters per foot of run" line on the blade of the square. Look under the pitch mark to find the length each rafter must be to cover a foot of run. On a 3/12 roof, that is 31 cm (12.37 inches), or roughly 3.7 cm (1 1/2 inches) per 30 cm (1 foot). Multiply 31 times the run. On a 6 by 7.2 m (20 by 24 foot) building, the run would be 3 m (10 feet), so the rafter must be 28.1 cm (11 1/4 inches) long. Measure that length from the bottom of the plumb cut line and mark a point. Measure 2.5 cm (1 inch) up into the rafter and 8.7 cm (3 1/2 inches) back up and make a triangle, called a birdsmouth notch, to fit over the wall cap board.
Add any overhang desired to make an eave, mark another angle, like the plumb cut but putting the heel of the square at the top of the board. Measure another 1.8 cm (3/4 inch) below the plumb cut line and mark an identical angle for the real cut; this allows for a ridge board at the peak to connect two rafters. Cut all angles with a circular saw. Test fit the rafter by putting the birdsmouth securely over the wall cap and making sure the plumb cut is in the centre of the building. Mark and cut all other rafters.
Install the first pair of rafters at the back of the building. Nail the birdsmouth of both rafters to the wall caps with framing nails and a hammer. Let the plumb cuts rest against each other. Go to the other end of the roof and install another pair of rafters. Get a ridge board to fit between the two end rafters, probably a 5 by 15 cm (2 by 6 inch) beam but refer to building regulations. Splice the board if one length will not cover the roof. Cut a 60 cm (2 foot) piece of matching board and nail it across the joint where the two ends meet. Space it to fit between pairs of rafters so each board is fully supported by a rafter pair; the splice should fit exactly between 60 cm (24 inch) spaced rafters.
Slide the ridge board into the end rafters from the bottom. Hold it in place (with help or temporary nails) and use a level to plumb the rafters perfectly. Brace the end rafters temporarily with 5 by 10 cm (2 by 4 inch) beams nailed to the rafters and to stakes in the ground outside. Install other rafters at 60 cm (24 inch) intervals the length of the roof, nailing birdsmouths to wall caps and plumb cuts to ridge board.
Add gable studs and cross ties for extra support. Cut 5 by 10 cm (2 by 4 inch) gable studs to fit vertically between the bottom of the ridge board and the end wall cap on each end. Install cross ties, 5 by 10 cm (2 by 4 inch) beams cut with angled ends to connect rafters on each side. Space these according to the height of the space under the rafters, just below the peak on low-slope, a third of the way below the peak for higher pitches. Spaces to be used for rooms will also require vertical braces.
A flat roof on one part of a house that abuts a wall may be converted to a single-pitch roof by installing a ledger board, a support fastened to wall studs, at the appropriate height. That replaces the ridge board, and rafters are nailed to the ledger board and wall caps.