Rafters and a ridge board are the key components of a roof-framing system. The rafters support the sides or slope of the roof. The ridge board forms the peak and ties the rafters together, side to side and end to end. Other elements, like gable studs and cross ties, supply additional bracing. Gable studs are placed between the bottom of the ridge board and the top of the wall at each end of the roof. Cross ties are fastened across the two rafters, below the ridge board, to hold them more firmly in place.
Mark the cap boards on the both side walls, starting at one end and measuring with a tape measure 24 inches apart (on rafter centres) the length of the roof. The last space may be slightly less than 24 inches to conform to 4-by-8-foot sheathing panels. Lay the rafters out in pairs on each side of the roof. Check all rafters to make sure top or plumb cuts are correct and the same on both rafter sides and that the "birdsmouth" triangle notches on each rafter will fit precisely over the wall cap board.
Set the first pair of rafters upright at the back end of the roof. Nail them with framing nails and a hammer through the birdsmouth into the wall cap (this will require three carpenters, one on each side and one in the centre). Let the plumb cuts rest against each other. Repeat this installation with rafters on the other end of the roof.
Use a level to plumb the first end rafters vertically, then brace them with 2-by-4-inch boards nailed temporarily to each rafter and to stakes in the ground outside. Do the same with the other end rafters. Get a ridge board the exact length between the ends of each rafter pair. Splice two boards if one won't cover that length; cut "gussets" of 1-inch lumber the same width as the ridge board and a foot beyond the joint on either side, Nail gussets on either side of the joint while the two boards are supported and level from end to end.
Lift the ridge board to the roof and slide it between the rafter ends on each end from the bottom. Secure each end with one nail on each side. Check the ridge board to make sure it is level and the end rafters are still plumb, then add a second nail to each birdsmouth and ridge board connection. Make sure the splice will fall between two rafter pairs, so each end is fully supported by rafters.
Install other rafters, one pair at a time, checking each set for plumb and the ridge board for level. Make "gable studs" of 2-by-4-inch boards to fit between the bottom of the ridge board and the wall top on each end set of rafters. Measure and cut 2-by-4 cross ties to connect the rafters on each side about a third of the way between the ridge board and the wall top. Cut ends at angles to conform to the slope of the rafters. Nail them in place with framing nails.