A ridge board forms the peak of a gable roof. It holds the tops of roof rafters on both sides and ties rafter pairs together from end to end. It should be of larger lumber than the rafters -- a 2-by-6-inch ridge board with 2-by-4 rafters, for instance. It must span the entire length of the roof, end wall to end wall, plus any desired overhang on each end to form a gable eave. Installing rafters and a ridge board is a major endeavour which will take at least three workers on the roof.
Use a tape measure and marker to mark the locations for rafters on both wall caps and both sides of the ridge board while it is on the ground. Splice two boards if one will not span the length. Put two boards end to end and nail 1-inch splicing boards 18 inches long on either side. Place the splice so it will fall between two rafter pairs, so each board is fully supported by rafters. Space rafter centres 24 inches apart, from back to front. The last rafter location may be slightly less than 24 inches.
Set a pair of rafters in place at the back of the roof. Nail them to the wall caps through the notch at the bearing point or birdsmouth, using framing nails and a hammer. Plumb the rafters with a level and let the tops rest together. Install a second set of rafters at the other end. Slide the ridge board between each set of rafters from the bottom, allowing for overhang. Use a level to set the ridge level and doublecheck the rafter plumb.
Nail the tops of each rafter to the ridge board with two framing nails on each side, making sure the top of the rafter is flush with the top of the ridge. Add other rafters at marked intervals, nailing the bearing point at the wall cap and the top of the rafter to the ridge board. Keep the tops of the rafters precisely even with the top of the ridge board, set the bearing notch firmly on the wall cap and check the ridge level at each rafter location.
Add a "gable" stud at each end. Measure the distance between the bottom of the ridge board and the wall cap and cut a 2-by-4 to fit. Toenail this stud to the ridge board and the wall cap. Some roof framers start ridge board installation with this step. Do this by cutting the gable stud and nailing it in place to the wall cap and bracing it plumb with 2-by-4s nailed from the stud to stakes in the ground. Set the ridge board on it, nail it in place and brace it with temporary supports between the ridge and the roof joists, then install the rafter pairs and remove the braces.