There are many styles of homes that are built with gable roofs. It is the simplest roof of all to frame. It is also the most common roof built for that reason alone.
All the rafters cut for a gable roof are all cut to the same length. The birdsmouth, which is the cutout that sits on the top plate, is located in the same place on all rafters. The overhang, if there is one, is also the same.
The ridge board, which is usually a 2x, runs the full length of the house. This can be cut ahead of time and have the rafter layout marked on it. The rafter layout usually matches the wall stud layout. This transfers the roof load down to the foundation.
Before any framing begins, some of the plywood that will be used for roof sheathing is laid across the ceiling joists the whole length of the building. This gives the carpentry crew something to comfortably stand and walk on during the construction of the roof. It's also a good idea to pass all the roof plywood up to the ceiling joists. This makes passing it up to the roof much easier.
Next, pass the ridge board (s) up and lay them on the plywood walkway, making sure the layout is going the right way and matches the wall layout. Now pass the rafters up. Lay them with the plumb cuts on the plywood and the tails on the top plate. If the house is a ranch, the rafters can be leaned against the wall between ceiling joists to keep them from sliding of the wall. Send up a couple of 2×4's for bracing and you are ready to frame.
Before the ridge board is even raised, nail two rafters to one side, nailing on the first one and the last one on the layout. The ridge can now be raised. Pull the ridge so the birdsmouths are tight to the wall and raise it high enough so the level cuts of the birdsmouths are flat on top of the wall. Once the rafters are in place tack them to the wall. Now nail two rafters on the other side of the building to the top plate. These will also be the first and last rafters on the layout and will mirror the ones you put up on the other side of the building. Raise or lower the ridge so the top of the plumb cuts on the rafters meet the top edge of the ridge. If the rafters where cut correctly,
the plumb cuts should fit snugly against the ridge with little or no gaps.
With the ridge in place, install a stud in the gable end from the ridge down to the top plate, using a level to plumb it up. Take one of the 2×4 braces and nail it near the top of the stud. Nail a block to the plywood walkway being sure to catch a couple ceiling joists. Now put the level on the 1 1/2 "side of the stud and plumb it that way. When the stud is plumb, nail the 2x brace to the block. This will square up and position the roof where it needs to be. The rest of the rafters can now be installed, alternating from one side to the other, pulling the rafters tight to the wall and bringing the plumb cut to the top of the ridge. By alternating from one side to the other the ridge will stay straight. If one side is installed all at once, the weight will push on the ridge creating a bow. This will make it tough to straighten when the other side is put on.
After the roof is framed, collar ties, fascia boards, and overhangs are put on. With those items in place, the plywood can be put on. Frame in the gable ends and your ready for shingles.
Mike Merisko (c) 2006