You aren't likely to attempt a complex roof on your own, but a simple gable roof is within the abilities of many do-it-yourselfers.
The basic structure is a triangle consisting of a ceiling joist and two rafters. Additional components include ridge board, blocking, and lookouts (or outriggers) for supporting wide overhangs. The techniques for framing the roof of a room addition are the same as for new construction, except that you have to tie the new roof into the old. Use the following techniques for framing a gable roof. You will need two helpers.
Laying Out and Cutting Rafters
From your plans you can determine the slope of the roof (number of inches of rise for every fool of horizontal run). Slope is expressed as “something in 12.”
Choose a straight piece of rafter stock for the first rafter, and mark a plumb cut by setting framing square near one end of board. If slope were 4 in 12, tongue (short leg) and blade (long leg) of square would intersect bottom edge of board at 4-inch and 12-inch mark, respectively. Hold square this way to make all marks.
You can determine rafter length with the formula a^2 + b^2 = c^2 (where a and b are rise and run). Or you can use the tables stamped on the blade of the rafter square. The 12.65 under the 4-inch mark means that for every inch of run, you need 12.65 inches of rafter length. If run is 8 feet (half of total span of 16 feet), rafter length for a 4-in-12 roof is 101.2 inches, or 8 feet 5¼-inches.
Measure from plumb cut a distance equal to rafter length and mark top edge of board. Holding tongue of square against this mark, set square at 4-in-12 and scribe a line.
Then slide the square, in 4-in- 12 position, back toward original plumb cut until it creates a line for seat of bird’s mouth exactly the width of 2 by 4 cap plate. Mark it.
Next, mark tail cut by sliding square beyond bird’s mouth the length of overhang. Move plumb cut mark down toward bird’s mouth exactly half the thickness of ridge board (3/8-inch for ¾” board).
Lay out and cut first pair of rafters. Test them by placing in position with scrap of ridge board material between plumb cuts. Make layout adjustments to ensure snug fit.
You can then cut rest of the rafters, using the first rafter as a pattern.
Installing Ceiling Joists
Begin by marking locations of ceiling joists and rafters along tops of walls. The outside of first rafter should be flush with the gable wall. Set first ceiling joist in from wall to make room for gable studs and to provide a nailer for finish ceiling material.
Next, toenail ceiling joists to wall plates or, where earthquakes or high winds are a factor, attach joists with metal framing clips. Trim top corners of joists flush with rafter slope. This framework acts as a scaffold.
The way you attach a gable roof to the existing house roof depends on whether the new ridge lies in the same direction as the old (parallel) or intersects it (perpendicular).
New Ridge Parallel to Old
The ridge board you use should be one size larger than the rafters (2 by 8 for 2 by 6 rafters, and so on).
1. Butt ridge board to edge of house rafters, holding it in place with a joist hanger or by nailing two new rafters against house rake rafters. Hold other end in place with temporary bracing.
2. Install rafters in pairs. Toenail lower ends to cap plates with three 8d nails at each connection and to ceiling joists with three 16d nails. Nail top ends into ridge board, facenailing one side and toenailing other. Alternate this nailing sequence at ridge as you work your way along rafters. If you live in an area of earth quakes or hazardous winds, use metal framing clips and hurricane anchors to secure rafters to top plates and ridge board.
3. Install rake (or barge) rafter. If ends of rafters will be covered by fascia boards, extend fascia boards beyond end rafter far enough to sup port rake rafters. Cut and nail blocks between rake and end rafters every 16 inches. If there will be no fascia boards, support rake rafter with 2 by 4 lookouts, spaced 2 feet apart and notched into last pair of rafters.
4. Nail blocking to fit between rafters above walls, except where eave vents will be installed.
New Ridge Perpendicular to Old
Installing rafters with the new ridge board perpendicular to the old one is a more complicated procedure than installing a parallel ridge.
1. Begin by cutting two pairs of rafters. Position one pair at gable end of addition, with a scrap of ridge board material between rafters. Nail rafters to wall plates only. Follow same steps with other pair of rafters at opposite end of wall.
2. Remove scraps and slide ridge board up between pairs of rafters so one end rests on house roof and top is flush with tops of rafters. Mark roof where ridge board touches it.
3. Scribe a line at end of ridge board parallel to roof slope. Remove ridge board and cut along line, saving scrap piece. (See step 10.)
4. Snap a chalk line between mark on roof and point where each new addition wall intersects house. Snap second pair of lines parallel to and 1 foot below first ones and remove all shingles between these lines.
5. Slide ridge board back in place and nail it securely to roof sheathing. If sheathing is springy, go into attic and nail blocking between rafters at that point.
6. Mark rafter layout (16 inches or 24 inches on center) on both sides of ridge board and on each wall plate, starting at same end.
7. Install all common (full length) rafters.
8. Cut and place two 2 by 6 plates on roof sheathing so top edges line up between point where top of ridge board intersects roof and addition walls. To check alignment before nailing, lay a long, straight board across common rafters at several points. Board should barely touch top edge of plates. Adjust plates and nail to roof sheathing.
9. Mark spacing for valley jack rafters along 2 by 6 plates by hooking tape measure on last common rafter and pulling it out parallel with ridge board. Start near bottom of common rafter, where first measurement intersects top edge of plate (at 16 inches or 24 inches, depending on rafter spacing). Slide tape mea sure up rafter for each succeeding measurement (32 inches, 48 inches, and so on), always keeping it parallel with ridge board.
10. Plumb-cut valley jack rafters at same angle as common rafters. From the point of the plumb cut, measure and mark a distance equal to distance between ridge board and top edge of plate at layout marks (upslope side). Scribe angle to match seat cut angle of bird’s mouth (perpendicular to plumb cut).
Set blade angle on circular saw to match fail of ridge board you saved from step 3. Cut along line so bottom of blade bevels away from line.
To avoid wasting stock, cut only one valley rafter from each board and use remainder for opposing rafters. (E.g., use cutoff from longest rafter for shortest rafter on opposite side.)
Nail valley jack rafters in same way as common rafters.