How to Make Rafters Fit
Rafters support a roof, holding the decking and shingling or other covering. There are three basic kinds of rafter: common, hip and jack. Common rafters are used in every type of roof, running from a peak to a wall. Hip rafters are used in hip roofs, which slope on four sides, to connect a peak to a corner wall. Jack rafters are short, variable length rafters that connect hip rafters and common rafters in a hip roof. The most frequently used rafters are, of course, common rafters. They are fairly easy to fit to a roof. Does this Spark an idea?
Other People Are Reading
Things You'll Need
- 2-by-4-inch rafter boards
- Framing square
- Pencil
- Tape measure
- Circular saw
Instructions
-
-
1
Fit common rafters using three elements: the roof span -- or width -- from wall to wall; the rafter run -- half the span and the space each rafter must support on its side of the roof; and the pitch -- or angle of slope -- from wall to peak. Those numbers will be given on a house plan, expressed in feet (such as 24-foot span and 12-foot run) and slopes, such as 5/12, 6/12 or 7/12, for roofs that rise at 5, 6 or 7 inches per foot.
-
2
Use those numbers and a framing square to make one rafter to be a pattern for all others. Lay a 16-foot, 2-by-4-inch rafter board on supports, such as sawhorses, with the 4-inch face up. Put the corner (heel) of the framing square at the bottom edge of one end of the board. Put the pitch mark (5, 6, etc.) on the thin side (tongue) of the square and the 12-inch mark on the wide side (blade) at the top of the board, so they form an angle on the tongue at the end of the board. That is the top or plumb cut. Mark that line with a pencil.
-
-
3
Figure the length of the rafter with the table on the blade of the square. The top line is "length of common rafter per foot of run." Find the pitch mark and look at the line under it to see how long each rafter must be per foot of run. For a 5/12 pitch on a 12-foot run, for instance, that would be 13, or 13 inches of rafter for every foot of run for a total of a 13-foot rafter. (Calculations are similar for all pitches, but distances will vary with the pitch).
-
4
Measure that distance (156 inches) along the bottom of the rafter board from the bottom of the plumb cut line. Mark that point. Measure 3 1/2 inches back up the board, mark that point, then measure 1 inch up into the board at that point. Draw a triangle from the top of the 1-inch mark to the 156-inch mark. That forms a notch called a birdsmouth, which will fit exactly on the top board on the wall.
-
5
Add any overhang desired for an eave, measure that from the bottom of the birdsmouth and calculate an end-cut angle by placing the framing square at the top of the rafter board and the pitch and foot marks, as on the plumb cut. That will create an angle similar to the plumb cut, but reversed. Move back to the plumb cut, measure 3/4-inch down the board and mark an identical plumb cut angle. This allows for a ridge board between rafters.
-
6
Cut all those angles with a circular saw. Test that pattern rafter by lifting it in place on the roof, with the birdsmouth firmly on the wall cap board and the plumb cut to the top. Use a short piece of 2-inch board to test the plumb cut angle; make sure it is in the center of the roof. If that rafter fits, use it as a pattern to mark and cut all other rafters. Then all rafters should fit exactly on the wall cap and onto the center ridge board.
-
1
Tips & Warnings
Use other lines on the framing square table to calculate length of hip and jack rafters and also to figure cut angles for those rafters. Calculate rafters for gambrel or other multiple slope roofs or for other styles such as saltbox or shed roof. If you know the pitch and run, you can use the square to figure rafter lengths and angles.
