How to Find Angles for Rafters
Most houses today are built with pre-fabricated trusses, but there is still a place for traditional rafters. Some people prefer rafter roofs because they provide a bit more attic space. Architects and house designers also are more creative in roof styles, mixing gable, hip and other elements to create a distinctive look which may not fit truss designs. Thus, rafters. Tables are available showing angles and other facets of rafter creation, but the easiest solution is the most basic tool in a carpenter's kit -- the framing square, also called a builder's, carpenter's or roofer's square. Does this Spark an idea?
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Instructions
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Learn the square. It has two sides, a blade 2 inches thick and 24 inches long, and a tongue 1-1/2 inches wide and 16 inches long, that form a 90-degree angle at a corner called the heel. Hold the blade in the left hand to show the face side. A table on the blade face will show numbers for common, hip, valley and jack rafters. Common rafters run from a peak to an eave. Hip and valley rafters run diagonally from a peak to an eave. Jacks are short rafters which fill in around hips and valleys.
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Find the run of the rafter and the pitch of the roof. Calculate the run on a gable roof by measuring the distance between walls (the span) and dividing that by 2 (the run). Figure the common run on a hip roof by measuring from below the peak to the outer wall; hip and valley rafter runs are figured the same way, by measuring diagonally from peak to wall. The pitch is the slope or rise of the roof, figured in inches per foot. It can be varied as desired to get the angle and look you want.
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Calculate a common rafter, for example, a 16-foot run for a 5/12 pitch (roof rises 5 inches in every foot), which is a common pitch on residences. Place the heel of the square on a 2-by-4-inch board with the 6-inch mark on the tongue and the 12-foot mark on the blade at the top of the square. That will give the angle for the top cut of the rafter (it also will generally be the end cut, but in reverse).
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Look at the table under the 5-inch mark to find the length of rafter per foot of run; that will show 13, meaning a rafter must be 13 inches long for every foot of run. Multiply 13 times the run, 16 feet, to get 208 inches. Measure 208 inches on a rafter board and mark a triangle 1 inch deep into the bottom of the board sloping down 4 inches. That is a notch (called a bird's mouth) to fit over the 2-by-4-inch cap board on the top of the wall.
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Figure lengths for hip and valley rafters similarly, using the table; the length differential for a 5/12 pitch hip or valley rafter will be 17.69 inches. Figure jack rafters lengths the same way, but note the dimensions will vary depending on whether jack rafters are spaced 16 or 24 inches apart.
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Go back to the square to get the side cut angles for hip, valley and jack rafters. Look at the two bottom lines of the table for the side cut measurements. For a 5/12 hip rafter that is 11-1/2. Turn the square over and put the 1-foot mark on the tongue and the 11-1/2 mark on the blade at the bottom of the rafter. That will show the side cut angles for both top and bottom ends of the rafter.
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