DIY Rafter
- 1). Measure the width and length of the building to be roofed with a tape measure. The width will be the span of the rafter; half that distance will be the run, the space each rafter must cover. The length will determine the number of rafters needed. Divide the length by 24, because most rafters are spaced 24 inches apart, to get that number.
- 2). Decide on a pitch, the angle at which the sides of the roof will slope. Figure pitch in inches per foot of rise--a 4/12 roof, for instance, will go up 4 inches in every foot of run, while a 6/12 will rise 6 inches per foot. The pitch is designer's preference, influenced by the amount of snow and similar factors; heavy snow areas will use steeper pitches. Steeper pitches also provide more inside room.
- 3). Cut a pattern rafter first in any roof framing, using a framer's square. That square has a thin side, tongue, and a fat side, blade, with inches marked on both edges. Figure a 5/12 rafter, for instance, by putting the 5-inch mark on the tongue and the 12-inch mark on the blade at the top of a 2-by-4-inch board, with the heel or point of the square at the bottom of the board. That measurement will show you the angle for the top or plumb cut of the rafter, the edge that will fit against a ridge board at the peak of the roof.
- 4). Figure the length of the rafter using the table on the blade showing "length of common rafters per foot run." Look under the 5-inch mark; it will show 13, which means a rafter must be 13 inches long for every foot of run. Multiply the run by 13 to get the length; for a 12-foot run (which would cover half of a 24-foot wide building) that equals 156 inches.
- 5). Measure 156 inches down the pattern rafter and mark 1-inch deep into the bottom of the board. Draw a diagonal line from the top of that mark to the bottom of the rafter 4 inches down from the mark. That configuration will create a triangle, called a bird's mouth, which when cut out will make a notch on the rafter to fit on the top cap board of the wall.
- 6). Add the amount of overhang wanted for an eave on the building to the bottom of the bird's mouth. For a 12-inch overhang, that will make the total rafter length 172 inches--156 plus 4-inch bird's mouth plus 12-inch overhang. Cut the rafter that length; generally the bottom cut will be the same angle as the top cut, but with the angle reversed. Go back to the plumb cut and take off another inch to allow for a 2-inch ridge board where rafters on both sides will meet. Test fit that rafter. Cut all others to that pattern.
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