What Angle Do You Cut for a Gable Roof Shed?
Most small household sheds are built with basic gable roofs, which slope from a center peak to the shed walls on each side. A common variation is the saltbox roof, which is a gable with the peak offset so the side slopes are different widths. Gable roofs are popular because all you need to cut those rafters is a framing square, a tape measure, a pencil and a circular saw. Does this Spark an idea?
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Three Angles
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Common gable rafters have three angle components: a top or plumb cut, a bearing point notch or birdsmouth and an end or tail cut. The plumb cuts of two opposing rafters meet at the center of the roof to form the peak. The birdsmouth is a triangle cut in the bottom of a rafter to fit atop the wall cap. The end or tail cut is at the end of the rafter that hangs over the side of the building.
Run and Pitch
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You need two elements to calculate gable roof rafter cuts -- the run of each rafter, or the space it covers between the peak and the wall, and the pitch, or slope of the roof. The run is easy to figure -- half the width of the shed, 4 feet for an 8-foot shed, for instance. The pitch is expressed in inches of rise per foot, as in 4/12 for a roof whose sides slope up 4 inches per foot. The pitch can be anything you want -- sheds usually have a medium pitch, 4/12 or 5/12.
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Framing Square
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A framing square is the key tool to figure rafter cuts. It has a 1 1/2-inch wide tongue 16 inches long and a 2-inch wide blade 24 inches long which form a 90-degree angle at a point called the heel. The blade also has a table for calculating the length of rafters per foot of run; an angled rafter will be longer than the flat run. The square will form the plumb cut, be used to mark the birdsmouth and be used to figure the tail cut angle.
Plumb Cut Angle
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To figure the plumb cut angle, lay a 2-inch by 4-inch rafter board on a flat surface with a 4-inch face up, with the square at the end of the board. Set the heel at the bottom of the board, with the pitch mark (4-inch for a 4/12 roof) on the tongue and the 12-inch mark on the blade aligned at the top of the board. The plumb cut is the angle on the tongue at the end of the board.
Birdsmouth Angle
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Refer to the rafter table to find the "length of common rafter per foot of run." Use that to calculate the location of the birdsmouth; multiply that length times the run (12.65 for a 4-pitch), convert that to inches and measure it for the bottom of the birdsmouth. Cut a birdsmouth with a 1-inch vertical edge in the rafter to fit the outside edge of the wall top, sloping to the bottom of the rafter 3 1/2 inches back toward the plumb cut. That 1-inch by 3 1/2-inch triangle will fit exactly on a 2-by-4-inch board on top of the shed wall.
Tail Cut
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Add any length of overhang needed to form an eave on the side of the shed and calculate a tail cut angle by putting the heel of the square at the top of the rafter and using the same plumb cut alignment. Plumb and tail cut angles will vary with the pitch of the roof; the steeper the pitch, the more severe each angle will be.
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