Laying out and cutting roof rafters for a small shed is the same as making rafters for a big shed or a house. The essential tool is a framing square. This square, usually steel, has a tongue 16 inches long and 1 1/2 inches wide and a blade 24 inches long and 2 inches wide which form a 90-degree angle called a heel. The blade has tables, developed over many years by many carpenters, with the information needed to calculate angles and lengths for rafters.
- Skill level:
- Moderately Challenging
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Things you need
- Framing square
- Tape measure
- Rafter boards
- Circular saw
Select a pitch, or angle of slope, for the small shed roof. Sheds usually have fairly low slopes, like 4 inches of rise per foot, called a 4/12 roof. Use that pitch and the run, or basic length, of the rafter to figure all angles. The run is half the width of the shed or the length each rafter must cover. On an 8-foot by 10-foot shed, for example, that run would be 4 feet.
Lay a 2-inch by 4-inch rafter board on sawhorses or similar supports, with the 4-inch face up. Use a board longer than the run, a 6-footer on a 4-foot run, for example. Place the heel of the square at the bottom edge on one end of the board. Line up the pitch mark (4-inches for a 4/12) on the tongue and the 12-inch mark on the heel at the top of the board. That forms an angle on the tongue called the top or plumb cut. Mark that line with a pencil.
Use the table on the blade to figure the length the rafter needs to be to fasten to the wall. Look on the top line of the table "length of common rafter per foot of run" under the 4-inch mark. That will show a rafter must be 12.65 inches for every foot of run, or 50 1/2 inches for a 4-foot run. That is where to cut a triangle called a birdsmouth to fit over the shed wall.
Use a tape measure to mark that 50 1/2-inch point from the bottom of the plumb cut on the bottom of the rafter. Mark that spot. Make a vertical line with the square 1 inch up into the bottom of the rafter and mark that point. Measure 3 1/2 inches back up the bottom of the board and mark that point. Connect the 1-inch mark and the 3 1/2-inch mark to form a triangle which will fit exactly on the shed wall.
Add an overhang, or tail, to extend the rafter beyond the shed wall. That typically is 1 foot, which would make the final rafter length in this example 62 1/2 inches. Draw a line at that length, either square across the board or at an angle like that of the plumb cut but reversed by putting the heel of the square at the top of the board.
Use a circular saw to cut all those angles and the rafter is made. Make one rafter and use it as a pattern to lay out and cut all the others. Determine the number of rafters needed by dividing the length of the roof by 24, to space rafters 24 inches apart to support roof decking. Multiply by two for one rafter on each side of the roof.
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