How to Lay Out Roof Rafters
The origins of the framing square seem lost in history, although a U.S. patent was issued for one metal version in 1819. But some carpentry historians think elements of the square date to when carpenters would do calculations and write numbers down on the edges of wooden squares. All that history has evolved into a tool today that is essential in framing a house and can simplify laying out roof rafters so almost anybody can do it. Cutting and installing the rafters, however, still requires carpentry skill. Does this Spark an idea?
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Things You'll Need
- House plans
- Framing square
- Tape measure
- 2-by-4-inch board, at least 16 feet
Instructions
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Lay out a basic common rafter on a 2-by-4-inch board at least 16 feet long. This will work for rafters of 4/12, 5/12 or 6/12 pitch roofs, with slopes of 4, 5 or 6 inches per foot from outside wall to peak. Obtain the specific pitch from a house plan. Set a framing square at one end of the board laid flat with a 4-inch side face up. Put the blade, the long side of the square, with the 12-inch mark at the top of the board. Align the shorter tongue with the pitch mark, 4, 5, 6 or whatever the house plan shows. The tongue will form an angle at the end of the board, called a top or plumb cut. Mark the line with a pencil.
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2
Get the span, the width of the roof from wall to wall and the rafter run, the distance each rafter must support. This is half the span. Obtain these dimensions from the house plan. Figure the length needed for the rafter with a table on the blade. Look up the pitch on the "length of common rafters per foot of run" line on the table. That will be 13 for a 5/12 roof, or 13 inches per foot of run. On a 24-foot roof with a 12-foot rafter run, each rafter must cover 13 feet from peak to wall.
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Measure this distance (156 inches) from the bottom of the plumb cut line and mark the point on the bottom of the board. Measure up 1 inch and mark that point. Go back to the first mark, measure back up the board 3 1/2 inches and mark that point. Use the square to draw a line between that mark and the one an inch up. This makes a triangle called a birdsmouth, which will fit over the 2-by-4 on the top of the wall. (Measurements will vary with the roof pitch, but the technique is the same).
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Add any overhang desired for an eave, typically at least 18 inches, and mark another angle, such as the plumb cut but figured with the heel of the square at the top of the board. Measure down the board an additional 3/4 inch from the original plumb cut line and mark a duplicate line to create space for a ridge board between two rafters. This completes the layout of one rafter; transfer these marks to all other rafters. Figure the number of rafters needed by dividing the length of the building by 24 for rafters spaced 24 inches apart; multiply this by two since a rafter will be needed on each side of the roof.
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5
Calculate rafters for hip roofs, which slope on four sides, in similar fashion. Use the "difference in length of hip and valley rafters" on the table to figure the length of hip rafters from the peak to the wall corners. Use the "difference in length of jacks" line to calculate jack rafters, which go between the hip rafters and common rafters on either side. Use the table to figure the angle cuts for hip and jack rafters, which must be cut at angles rather than straight across a board.
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Figure rafters for special roofs like gambrel and mansard, which have more than one slope per side, or for offset roofs, such as saltbox styles, using the same rafter tables. Get the pitch and run for each rafter element and calculate the specifics just like common rafters. Use the same basic technique for wider rafter boards, such as 2-by-6s.
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