How to Use Frame Square Rafter Tables
Many households have a framer's square, but not many really appreciate how remarkable the device is. It's also called a builder's square, a carpenter's square or a roofer's square. No matter what the title, the tool and its uses are the same. The square has two sides, a thick one called a blade and a thin one called a tongue. The blade typically is 24 inches long on its outer edge, 22 1/2 on the inside. The tongue is 16 inches on the outside, 13 inside. Blade and tongue meet at 90-degree angle called the heel. Does this Spark an idea?
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Instructions
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Use the table on the blade to figure rafter angles and cuts. It offers simple basic solutions to complex mathematical calculations to provide the correct angles for rafter and other construction intersections. It shows figures for common rafters, hip roof or valley rafters and jack or short intermediate rafters. These are shown as the length of run, difference in length depending on rafter spacing and side cuts for hip or valley rafters.
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Figure common rafters with the square and the table. Use the pitch or slope of the roof to determine the basic angle for the top of the rafter, at the peak. For a 6/12 roof, which slopes up 6 inches for every 12 feet of run, put the heel of the square on the bottom of a rafter board with the 6-inch mark on the tongue and the 12-foot mark on the blade at the top of the board. That will show the angle for the top or plumb cut.
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Calculate the length of rafter needed by looking at the common rafter line under the 6, for pitch, mark. That will show 13.42, meaning a rafter must be 13.42 inches for every foot of run, or distance between the peak and wall. The .42 is about 1/8 inch. If the rafter run is 12 feet, that means the rafter must measure 161 inches between the peak and the wall.
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Figure rafters for hip roofs or roof valleys in similar fashion. For a 6-pitch hip rafter, that difference will be 18, meaning the rafter must be 18 inches for every foot of run. Lengths of jack rafters, which run between common and hip rafters, are shown differently, depending on whether spacing is 16 inches or 24 inches apart. The bottom lines of this table show the differences in side cuts, since hip, valley and jack rafters often do not butt up evenly but must be trimmed at an angle.
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Find other dimensions on the square. Scales on the face of the square are in 1/8 and 1/16 gradients, most commonly used in construction. Most squares also have tables in 1/10, 1/12 and 1/32 of an inch, but these are not frequently used in wall or roof framing or most other applications.
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