The Swanson Speed Square is a popular tool with most carpenters. The triangle-shaped "square" was invented in 1925 by Albert Swanson to provide a quick, accurate method for marking rafters. Because of its compact size, it is frequently used as a saw guide when cutting framing members as well as a layout tool for quick 45- and 90-degree cuts. Using this tool to determine the pitch of an existing roof is simple once you find the correct scale markings.
Things You'll Need
- Speed Square
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Use a level and a pencil to make a plumb, vertical line on one face of an existing rafter.
Place the pivot point on the square against the bottom of the rafter and align the 90-degree angle side of the square with the pencil mark.
Note the number on the “Common” scale parallel with the 45-degree angle of the square. The hash marks with corresponding numbers between 1 and 30 refer to the roof’s rise angle. The pitch of the roof is read where the bottom of the rafter crosses this scale.