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How to measure roof pitch with a speed square

Updated February 21, 2017

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.

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.

Tip

Roof pitch is commonly referred to in terms of the amount of vertical rise per foot of horizontal length. A roof with a 6:12 pitch rises 6 inches for every foot between the eave and the peak.

Things You'll Need

  • Speed Square
  • Level
  • Pencil
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About the Author

Finn McCuhil is a freelance writer based in Northern Michigan. He worked as a reporter and columnist in South Florida before becoming fascinated with computers. After studying programming at University of South Florida, he spent more than 20 years heading up IT departments at three tier-one automotive suppliers. He now builds wooden boats in the north woods.