How to Measure Roof Pitch With a Speed Square

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Finding the pitch of a roof is simple with a Speed Square.
Finding the pitch of a roof is simple with a Speed Square. (Image: Jupiterimages/Photos.com/Getty Images)

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
  • Level
  • Pencil

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.

Tips & Warnings

  • 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.

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