Framing Square

Where did this system of slope designation come from? It refers to the measuring system that carpenters have traditionally used to lay out and cut roof rafters. The x-in-12 ratio told the carpenter that for every 12 in. of run--the horizontal distance covered by the rafter--the rafter would rise a certain number of inches .
Once understood, this ratio gives us an immediate visual image of the roof slope.

Roof Slope related to the framing square. Carpenters describe the slope of the roof with an equation that tells the number of inches a rafter rises for each 12 in. that it spans horizontally. This example shows the slope of a 9-in-12 roof.

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