The real challenge in roof frame layout is not the math; it’s visualization. You have to be able to see how the triangle fits the roof you’re trying to build. Using quaint language (the framing square) or high-tech calculators that work in feet and inches doesn’t help if you don’t know how the geometry fits the roof. In a previous article (see “Rafter Square Basics: Foundation Layout,” Apr/22), I explained how the brace table etched on a framing square can be used to unlock the geometry needed to quickly and accurately lay out a foundation. In this article, I’ll focus on the rafter table on the front of the square, and how the numbers it contains can be used to lay out the rafter cuts needed to frame a roof.

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The front of a standard framing square is etched with rafter tables on the wide blade and the octagon scale on the narrower tongue.
The front of a standard framing square is etched with rafter tables on the wide blade and the octagon scale on the narrower tongue.

Unlike the brace table, which uses the same isosceles triangle for the first 13 entries and the familiar 3-4-5 triangle for the final entry, the rafter table provides the base, altitude, and hypotenuse for 34 different right triangles, each with angles that are different from all the rest. These triangles are well-camouflaged on the square, but they are there, and learning how to decipher them in the 1970s proved to be a turning point in my own quest to learn how to frame a roof.

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