Rafter Layout With a 12-Inch Square
Laying out a common rafter's plumb, seat, and tail cuts with an oversized square — the pitch scale for plumb, the slot for level.
Read Any Pitch on Site With a Speed Square — Two Ways
Set the speed square's pivot on the slope, rest a torpedo level on the square's top edge, and rotate the square until the bubble reads level — the pitch reads off the square's degree/rise scale (a 4/12 in the demo). No level handy? A hanging chalk line is a plumb reference that does the same job.
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The Problem
Matching an existing roof pitch — for a repair, an addition, a ladder bracket, a rafter template — means reading the angle off the real thing, on a surface with no reference edge in sight.
The Techniques
With a torpedo level. Set the speed square's pivot corner on the slope. Rest the torpedo level on the square's top edge and rotate the square until the bubble centers. The square's scale now reads the pitch where the slope crosses it — the demo roof reads a common 4, a 4/12.
With a chalk line. No level in the pouch: hook the chalk line so it hangs on the far side from your pivot point, let it settle — a hanging string is dead plumb — set the square's pivot on the slope, and rotate until the square's edge sits right on the string. Read the pitch the same way. In the video both methods return the same 4/12.
Pro tip: A plumb reference and a
level reference are interchangeable here because they're 90 degrees
apart and the square's scale accounts for which edge you read — which is
why anything that hangs (chalk line, string with a nut on it) can stand
in for the level in a pinch.
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