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.
Plumb Cut vs. Heel Cut: The Two Rafter Angles and How to Figure Them
The plumb cut at the top of a rafter matches the roof pitch angle; the heel (seat) cut at the bottom is its complement — 90 minus the plumb angle. On a valley against an existing roof, the heel cut also picks up a bevel equal to that roof's pitch.
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The Problem
Rafter cuts have names that get thrown around as if everyone was born knowing them — plumb cut, heel cut, seat cut — and if you don't know which is which, you can't follow a cut list, a framing square table, or the person teaching you. The two cuts are also related by simple math that nobody bothers to say out loud.
The Technique
The plumb cut is the angle at the top of the rafter — where it meets the ridge, or on a porch roof, the wall or valley. It's called that because when the rafter is installed, that cut face sits plumb, straight up and down. Its angle comes from the roof pitch: on a 5-pitch roof (5 inches of rise per 12 of run), the plumb cut is about 22.5 degrees.
The heel cut — some framers say seat cut — is at the bottom of the rafter, and it's the opposite of the plumb cut: when the rafter is installed, this face sits level. Because the two faces are perpendicular, the math is one subtraction:
Heel cut angle = 90 − plumb cut angle. For that 5-pitch rafter: 90 − 22.5 = 67.5 degrees.
When a bevel joins the party: framing a valley against an existing roof plane, the heel cut also has to lie flat on that roof — so you cut it with the saw beveled to the existing roof's pitch angle. Existing roof at 5.5 pitch, roughly 25 degrees? Set a 25-degree bevel and cut the heel at its 67.5, and the rafter seats into the valley.
Pro tip: The pitch-to-degrees conversions framers rattle off are rounded — 5/12 is really 22.6 degrees, 5.5/12 is 24.6 — and the rounding is fine, because both cuts come off the same reference. Get the plumb cut from your speed square's pitch scale and derive the heel from it, and any small rounding cancels itself out.
Safety note: Steep angle and bevel cuts on rafter stock leave less shoe on the wood than a square crosscut — support the work so the offcut can't pinch the blade, and keep the saw's weight on the supported side of the cut.
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