Plumb Cut vs. Heel Cut: The Two Rafter Angles and How to Figure Them

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.