What angle is a 6/12 roof pitch?

A 6/12 pitch is about 26.57 degrees. Pitch is rise over run, so 6 inches of rise for every 12 inches of run works out to an angle whose tangent is 6 divided by 12.

What is the plumb cut on a rafter?

The plumb cut is the vertical cut at the top of the rafter where it meets the ridge board. Its angle off level equals the roof angle, so a 6/12 roof has a plumb cut of about 26.57 degrees.

What is a bird's mouth cut?

The bird's mouth is the notch cut into the rafter where it sits on the top wall plate. It has a level seat cut and a vertical heel cut, and the seat angle is 90 degrees minus the roof angle.

How do I find rafter length from pitch and span?

Take half the span as the run, then multiply by the rafter factor, which is the square root of 12 squared plus the pitch squared, divided by 12. The calculator does this and also subtracts half the ridge thickness.

AngleWise / Roof Rafters

How to Cut Roof Rafters:
Pitch, Plumb & Seat Cuts

A common rafter is one straight board with two cuts: a plumb cut up top where it meets the ridge, and a bird's mouth where it sits on the wall. Give the calculator your pitch and span, and it hands you the rafter length and both cut angles — set your saw and go.

Rafter Calculator

Enter slope as

Pitch — rise in 12

Building Span — outside wall to outside wall (inches)

Ridge Board Thickness — deducted from run (inches)

The Terms, In Plain English

Pitch
How steep the roof is, written as rise-in-12 — like 6/12, meaning 6 inches of rise for every 12 inches of run. It's the number everything else comes from.

Run
The horizontal distance the rafter covers — half the building span for a standard gable roof. Subtract half the ridge thickness so the rafters meet cleanly.

Plumb Cut
The vertical cut at the top of the rafter, where it leans against the ridge board. Its angle off level is the same as the roof angle.

Bird's Mouth (Seat Cut)
The notch where the rafter sits on the top of the wall. It has a flat seat and a vertical heel — the seat angle is 90° minus the roof angle.

How to Cut a Common Rafter, Step by Step

Set your pitch. Decide your rise-in-12 (or enter the angle). 4/12 is a gentle roof; 12/12 is a steep 45°.
Find your run. Take half the building span. Subtract half the ridge board thickness so the two rafters don't overlap at the peak.
Get the rafter length. The calculator gives the true sloped length from run and pitch — that's the board length before cuts.
Mark the plumb cut. At the top, this vertical cut leans against the ridge. Use the plumb angle from the calculator (it equals the roof angle).
Mark the bird's mouth. Where the rafter crosses the wall, cut the level seat and vertical heel. The seat angle is 90° minus the roof angle.
Add a tail (overhang) if you want eaves, then cut a plumb or square tail at the end.

Reading Pitch as an Angle

Pitch and degrees are two ways of saying the same thing. A few common ones worth knowing by heart:

4/12 ≈ 18.4° — a low, walkable roof.
6/12 ≈ 26.6° — the most common residential pitch.
8/12 ≈ 33.7° — steeper, sheds snow well.
12/12 = 45° — a dramatic, steep roof; rise equals run.

Always check your local building code and load requirements. Span tables, rafter sizing, and snow/wind loads vary by region. This calculator gives you the geometry — lengths and angles — not structural sizing.

Common Mistakes

Forgetting the ridge deduction. If you use the full half-span as your run without subtracting half the ridge thickness, your rafters come out long and the peak won't close.
Confusing plumb and seat cuts. The plumb cut is vertical (off level by the roof angle); the seat is horizontal. Mixing them up gives a rafter that won't sit right.
Measuring along the wrong line. Rafter length is the sloped distance, not the run. Always use the calculator's rafter length, not the horizontal run.
Cutting all rafters before testing one. Cut and fit a single rafter, check the ridge and bird's mouth, then use it as a pattern for the rest.

Frequently Asked Questions

What angle is a 6/12 roof pitch?: About 26.57°. Pitch is rise over run, so 6 in 12 gives an angle whose tangent is 6÷12. The calculator converts pitch to degrees both ways.
What is the plumb cut on a rafter?: The vertical cut at the top where the rafter meets the ridge. Its angle off level equals the roof angle — so 26.57° on a 6/12 roof.
What is a bird's mouth cut?: The notch where the rafter sits on the wall plate, with a level seat and vertical heel. The seat angle is 90° minus the roof angle.
How do I find rafter length from pitch and span?: Use half the span as the run, multiply by the rafter factor (√(12² + pitch²) ÷ 12), and subtract half the ridge thickness. The calculator does it all.

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