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A CLEARER VIEW OF EVERY CUT

Birdsmouth & roof angles.

Understand the pitch. See the notch. Check your dimensions.

Free to use No signup needed Calculations in your browser
Results update as you type
Birdsmouth & rafters

Understand your birdsmouth cut.

A pitch, a seat length and the actual timber depth.

Start with the roof pitch

Choose the angle format

Angle above horizontal. Both ratio dimensions must use the same unit.

Add the timber details

Two dimensions, measured in different directions. The diagram shows both.

The horizontal bearing length of the notch.

Perpendicular to the timber edges, not vertical.

Depth comparison Planning only

These are comparison values, not automatically chosen code limits. Confirm the permitted detail for the project, timber and local requirements. A result within the comparison is not structural approval.

Your calculations stay in this browser tab.
YOUR CALCULATIONLive preview
DA · Horizontal seatHAPN · NotchRafter edgeTimber
N and D are perpendicular to the rafter edge.Schematic only · not a cutting template
Perpendicular notch depth
—mm

Enter horizontal seat length, actual rafter depth to see your result.

Planning estimate · verify before ordering

These tools calculate geometry and estimate quantities. They do not assess structural suitability, fixing schedules or manufacturer requirements.

A LITTLE CLARITY GOES A LONG WAY

Know what goes into your numbers.

The calculator keeps the working visible, so you can check your measurements and make an informed next step.

Which timber dimensions do I enter?

Seat length is the horizontal bearing length of the cut. Rafter depth is the actual section depth measured perpendicular to the rafter edges, not its width or a vertical measurement. Use millimetres or decimal inches, as labelled.

What is the difference between notch depth and the heel cut?

Perpendicular notch depth is seat length × sin(pitch). The vertical heel cut is seat length × tan(pitch). They measure the same notch in different directions and must not be treated as interchangeable.

How is height above the seat calculated?

At the deepest heel, the vertical height above the seat is (rafter depth − perpendicular notch depth) ÷ cos(pitch). The diagram labels this as HAP. It is not the vertical depth of the cut.

Does a result within the selected allowance mean the cut is approved?

No. The 1/3, 1/4 and custom comparisons are numerical planning references, not automatically selected building-code limits. Local rules, bearing, timber product, overhangs, loads and the specific detail need project verification. This tool does not approve a rafter, engineered joist or stair stringer.

Are these the settings for my saw?

No. Seat and plumb angles are measured from the rafter edge, and add to 90°. Tool scales and cutting orientation vary. The illustrated detail is a labelled schematic, not a full-size cutting template.

Can I enter roof pitch as rise and run?

Yes. Switch from Degrees to Rise : run. Enter both in the same unit; 6 : 12 is about 26.565°. The pitch is calculated using the arctangent of rise divided by run. Rafter length takes a separate horizontal run, not the full building span.

Are these calculators free?

Yes. The calculations, copy, CSV and print tools run in your browser without signing up. Reusing a Smart Component in the paid QuoteCore+ app is a separate next step.

What happens when I change units?

Existing dimensions and per-unit material rates are converted. Metric uses metres for lengths and millimetres for small details; imperial uses feet and inches. Currency changes only the currency label, not the exchange rate.

Can I turn the result into a quote?

Yes. Send a length, area or priced material to the Free Quote Generator and review the details there. Tax, margin and delivery are not included in the calculator estimate. Cut depths and angles are not transferred as material quantities.

FROM A MEASUREMENT TO A WORKFLOW

There’s more to a job than the numbers.

See how the paid QuoteCore+ app connects digital takeoff, Smart Components, quotes, orders and invoices. Start with the demo, without signing up.

Try the Demo Explore QuoteCore+