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What the birdsmouth calculator measures
This birdsmouth calculator works out the vertical heel, perpendicular notch depth, notch percentage and height above plate from your roof pitch, actual rafter depth and chosen seat length · Seat length is an input you choose from your project design
What to enter
Enter the pitch rise per 12 units of horizontal run, such as 6 for a 6:12 roof · Measure the actual rafter depth across its wide face, at right angles to its edges · Enter the horizontal seat length and the available support width in the direction the rafter runs · All timber dimensions use inches or millimetres according to your unit selection
The seat must fit on the entered support · The tool rejects a seat wider than that support or a notch reaching the full rafter depth, but those checks do not establish that a cut is structurally suitable
Understand the measurements
The seat is horizontal when installed · The heel is vertical · Notch depth is measured perpendicular to the rafter, while height above plate is measured vertically
Roof angle θ = arctan(pitch rise ÷ 12)
Vertical heel = seat × tan(θ)
Perpendicular notch depth = seat × sin(θ)
Height above plate = actual rafter depth ÷ cos(θ) − vertical heel
Worked example: 2×8 rafter at 6:12
With an actual depth of 7¼ in and a 3½ in seat, the angle is 26.57° · The vertical heel is 1¾ in, perpendicular notch depth is 1.565 in and height above plate is 6.356 in · Equivalent metric inputs are 184.15 mm and 88.9 mm
Notch percentage = perpendicular notch depth ÷ actual rafter depth × 100 · In this example the notch is about 21.6% of the actual depth · Compare the geometry with the limits specified for your project
If you only know the roof rise and run, use the roof pitch calculator first · This tool calculates geometry and does not check rafter span, loads, connections or structural compliance
A little help
Make a checked birdsmouth template
Mark the layout on one sound pattern rafter, locate the heel from the drawing’s reference, check the notch and test both bearing points · Once the complete rafter fits, mark it MASTER and copy from that same piece
The diagram is not to scale and this tool does not generate a universal cut-out template · Different roof pitches, actual timber depths and support arrangements require different layouts
Does this check whether the rafter is safe?
No · The notch percentage helps you compare geometry with the limit specified for your project · It does not verify span, loads, bearing, uplift or local requirements · Follow the complete design and use the illustrated birdsmouth cutting guide before marking timber