How to use the rafter and birdsmouth calculator
Choose Symmetrical gable span only when the ridge is centered and both bearing walls are level and equally high. Otherwise choose Direct horizontal run and enter the horizontal distance from the outside wall bearing line to the ridge-face plumb line. Then enter roof pitch, horizontal overhang, the member’s actual depth and the planned seat length. Review the calculated line lengths, angles and birdsmouth geometry, then verify structural requirements separately before cutting.
The roof dimensions in this calculator are processed in your browser. They are not sent to a calculation server or saved in an account.
Span, run and ridge-board thickness
For a centered symmetrical gable, each side begins with half the building span. The ridge board occupies part of that half-run, so the distance to the ridge face is:
effective run = span ÷ 2 − ridge thickness ÷ 2
Only half the ridge thickness is removed from one side because the ridge is centered. Direct-run mode starts at the ridge-face plumb line already, so it applies no ridge deduction. A roof does not always use a ridge board; enter its thickness only when that is the reference used for the layout.
How common-rafter length is calculated
Roof pitch describes rise divided by horizontal run. For an X-in-12 pitch, the angle above level is θ = atan(X ÷ 12). The vertical rise and straight rafter line to the outside wall bearing line are:
rise = run × tan(θ)line length = run ÷ cos(θ), equivalent to√(run² + rise²)
These are right-triangle geometry. They do not include a cutting allowance or choose the stock length to purchase.
Horizontal overhang vs. rafter-tail length
Horizontal overhang is a level distance from the outside wall bearing line to the tail or fascia plumb line. It is not measured along the sloping rafter. Convert it to the sloping tail length using tail length = horizontal overhang ÷ cos(θ). Total ridge-face to tail-plumb line length is (effective run + horizontal overhang) ÷ cos(θ).
What the birdsmouth measurements mean
- Seat: the horizontal bearing surface entered for the layout.
- Heel/plumb line: the vertical line at the seat edge.
- Heel rise: the vertical difference across the horizontal seat caused by the roof slope.
- Perpendicular notch depth: removed material measured square to the rafter faces.
- Remaining rafter depth: actual perpendicular member depth minus perpendicular notch depth.
- Height Above Plate (HAP): vertical distance from the horizontal seat plane to the rafter’s top edge at the heel/plumb line.
Heel rise and perpendicular notch depth describe different directions, so they are not interchangeable.
Birdsmouth formulas
heel rise = seat length × tan(θ)perpendicular notch depth = seat length × sin(θ)remaining depth = actual rafter depth − notch depthHAP = actual rafter depth ÷ cos(θ) − heel rise
The notch percentage compares the calculated perpendicular notch depth with the entered actual rafter depth. It is shown as a geometric ratio only; the tool does not compare it with a universal limit.
Why rafter cut angles can look different
An angle can be described from level, from a rafter edge or from a square cut. The calculator labels each reference: roof angle above level is θ; seat-line angle relative to the rafter edge is θ; plumb-line angle relative to the rafter edge is 90° − θ. For 6/12, those are approximately 26.565°, 26.565° and 63.435° respectively.
Worked example: 12 ft run at 6/12
With a 12 ft direct run, 6/12 pitch, 18 in horizontal overhang, 7.25 in actual rafter depth and 3.5 in horizontal seat, the calculated roof angle is about 26.565°. The run rises 6 ft; its rafter line is about 13.416 ft. The sloping tail is about 1.677 ft, making total ridge-face to tail-plumb line length about 15.093 ft.
For the birdsmouth, heel rise is 1.75 in, perpendicular notch depth is about 1.565 in, remaining perpendicular depth is about 5.685 in, HAP is about 6.356 in, and notch depth is about 21.59% of the entered actual depth.
These values describe geometry only. They do not determine whether the rafter size, bearing or notch is permitted for a particular structure.
Use actual rafter depth
A nominal U.S. 2×8 is not 8 inches deep. The dry minimum dressed depth listed for dimension lumber in the current NIST Voluntary Product Standard PS 20-25 is 7.25 inches for that nominal size. The preset is a reference only: check the actual member dimensions before laying out a cut. Other countries, products and material conditions can differ.
Geometry is not structural approval
This calculator does not determine rafter span capacity, species or grade suitability, required bearing, allowable notching, connection requirements, snow or wind capacity, ridge-beam requirements or local-code compliance. It does not size members or approve the entered geometry. Check approved plans, manufacturer information and applicable project requirements; consult the project designer or a qualified professional where required.
Frequently asked questions
How do I calculate rafter length?
Find the horizontal run and roof angle, calculate rise as run × tan(angle), then calculate the sloping line as √(run² + rise²) or run ÷ cos(angle).
What is a 6/12 roof angle?
A 6/12 pitch rises six units for every twelve horizontal units. Its angle is about 26.565° above level.
Does the overhang count in rafter length?
It can be included in the total ridge-to-tail line length. First convert the horizontal overhang to its sloping tail length.
What is a birdsmouth cut?
It is a seat and notch layout at a common rafter’s wall bearing. Required bearing and allowable cuts depend on the project and are not established by this geometry calculator.
Is birdsmouth depth the same as heel rise?
No. Heel rise is vertical. Notch depth in this calculator is measured perpendicular to the rafter faces.
How deep can I cut a birdsmouth?
Allowable notching depends on the applicable code, member, structural design and project conditions. This calculator shows notch geometry so it can be checked against the requirements that apply to the project; it does not provide a universal limit.
Can this calculator size my rafters?
No. It calculates common-rafter geometry only. It does not calculate structural spans, loads, member size, connections or code compliance.
Can I use Metric?
Yes. Choose Metric to enter building geometry in metres and rafter, seat and ridge dimensions in millimetres. Unit switching converts the values while preserving their physical dimensions.
Calculate your own rafter geometry
Use the Rafter & Birdsmouth Calculator to calculate common-rafter length, overhang and birdsmouth geometry. If you need to establish the roof slope first, use the Roof Pitch Calculator. For sloped surface area, continue to the Roof Area Calculator; for shingles and other roofing material estimates, use the Roofing Material Calculator. Browse other Roofing & Construction tools. Found a content or calculation issue? Report it to FivoTools.