Calculator
Example Data Table
| Pitch | Run | Seat Cut | Rafter Depth | Approx Heel Depth | Basic Check |
|---|---|---|---|---|---|
| 4 / 12 | 120 in | 3.5 in | 7.25 in | 1.167 in | Common light roof layout |
| 6 / 12 | 144 in | 3.5 in | 7.25 in | 1.750 in | Near one fourth depth |
| 8 / 12 | 168 in | 3.0 in | 9.25 in | 2.000 in | Often needs review |
Formula Used
The calculator uses right triangle roof geometry. Pitch ratio is rise divided by run.
pitch ratio = pitch rise / pitch run
roof angle = atan(pitch ratio)
effective run = horizontal run - half ridge thickness, when ridge deduction is selected.
rafter rise = effective run × pitch ratio
common rafter length = √(effective run² + rafter rise²)
tail length = overhang run / cos(roof angle)
heel depth = seat cut length × tan(roof angle)
remaining depth = rafter depth - heel depth
maximum notch depth = rafter depth × maximum notch percent / 100
How to Use This Calculator
- Choose the unit used on your plan or tape measure.
- Enter roof pitch as rise over run, such as 6 over 12.
- Enter the horizontal rafter run from wall plate toward the ridge.
- Add ridge thickness and choose whether to deduct half of it.
- Enter the overhang, rafter depth, rafter thickness, and seat cut.
- Set the notch limit you want to check.
- Press calculate and review heel depth, remaining depth, angles, and stock length.
- Download the CSV or PDF report for shop notes.
Common Birds Mouth Rafter Planning Guide
Understanding a Common Birdsmouth Cut
A common birdsmouth cut lets a sloped rafter sit flat on a wall plate. It also gives the heel cut a clean face against the outside edge. Good layout keeps the roof line true. It also protects the rafter from a deep notch.
Why the Notch Matters
The seat cut should support the rafter without removing too much wood. A deeper heel may look neat, but it reduces remaining depth. Many builders limit birdsmouth depth to one fourth of the rafter depth. Local codes, span tables, and engineered drawings can require stricter rules. Treat this calculator as a planning aid. Check the final design before cutting valuable material.
Measurements to Collect
Measure the horizontal run from the wall plate to the ridge line. Deduct half the ridge board when your layout uses that method. Enter the roof pitch as rise over run. For common layout, rise per twelve units is typical. Add the rafter depth, actual board thickness, desired seat length, overhang, and waste allowance. Use actual lumber sizes, not nominal names. This keeps the heel depth and board length closer to the real cut.
How Results Help
The angle result gives the plumb cut. The seat cut is level. The heel depth shows how much wood is removed by the notch. The remaining depth shows how much rafter remains above the seat. The common rafter length helps you place the birdsmouth along the board. The tail length estimates the eave section beyond the wall.
Planning Before Cutting
Run several seat lengths before you choose one. A wider seat can improve bearing, but it can also deepen the notch. Compare the limit result with your plan. Keep notes with each roof section. Different spans, pitches, and overhangs may need separate patterns on complex roofs.
Good Shop Practice
Mark one rafter first. Cut it carefully. Then place it on the wall plate and ridge to confirm fit. Check crown direction, plate width, ridge thickness, and overhang. When the test piece fits, use it as a pattern. Keep the saw kerf on the waste side. Recheck the seat depth often. Small layout errors can grow across a roof. For structural work, follow stamped plans and local rules.
FAQs
What is a birdsmouth cut?
It is the notch cut into a rafter so the rafter can sit flat on the wall plate. It usually includes a level seat cut and a vertical heel cut.
What does heel depth mean?
Heel depth is the vertical depth removed by the birdsmouth notch. This calculator estimates it from the seat cut length and roof angle.
How much birdsmouth depth is allowed?
Many field layouts use one fourth of rafter depth as a check. Your local code, span tables, or structural drawings may require a different limit.
Should I deduct the ridge board?
Deduct half the ridge board when your run is measured to the ridge centerline. Do not deduct it when your run already ends at the ridge face.
Can I use millimetres?
Yes. Select millimetres and keep every length input in millimetres. The pitch ratio still works because rise and run use the same unit basis.
What is the seat cut length?
Seat cut length is the flat bearing length that sits on the wall plate. Wider seats add bearing, but they can also increase notch depth.
Does this replace a framing plan?
No. It is a layout and checking tool. Use stamped drawings, local rules, and professional review for structural roof framing decisions.
What do the export buttons save?
The CSV saves input and result rows for spreadsheet use. The PDF saves a simple report that can be printed or kept with job notes.