Birdsmouth Cut Calculator
Lay out the birdsmouth notch where a rafter sits on the wall plate. Enter the roof pitch, rafter size and seat length, and get the heel (plumb) cut height, seat cut, height above plate, notch depth and speed-square setting — with a notch check, a live diagram, by-pitch and by-rafter tables and a downloadable PDF.
| Pitch | Angle | Heel cut | HAP | Notch | Status |
|---|---|---|---|---|---|
| Enter the pitch and rafter to compare | |||||
| Rafter | Depth | HAP | Notch | Max seat | Status |
|---|---|---|---|---|---|
| Enter the pitch and rafter to compare | |||||
How to use the birdsmouth cut calculator
Four quick steps turn your roof pitch, rafter size and seat length into the heel cut, seat cut, height above plate and notch depth — with a pass/fail notch check, a live diagram, by-pitch and by-rafter tables, a chart and a downloadable report.
Set the roof & rafter
Enter the roof pitch (rise per 12 in of run) and pick the rafter size — the actual depth fills in automatically.
Set the seat & limit
Choose the wall plate (seat length) and the notch limit — one-third is common, one-quarter is conservative.
Add overhang & count
Optionally enter the eave overhang for the tail length and the number of rafters you are cutting.
Read results & save a PDF
Get the heel cut, seat cut, HAP, notch depth and speed-square setting, with a notch check and a downloadable report.
How a birdsmouth cut is calculated
Everything starts from the roof angle, the angle the rafter makes with the level plate. The heel cut comes from the pitch and the seat length; the notch depth is checked square to the rafter edge against your limit. Here is the exact math the calculator runs.
Worked example — 6/12 pitch, 2×8 rafter (7¼”), 3½” seat, ⅓ notch limit, 12″ overhang
Roof angle: atan(6/12) = 26.6° · Heel cut: 3.5 × 6/12 = 1¾"
Plumb depth: 7.25 ÷ cos 26.6° = 8.11" · HAP: 8.11 − 1.75 ≈ 6⅜"
Notch depth (square): 3.5 × sin 26.6° = 1.57" ≈ 22% · Max seat at ⅓: ≈ 5⅜" · Tail: 12 ÷ cos 26.6° ≈ 13⁷⁄₁₆"
≈ 1¾” heel cut with a 3½” seat — notch about 22% of the rafter depth, well within the ⅓ rule, leaving 6⅜” above the plate.
Birdsmouth cut reference charts
Handy lookups for how pitch, rafter size and seat length change the heel cut, height above plate and notch depth.
| Pitch | Roof angle | Heel cut | HAP | Notch % |
|---|---|---|---|---|
| 3/12 | 14.0° | ⅞” | 6⅝” | 12% |
| 4/12 | 18.4° | 1³⁄₁₆” | 6½” | 15% |
| 5/12 | 22.6° | 1⁷⁄₁₆” | 6⅜” | 19% |
| 6/12 | 26.6° | 1¾” | 6⅜” | 22% |
| 7/12 | 30.3° | 2¹⁄₁₆” | 6⅜” | 24% |
| 8/12 | 33.7° | 2⁵⁄₁₆” | 6⅜” | 27% |
| 9/12 | 36.9° | 2⅝” | 6⁷⁄₁₆” | 29% |
| 10/12 | 39.8° | 2¹⁵⁄₁₆” | 6½” | 31% |
| 12/12 | 45.0° | 3½” | 6¾” | 34% |
At 12/12 a full 3½” seat pushes the notch to about 34% — just over the ⅓ rule. Trim the seat to roughly 3⁷⁄₁₆” or step up to a deeper rafter.
| Rafter | Actual depth | HAP | Notch % | Max seat (⅓) |
|---|---|---|---|---|
| 2×4 | 3½” | 2³⁄₁₆” | 45% | 2⅝” |
| 2×6 | 5½” | 4⅜” | 28% | 4⅛” |
| 2×8 | 7¼” | 6⅜” | 22% | 5⅜” |
| 2×10 | 9¼” | 8⁹⁄₁₆” | 17% | 6⅞” |
| 2×12 | 11¼” | 10¹³⁄₁₆” | 14% | 8⅜” |
A 2×4 at 45% exceeds the ⅓ rule with a full 3½” seat — shorten the seat or step up a size. Deeper rafters leave more material above the plate.
| Seat cut | Heel cut | HAP | Notch % | Status |
|---|---|---|---|---|
| 3½” | 1¾” | 6⅜” | 22% | OK |
| 4″ | 2″ | 6⅛” | 25% | OK |
| 5″ | 2½” | 5⅝” | 31% | OK |
| 5½” | 2¾” | 5⅜” | 34% | Over ⅓ |
| 6″ | 3″ | 5⅛” | 37% | Over ⅓ |
On a 2×8 at 6/12 the seat can go up to about 5⅜” before the notch passes ⅓. A 2×6 wall (5½” seat) just tips over, so use a 2×10 or trim the seat.
What the birdsmouth calculator gives you
Pitch, rafter size and seat length give you every cut on the rafter plus a code-style notch check.
Birdsmouth cuts at a glance
Who uses a birdsmouth calculator
From a backyard shed to a porch roof or a full framing job, the same calculator lays out the notch and the tail.
Cutting common rafters for a shed or garage roof and wants the heel cut, seat and HAP right the first time.
- Set the pitch and rafter
- Read the heel cut
- Cut one as a pattern
Adding a covered porch or pergola roof and needs the tail length for the overhang plus a clean notch.
- Enter the eave overhang
- Check the tail length
- Keep the notch under ⅓
Framing a roof fast and wants the roof angle, speed-square setting and max seat before marking a batch.
- Grab the angle setting
- Confirm bearing & notch
- Batch-cut the run
Tips for cutting a clean birdsmouth
A birdsmouth works best when it is marked accurately, sized within the rules and cut without overshooting the corner.
Birdsmouth cut calculator FAQ
Quick answers about notch depth, heel and seat cuts, height above plate, small rafters and building code.
A birdsmouth (or bird’s mouth) is the notch cut into a rafter so it sits flat on the wall’s top plate. It has two faces: a level seat cut that rests on the plate and a vertical heel (plumb) cut against the plate’s outer edge.
On a 6/12 roof with a 3½” seat, the heel cut is 1¾”.
A common rule keeps the notch to no more than one-third of the rafter depth, leaving at least two-thirds above the plate, with at least 1½” of bearing on the plate.
On a 2×8 (7¼”) at 6/12 with a 3½” seat the notch is about 22% — within the ⅓ rule. Your local building code is the final word.
The heel (plumb) cut equals the seat length × rise ÷ 12. For a 3½” seat on a 6/12 roof that is 3.5 × 6/12 = 1¾”.
The level seat cut equals the plate width, and the notch depth square to the rafter edge is seat × sin(roof angle).
The seat cut is the level (horizontal) face that bears on the top plate — usually the full plate width. The heel cut is the vertical (plumb) face at the outer edge of the plate.
Together they form the notch; height above plate (HAP) is what’s left of the rafter above the seat.
Height above plate is the vertical rafter material left above the seat cut, measured plumb. It sets how high the roof sits on the wall and should stay consistent on every common rafter.
On a 2×8 at 6/12 with a 3½” seat, HAP is about 6⅜”.
You can, but a full 3½” seat on a 2×4 (3½” deep) removes far more than a third of the rafter, so it fails the common notch rule. Use a shorter seat, a deeper rafter, or no birdsmouth at all.
On a 2×6 the same seat is about 28% — within the rule.
The tail runs from the heel cut out to the plumb (fascia) cut. Its length along the rafter is the horizontal overhang ÷ cos(roof angle).
A 12″ overhang on a 6/12 roof gives about 13⁷⁄₁₆” of tail.
Yes. Rafter spans, how much you may notch, minimum bearing, and the ties and connectors that resist uplift are all set by your local code — and often an engineer for heavy snow loads or long spans.
Treat this calculator as a layout estimate and confirm against your code.
Layout estimate only — confirm on site. This calculator works in US units (inches) and uses actual lumber sizes (a 2×8 is 7¼” deep). Heel cut = seat × rise ÷ 12, and the notch depth is checked square to the rafter edge against your limit. A common rule keeps the notch to one-third of the rafter depth or less with at least 1½” of bearing; don’t overcut the inside corner. Rafter spans, notching limits, ties and uplift connectors are set by your local building code — check it, and use an engineer for heavy snow loads or long spans. Lay out one rafter, test-fit, then use it as a pattern.