Birdsmouth Cut Calculator
Turn your roof pitch into precise rafter cuts. Enter your pitch, rafter size, and wall plate width to accurately calculate the heel cut, seat cut, and height above plate (HAP).
Layout your rafters with confidence
A birdsmouth notch allows a roof rafter to sit flat and securely on the top plate of an exterior wall. It consists of a horizontal "seat cut" and a vertical "heel cut" (or plumb cut).
By locking in the seat cut length (which usually matches the wall framing width), you can determine the exact heel cut required based on the roof pitch. The remaining uncut vertical wood is the Height Above Plate (HAP), which is the most critical measurement for keeping your fascia line perfectly straight.
Height Above Plate (HAP)
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Heel (Plumb) Cut
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Height Above Plate
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Rafter Angle
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Degrees
Seat (Level) Cut
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Total Plumb Depth
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Full vertical line length
Remaining Wood
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Structural check
HAP at different roof pitches
Comparing dimensions for the same rafter size and seat cut.
| Pitch | Heel Cut | H.A.P. | Angle |
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Framing tip: Cut your first rafter and test fit it as a template (pattern rafter) before cutting the rest of your lumber. Wood dimensions vary, and walls are rarely perfectly straight.
How to use the birdsmouth calculator
Calculate a birdsmouth cut by using the roof pitch, rafter span, and wall plate width. The birdsmouth consists of a seat cut and a heel cut. For a roof with a 6:12 pitch, the rise equals 6 inches for every 12 inches of run. Calculate rafter length using the Pythagorean theorem, then determine the seat cut depth while maintaining structural code requirements.
- Enter Roof Pitch: Input your roof's rise over 12 inches of run (e.g., 6 for a 6/12 roof).
- Select Rafter Size: Choose the nominal size of the dimensional lumber you are using for the rafters (like 2x6 or 2x8).
- Set Seat Cut: The seat cut is the horizontal depth of the notch. This is typically set to match your exterior wall's top plate width (3.5" for a 2x4 wall, 5.5" for a 2x6 wall).
- Review the Results: The calculator will immediately provide the heel cut depth (the vertical line of the notch) and the HAP (Height Above Plate).
- Layout the Template: Use a framing square. Mark your plumb line first, measure down the plumb line using the HAP value, and square off your seat cut from that point.
Birdsmouth Math & Formulas
The math behind a birdsmouth cut is purely trigonometric, relying on the properties of right triangles established by your roof's pitch.
Angle (θ) = arctan(Rise ÷ 12)
Heel Cut = Seat Cut × (Rise ÷ 12)
Total Plumb Depth = Rafter Width ÷ cos(θ)
H.A.P. = Total Plumb Depth − Heel Cut
Example: For a 6/12 roof pitch using a 2x6 rafter (5.5" actual) and a 3.5" seat cut:
The heel cut is 3.5 × (6 / 12) = 1.75".
The angle is ~26.56°, making the total plumb depth 5.5 / cos(26.56°) ≈ 6.15".
The HAP is therefore 6.15 - 1.75 = 4.40" (roughly 4 3/8").
Marking & Cutting Checklist
A birdsmouth notch weakens the end of the rafter, so precision is important to ensure the rafter is fully supported by the wall plate without compromising its structural integrity.
Crown the Lumber
Sight down the edge of your board. Position the rafter so the "crown" (the natural bow) faces up. Gravity will straighten it over time.
Use a Framing Square
Use a framing square with stair gauges set to your rise and run (e.g., 6 and 12) to draw perfectly accurate plumb and level lines.
Mark HAP from the Top
Always mark your HAP (Height Above Plate) measuring down from the top edge of the rafter. This keeps the roof plane perfectly flat even if the lumber width varies slightly.
Don't Overcut
Use a circular saw to cut the lines, but stop before the blade crosses the inside corner. Finish the cut with a handsaw so you don't weaken the remaining wood.
Building Codes and the "2/3 Rule"
One of the most common mistakes in roof framing is over-cutting the birdsmouth notch. Because the notch is located at the exact point where the roof load transfers to the walls, taking out too much wood can cause the rafter tail to split, crack, or the roof to sag over time.
The International Residential Code (IRC) provides specific guidelines for notching structural lumber. For rafters, the golden rule is the "2/3 Rule": you must leave at least two-thirds of the rafter's actual depth intact directly above the birdsmouth cut (measured perpendicular to the grain).
Minimum Remaining Wood Example:
If you are using a 2x8 rafter (which measures 7.25 inches deep), the absolute minimum amount of wood that must remain above the notch is 4.83 inches. If your combination of pitch and seat cut leaves less wood than this, you must either decrease your seat cut size or use larger lumber (like a 2x10).
Code reference: ICC - IRC Chapter 8: Roof-Ceiling Construction.
Steep Roofs: When to Use a Partial Seat Cut
On shallow roofs (like a 4/12 pitch), you can usually cut the seat to match the exact width of your wall plate (e.g., 3.5" or 5.5") without violating the 2/3 rule. However, as the roof pitch gets steeper, the angle of the notch becomes sharper, and a full-width seat cut drives the heel cut dangerously deep into the lumber.
If you are framing an 8/12, 10/12, or 12/12 roof, you will almost always need to use a partial seat cut to maintain the structural integrity of the rafter.
- The minimum requirement: Building codes typically require a minimum bearing surface of just 1.5 inches on wood framing. You do not strictly need the seat cut to cover the entire width of the wall plate.
- The practical approach: If your 2x6 wall top plate is 5.5" wide, but an 8/12 pitch makes a 5.5" seat cut too deep, safely reduce your seat cut input to 2.5" or 3". The rafter will only bear on the outer half of the top plate, which is perfectly acceptable and allows the overhang to clear the wall.
Framing reference: The Journal of Light Construction (JLC) - Framing.
Speed Square vs. Framing Square
Once you have your calculated Heel Cut and HAP values from the calculator above, you need to physically transfer those lines to your lumber. There are two primary tools carpenters use to layout birdsmouth cuts:
The Framing Square (Steel Square)
The traditional method. You attach "stair gauges" (small brass clamps) to the square—one on the 12-inch mark (the run) and one on your pitch mark (the rise). You slide the square along the top edge of the rafter to draw your plumb (heel) and level (seat) lines simultaneously. This method is incredibly fast once set up.
The Speed Square (Rafter Square)
The modern method. You hook the lip of the square on the top of the board, pivot it until the edge aligns with the "Common" roof pitch number (e.g., 6) stamped on the square, and draw your plumb line. From that plumb line, you measure down your HAP, then use the square to draw a 90-degree line for the seat cut.
Tool reference: Wikipedia - Speed Square.
Interesting Fact
The rafter-to-wall connection at the birdsmouth cut is considered the most vulnerable joint for a roof's wind resistance during extreme weather. While traditional toenailing provides limited hold-down strength, installing metal hurricane ties over this joint can increase the roof's uplift resistance by up to 500%. According to the Federal Emergency Management Agency (FEMA), reinforcing this specific framing connection is one of the most critical and cost-effective ways to prevent catastrophic roof failure during severe storms.
Frequently Asked Questions
What is a birdsmouth cut?
A birdsmouth cut is a precise triangular notch cut into a common rafter or other timber where it meets the exterior wall plate. It allows the angled rafter to sit securely and flatly on the horizontal top plate of the wall, transferring the load directly downwards regardless of the roof slope. Whether you are building a standard gable or a simple lean-to roof (also known as a shed roof), this notch is essential in traditional roof framing.
What is HAP (Height Above Plate)?
The Height Above Plate (HAP) is the vertical length of the un-cut portion of the rafter remaining directly above the birdsmouth notch (specifically, in line with the vertical plumb cut or heel cut). It is the most important measurement to keep consistent across all your rafters to ensure your roof decking and the fascia board along the overhang remain perfectly straight and level.
How deep is a birdsmouth allowed to be?
Building codes restrict how deeply you can notch a structural member. For roof rafters, the depth of the notch generally shouldn't exceed 1/4 of the lumber's depth, and you must leave a specific amount of "remaining wood" perpendicular to the rafter above the cut. A common rule of thumb is that the remaining wood should be at least 2/3 of the total rafter depth, or minimum 3.5 inches, but you should always verify with your local building codes. Our birdsmouth cut calculator automatically checks this remaining wood depth for you to help ensure structural safety.
Should the seat cut match the wall width exactly?
Most carpenters prefer the seat cut to match the wall top plate width exactly (e.g., 3.5" for a 2x4 wall). This ensures maximum bearing surface without the rafter notching into the interior drywall space, which is especially important for roofs with a wide span. However, on roofs with a very steep pitch, the sharper angle means a full-width seat cut might cause the heel cut to be too deep. In those cases, the seat cut is shortened so the structural integrity of the rafter is maintained.
Do I cut birdsmouths for trusses?
No. Manufactured roof trusses are engineered differently and are designed to sit directly on top of the wall plates without any notching. Birdsmouth cuts are strictly for traditional "stick-built" roofs where individual rafters are measured by their rise and run, and then laid out on-site using a speed square or framing square before being cut.
Other useful calculators
Disclaimer: This birdsmouth calculator provides theoretical layout dimensions based on perfect geometrical inputs. Dimensional lumber can vary significantly in actual width and straightness. Always verify your calculations by cutting and test-fitting a single pattern rafter before cutting the rest of your materials. It is the user's responsibility to ensure all framing notches comply with local building codes and structural requirements.