Birdsmouth Cut Calculator

Free Birdsmouth Cut Calculator for Roof Rafters

This birdsmouth cut calculator works out the exact seat cut and heel cut dimensions for a rafter, based on your roof pitch, the rafter size you're cutting, and the width of the wall plate it sits on. Type in your numbers and the tool instantly shows the seat cut length, the heel cut height, the notch depth, and whether that notch stays inside the safe rafter-depth limit, along with a live diagram and a full step-by-step breakdown of the math.

A birdsmouth cut is one of those details that looks simple on paper but is easy to get slightly wrong on the actual board, especially if you're cutting a stack of rafters for a shed, porch, or full roof frame. Getting the seat cut and heel cut right the first time means every rafter sits flat, bears evenly, and lines up at the same height along the wall, so you're not shimming or trimming rafters one at a time up on the ladder.

What Is a Birdsmouth Cut?

A birdsmouth cut is a small triangular notch cut into the underside of a rafter at the point where it crosses the top of the wall plate. The notch has two parts: a level seat cut, which is the flat horizontal surface that actually rests on the plate, and a plumb heel cut, which is the vertical face that sits against the outside edge of the plate. Together they look a little like an open bird's beak biting into the bottom of the board, which is exactly where the name comes from.

Without this notch, a sloped rafter would only touch the wall plate at a single sharp corner, since the bottom of the rafter runs at an angle while the top of the plate is flat. That single point of contact carries the whole weight of the roof through a tiny area of wood, which is a recipe for splitting, sliding, and long-term sagging. The birdsmouth spreads that same load across a full flat surface instead, and gives you a clean spot to toenail the rafter down securely.

Why the Seat Cut and Heel Cut Both Matter

The seat cut is doing the structural heavy lifting here. It's the flat surface that actually transfers the roof's weight down into the wall plate and, from there, into the studs and foundation below. A seat cut that's too short leaves less bearing surface than the rafter needs, while one that runs wider than the plate leaves part of the notch hanging in open air with nothing underneath it.

The heel cut plays a smaller but still useful role. Because it sits flush against the outer face of the plate, it stops the rafter from sliding outward under load and gives you a solid vertical face to nail through when securing the rafter in place. Most codes and framing guides also expect a minimum bearing length, commonly around 1.5 inches, so the seat cut needs to be long enough to satisfy that on its own, separate from whatever the wall plate width happens to be.

How This Calculator Works

The math behind a birdsmouth cut comes down to one number: the roof pitch angle. Once that angle is known, both cuts follow directly from simple trigonometry.

• Step 1 — Find the pitch angle: θ = arctangent of (rise ÷ 12), or you can type the angle in directly if you already know it in degrees.
• Step 2 — Set the seat cut length: this is usually set equal to the wall plate width, so the rafter bears across the full top of the plate.
• Step 3 — Calculate the heel cut: Heel Cut = Seat Cut × tangent(θ). This gives the vertical height of the plumb cut.
• Step 4 — Calculate the notch depth: Notch Depth = Seat Cut × sine(θ). This is the depth measured perpendicular to the rafter, which is the figure building codes actually limit.
• Step 5 — Check the notch against the rafter depth: most framing references keep the notch to no more than one-third of the rafter's actual depth, with some jurisdictions requiring a stricter one-quarter limit.

How to Use the Birdsmouth Cut Calculator

Start by entering your roof pitch, either as a rise-per-12-inches ratio like 6/12 or 8/12, or as a straight angle in degrees if that's what your plans show. Next, pick the rafter size you're actually cutting from the dropdown, or choose custom if you're working with a non-standard depth. Then select your wall plate width; a single 2×4 top plate is 3.5 inches, and a 2×6 wall is 5.5 inches, or you can enter a custom width for anything else.

By default the seat cut is set to match the wall plate width, which is the setup most carpenters use, but you can switch to a custom seat cut length if your design calls for something different. Pick which notch-depth rule you want the calculator to check against, and if you're also working out the overhanging eave, add the horizontal overhang length to see the rafter tail's true length along the slope. Every result and the diagram above update instantly as you change any value.

Worked Example: A 2×8 Rafter on a 6/12 Roof

Say you're framing a small porch roof with 2×8 rafters (7.25 inches actual depth), a 6/12 pitch, and a standard 2×4 wall (3.5 inches actual width). The pitch angle works out to arctangent(6 ÷ 12), which is about 26.57 degrees.

Setting the seat cut equal to the wall plate width gives a 3.5-inch seat cut. The heel cut height is 3.5 × tangent(26.57°), which comes out to about 1.75 inches. The notch depth, measured perpendicular to the rafter, is 3.5 × sine(26.57°), or roughly 1.57 inches. Checking that against the one-third rule: 7.25 ÷ 3 is about 2.42 inches, and since 1.57 inches is well under that, this notch is safely within limits, leaving about 5.68 inches of solid rafter depth above the cut to carry the load.

How Deep Should a Birdsmouth Cut Be? (The One-Third Rule)

The most commonly cited guideline for a birdsmouth notch is that it shouldn't remove more than one-third of the rafter's total depth. Some building departments apply a stricter one-quarter limit instead, particularly for engineered or heavily loaded rafters, so it's always worth checking your specific local code before you start cutting.

This limit exists because every inch you notch out of a rafter is an inch that isn't there to resist bending. A rafter is strongest when its full depth is intact; cut too deep at the bearing point and you create a weak spot exactly where the roof's load and the wall's reaction force are concentrated. If your seat cut and pitch combination pushes the notch past the limit, the fix is usually simple: step up to the next rafter size, since a deeper rafter can absorb the same notch as a smaller percentage of its total depth.

How to Mark and Cut a Birdsmouth With a Speed Square or Framing Square

Once you have your numbers from the calculator, marking the cut on the actual board is straightforward:

• Lay the rafter on sawhorses with the crown (any slight bow) facing up, and mark your building line — the point where the rafter crosses the outer face of the wall — using your framing square set to the rise and run shown above.
• From that building line, use a speed square set to the plumb cut angle to mark the heel cut, a straight vertical line down into the rafter by the heel cut height from this calculator.
• From the bottom of that heel cut line, mark the seat cut running level (horizontal) back toward the ridge for the seat cut length shown above, meeting the natural slope of the rafter's underside.
• Cut along both lines with a circular saw, stopping just short of the inside corner, then finish the corner with a handsaw or jigsaw so the notch is clean and square rather than over-cut.
• Test-fit the first rafter on the actual wall plate before cutting the rest of the batch, then use it as a pattern to trace and cut the remaining rafters so every one matches exactly.

Common Mistakes When Cutting a Birdsmouth

A handful of errors account for most birdsmouth problems on real job sites:

• Cutting the notch too deep, which weakens the rafter right at its most heavily loaded point and can fail a framing inspection.
• Making the seat cut shorter than the wall plate, leaving part of the plate uncovered and reducing the actual bearing area below what the rafter needs.
• Over-cutting past the inside corner with a circular saw, which leaves a visible kerf mark and a slight weak point right where the two cuts should meet cleanly.
• Using the wrong pitch angle because a rise-per-12 ratio was converted to degrees incorrectly, which throws off both the seat cut and heel cut even if the arithmetic afterward is correct.
• Cutting one rafter as a pattern without test-fitting it on the wall first, then discovering a small error only after every rafter in the stack has already been cut the same way.

Birdsmouth Cut vs Other Rafter-to-Wall Connections

A birdsmouth notch isn't the only way to land a rafter on a wall plate, though it remains the most common method in traditional stick framing. Some builders use a plumb-cut-only rafter with a metal seismic or hurricane tie doing the structural connection instead of a notch, which avoids removing any wood from the rafter but relies entirely on the hardware and its nailing pattern being correct. Raised-heel or energy-heel trusses sidestep the birdsmouth question altogether by building the heel height into a manufactured truss, which is common where extra attic insulation depth is needed above the wall line. For standard site-built rafters, though, a properly sized birdsmouth remains the simplest and most widely used solution, especially when it's combined with a hurricane tie for extra wind and seismic resistance.

Who Uses This Calculator

This tool comes in handy across a range of framing and building situations:

• Framing carpenters and roofers use it to double-check seat cut and heel cut dimensions before cutting a full stack of rafters for a house, garage, or addition.
• DIYers and homeowners use it when building a shed, pergola, porch roof, or lean-to, where getting the birdsmouth right the first time saves lumber and rework.
• Contractors and estimators use it to confirm that a chosen rafter size will pass the one-third or one-quarter notch rule before ordering material.
• Students and apprentices use it to check hand calculations for framing coursework and to understand exactly how the seat cut and heel cut relate to the roof pitch.