Rafter Birdsmouth & Seat Cut Guide

Birdsmouth Anatomy: Heel Cut, Seat Cut & Plumb Cut

The birdsmouth is the notch cut into the bottom of a rafter so it seats firmly on the top plate of the wall. Learning how to calculate the rafter birdsmouth means understanding three geometric elements that meet inside the notch: the heel cut (a plumb line that mirrors the ridge plumb cut), the seat cut(a level line that bears flat on the wall plate), and the plumb cut angle (90° − pitch) that governs both. Together they form a right triangle inside the rafter whose legs are the seat length and the birdsmouth depth (BMD).

The three cuts are not independent: the seat cut length sets the birdsmouth depth through the pitch angle (BMD = seat × tan(pitch)), and the BMD in turn sets the HAP (HAP = depth − BMD). Change any one value and the other two shift instantly — which is exactly why the calculator above updates all of them together. This is also why a fixed seat length (equal to the wall plate width) is the standard starting point: it guarantees full bearing on the plate, and you then verify the resulting HAP is safe.

How to Calculate the Seat Cut Length on a Rafter

The seat length (sometimes mistyped as the "seed length" in search queries) is the horizontal bearing surface of the birdsmouth that sits flat on the wall plate. To calculate the seat length on a rafter, set it equal to the wall plate width so the rafter bears fully on the plate:

Once the seat length is fixed, the birdsmouth depth follows directly from the pitch:

How to Calculate HAP (Height Above Plate) on a Rafter

HAP (Height Above Plate) is the remaining solid rafter depth above the birdsmouth seat — the structural material that actually carries the load over the wall. To calculate HAP on a rafter, subtract the birdsmouth depth from the full rafter depth: HAP = Rafter Depth − BMD, where BMD = Seat Length × tan(pitch). The HAP ratio (HAP ÷ rafter depth) must stay at or above 2/3 (66.7%); below that threshold the rafter loses too much cross-section and the seat can crush or split under load.

Rafter height and HAP are related but distinct: the total rafter height (roof rise) = Run × (Pitch ÷ 12) and sets the ridge elevation, while the HAP is a local notching check at the wall plate. A steep roof can have a large rise but a small HAP if the seat cut is wide — which is exactly why high-pitch roofs (12/12 and above) often need 2x10 or 2x12 rafters to keep the HAP safe.

Why Don't All Rafter Calculations Use HAP?

Not all rafter calculations use HAP because HAP only matters where a birdsmouth notch is actually cut. Factory-built trusses, rafters sitting on a raised heel (energy-heel trusses), and rafters bearing on a beveled plate may have no birdsmouth at all, so the HAP check is irrelevant. In pure length math (line length + overhang) HAP never appears — it is a notching and structural check, not a length input. HAP enters the calculation only when you lay out the birdsmouth, because the seat-cut depth must leave at least 2/3 of the rafter depth solid above the plate. So rafter-length formulas ignore HAP, while birdsmouth layout must enforce it.

How to Calculate Rafter Length and Birdsmouth Together

Rafter length and birdsmouth are calculated in sequence because the birdsmouth position depends on the total rafter length. The birdsmouth does not change the rafter length — it only removes a notch so the rafter seats on the wall plate. The correct workflow is:

The only length adjustment the birdsmouth introduces is the plumb setback (rafter depth × tan(pitch)) at the ridge, which is already part of the standard length formula. For the full length-and-cuts workflow — including ridge deduction, overhang math, and the plumb setback in detail — see our companion guide on how to calculate rafter length and cuts.

How to Calculate Rafter Angles and Notches

The birdsmouth notch uses two angles: the plumb (heel) cut at 90° − pitch angle, and the seat cut at the pitch angle. For a 6/12 pitch (26.57°), the heel cut is 63.43° and the seat cut is 26.57°. The notch itself is the triangle formed where these two cuts meet inside the rafter. Additional notches — the ridge plumb cut, the tail cut, and any cheek cuts on hip/valley rafters — all derive from the same pitch angle. Set your circular saw to the pitch angle for plumb cuts and use a level (0°) reference for the seat cut.

Birdsmouth Cut Reference Table by Pitch

The table below shows birdsmouth dimensions for a standard 2x8 rafter (7.25" actual depth) on a double 2x4 top plate (3.5" seat) with a 10' run. Notice how the HAP ratio drops as pitch steepens — at 12/12 the HAP falls below the 2/3 safety line, which is why steep roofs need deeper rafters.

Reference: 2x8 rafter (7.25" actual), double 2x4 plate (3.5" seat), 10' run. At 10/12 and 12/12 the HAP drops below 66.7% — upgrade to a 2x10 or use a reduced seat length.

Step-by-Step: Marking the Birdsmouth on the Lumber

Once you know how to calculate the birdsmouth in rafter construction, the layout on the lumber follows a fixed six-step sequence. Each step builds on the previous mark, so work in order and double-check every measurement before the saw touches the board.

Common Birdsmouth Mistakes & Safety Limits

Two pitfalls catch beginners most often. First, using the nominal rafter depth (e.g., "8" for a 2x8) instead of the actual depth (7.25") — this inflates the HAP by 0.75" and hides a failing notch. Always use actual dressed dimensions. Second, forgetting that the seat cut angle equals the pitch angle, not 90° — a level seat cut on a steep roof produces a notch that does not bear flat, concentrating the load on a knife-edge. The seat cut must be parallel to the horizon (level), and the heel cut must be plumb (vertical), regardless of roof pitch.

Frequently Asked Questions