Birdsmouth Cut
Free birdsmouth cut rafter calculator. Get run, rise, total length, heel cut depth, min ridge board depth, height above plate for roof pitch and building width.
About This Calculator
The Birdsmouth Cut Calculator helps roofers, carpenters, framers, and DIY homeowners determine the precise dimensions needed to cut a birdsmouth notch in roof rafters. A birdsmouth cut is a critical structural notch on the underside of a rafter that allows it to sit securely on top of the wall plate while maintaining the strength of the rafter for the overhanging portion.
This calculator uses standard roof geometry and trigonometry to compute all essential birdsmouth dimensions. The rafter run is calculated as X = (W − t) / 2, where W is the building width and t is the ridge board thickness. The rafter rise Y = X × tan(θ) and the segment length Ls = X / cos(θ). The total rafter length adds the overhang: Lr = Ls + O / cos(θ). For the birdsmouth itself, the heel cut depth ch = cs × tan(θ), where cs is the seat cut length. The International Residential Code requires the minimum rafter depth to be at least 4 × ch to ensure the notch does not compromise structural integrity.
Regional Notes
United States (US): Dimensions are in inches. Standard lumber sizes are 2×4 (1.5" actual), 2×6 (5.5"), and 2×8 (7.25"). Common seat cut lengths equal the top plate width: 3.5" for single-ply walls. Ridge boards are typically 1" (nominal) or 1.5" actual thickness.
United Kingdom (UK): Dimensions are in millimeters. Common rafter sizes include 38×100 mm, 38×150 mm, and 50×200 mm. Seat cut lengths typically match the wall plate width at 90 mm or 100 mm. Roof pitches are often expressed as degrees or as a ratio.
India (IN): Dimensions are in millimeters. Common timber sizes vary by region. Standard wall thickness for brick masonry is 230 mm or 115 mm. Roof pitches typically range from 20° to 45° depending on the roofing material and local climate conditions.
Frequently Asked Questions
What is a birdsmouth cut in roofing?
A birdsmouth cut is a V-shaped notch cut on the underside of a rafter where it sits on top of the wall's top plate. It provides a flat horizontal surface (seat cut) and a vertical surface (heel cut) that allows the rafter to rest securely on the wall while maintaining the integrity of the overhanging portion of the rafter.
How do I calculate a birdsmouth cut?
To calculate a birdsmouth cut, first find the rafter run X = (building width - ridge board thickness) / 2 and the rafter rise Y = X × tan(roof pitch angle). Then compute the segment length Ls = X / cos(θ) and total rafter length Lr = Ls + overhang / cos(θ). The heel cut depth ch = seat cut length × tan(θ). The minimum ridge board depth must be at least 4 × ch per building code requirements.
What is the difference between seat cut and heel cut?
The seat cut is the horizontal cut of the birdsmouth that sits flat on top of the wall's top plate. The heel cut is the vertical cut that runs up from the back of the seat cut. Together they form the notch that allows the rafter to rest securely. The seat cut length is typically equal to or slightly less than the wall top plate width, while the heel cut depth is calculated from the seat cut length and roof pitch.
What is the depth limit for a birdsmouth cut according to building codes?
According to the International Residential Code (IRC), the rafter material should have a depth of at least four times the heel cut depth. This means the birdsmouth notch should not remove more than one-quarter of the rafter depth. The minimum ridge board depth is also at least four times the heel cut depth to ensure structural integrity of the rafter system.
How long should a seat cut be for a birdsmouth?
The seat cut length should match the width of the wall's top plate, typically 3.5 inches for a standard 2x4 wall or 5.5 inches for a 2x6 wall. In metric construction, common seat cut lengths are 90 mm or 140 mm. The seat cut can be slightly shorter than the top plate width but should never exceed it.
Is a birdsmouth cut necessary for all rafters?
A birdsmouth cut is necessary for timber-framed roofs where rafters rest directly on top of wall plates. It ensures the rafter has full bearing on the wall, prevents sliding, and properly transfers the roof load. However, roofs with trusses or those using rafter-to-joist hanger systems may not require birdsmouth cuts. Always consult local building codes for specific requirements.
What tools do I need to cut a birdsmouth in a rafter?
To cut a birdsmouth in a rafter, you typically need a circular saw for the main cuts and a handsaw or jigsaw for the final corner. A speed square is essential for marking the correct angles. For angled cuts on the rafter ends, a compound miter saw provides the most accurate results. Always wear appropriate safety gear including eye and hearing protection.
What is height above the plate in a birdsmouth cut?
Height above the plate (HAP) is the shortest vertical distance from the top of the wall plate to the top edge of the rafter. It is calculated as HAP = minimum ridge board depth - heel cut depth = 3 × heel cut depth. This dimension is critical for ensuring the rafter has sufficient structural depth above the notch.