Calculate the exact rafter length, plumb cut angle, and birdsmouth seat cut depth for your roof framing project. Simply enter your building span, roof pitch, and overhang — the calculator determines your rafter dimensions and recommends the right lumber size based on your span. Perfect for shed roofs, gable roofs, and home additions.
Total horizontal distance, wall-to-wall.
Rise in inches per 12 inches of run.
Eave extension beyond the wall. Typical: 12–24 in.
Approx. cost for your rafter lumber size.
📐 Rafter Dimensions
Rafter Length (per side)—
Plumb Cut Angle—
Birdsmouth Seat Cut Depth—
Recommended Rafter Size—
Total Rafter Length (incl. overhang)—
Rafters Needed (16" OC, both sides)—
Total Linear Feet of Rafter Lumber—
Estimated Lumber Cost—
Calculations assume a simple gable roof with a center ridge board. Always add 10–15% for waste and check local building codes.
How to Use the Rafter Calculator
Start by entering your building's total span — the horizontal distance from outside wall to outside wall. Select your roof pitch from the dropdown (expressed as X:12, meaning X inches of rise for every 12 inches of run). Then add your desired overhang (eave extension beyond the exterior wall, typically 12 to 24 inches).
The calculator computes the rafter length using trigonometry: rafter length = (span ÷ 2) ÷ cos(atan(pitch ÷ 12)). This gives you the length from the ridge board to the outside face of the wall. The overhang is added to this to get the total rafter length. The plumb cut angle is the angle at the top of the rafter where it meets the ridge board. The birdsmouth seat cut depth is the notch where the rafter rests on the top plate — critical for proper load transfer.
The lumber size recommendation follows standard span tables: 2×4 for spans up to 10 ft, 2×6 up to 14 ft, 2×8 up to 18 ft, 2×10 up to 22 ft, and 2×12 up to 26 ft. These assume 16-inch on-center spacing and standard residential loads. Always verify with your local building department.
Roof Framing Tips
Cut a test rafter first. Before cutting all your rafters, cut one, test-fit it, and use it as a pattern for the rest. A small error repeated across 20 rafters is a big problem.
Birdsmouth depth matters. The birdsmouth seat cut should not exceed one-third of the rafter's total depth. Cutting too deep weakens the rafter at its most critical load point.
Use a framing square. A framing square with stair gauges is the traditional tool for laying out plumb cuts and birdsmouth notches. Speed squares work well for smaller rafters.
Account for ridge board thickness. If using a 2× ridge board (1½" thick), subtract ¾" from each rafter's theoretical length so they meet flush at the ridge.
Check snow load requirements. In heavy snow regions, you may need to go up one lumber size or reduce spacing to 12" OC. Check your local building code for ground snow load values.
Rafter length is calculated from the ridge board center to the outside face of the wall plate. The formula is: rafter length = (span ÷ 2) ÷ cos(pitch angle). For example, a 24 ft span with a 4:12 pitch gives a rafter length of about 12.65 ft per side. Our calculator does this math automatically — just enter your span and pitch, and we compute the exact length including your overhang.
Rafter size depends on span, spacing, and load. As a general guide for 16" OC spacing: 2×4 works up to 10 ft, 2×6 up to 14 ft, 2×8 up to 18 ft, 2×10 up to 22 ft, and 2×12 up to 26 ft. Heavier roofing materials (tile, slate) or high snow loads may require larger lumber or closer spacing. Always check your local building code — span tables vary by jurisdiction and climate zone.
A birdsmouth cut is a notch at the bottom of a rafter where it sits on the top plate of the wall. It has two parts: the seat cut (horizontal, resting on the plate) and the heel cut (vertical, against the outside face of the wall). The birdsmouth transfers the roof load directly to the wall framing. The seat cut depth should not exceed one-third of the rafter's total depth to maintain structural integrity.
The plumb cut (or ridge cut) is the angled cut at the top of the rafter where it meets the ridge board — it matches the roof pitch angle. The seat cut is the horizontal portion of the birdsmouth notch that rests on the wall's top plate. Together with the heel cut, they form the birdsmouth. The plumb cut angle equals the roof pitch angle: for a 4:12 pitch, it's about 18.4 degrees.
Standard roof overhangs range from 12 to 24 inches beyond the exterior wall. A 12-inch overhang is common for simple gable roofs and provides basic water shedding. A 16- to 24-inch overhang offers better wall protection from rain and sun, and is typical for residential construction. Deeper overhangs (24+ inches) are used in hot climates for shade or in wet climates for extra weather protection, but require engineered support. Check your local building code — some jurisdictions have minimum overhang requirements.