Rafter Calculator

How to Use the Rafter Calculator

This rafter calculator simplifies roof framing by computing all the critical dimensions needed to cut and install common rafters. Enter your building width (the span from outside wall to outside wall), select your desired roof pitch, specify overhang length, and input your wall height. The calculator then determines rafter length, ridge height, birdsmouth cut dimensions, and angle cuts. Use these measurements to layout and cut your rafters accurately, ensuring a properly framed roof structure.

Understanding Roof Pitch

Roof pitch describes the steepness of a roof and is expressed as a ratio of vertical rise to horizontal run. A 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal run, creating a 26.57-degree angle. Lower pitches like 3/12 or 4/12 are common for ranch-style homes and areas with minimal snow, while steeper pitches like 8/12 to 12/12 are used in regions with heavy snow loads or for architectural style. Pitch affects both the aesthetic appearance and functional performance of your roof, including water drainage, snow shedding, and attic space.

Common Rafters vs. Other Roof Framing Members

Common rafters are the primary sloped members that run from the ridge board at the peak down to the top plate of the exterior walls. They are the most numerous framing members in a traditional stick-framed roof. Hip rafters run diagonally from corners at a 45-degree angle, while valley rafters form inside corners where two roof sections meet. Jack rafters are shortened rafters that run from hip or valley rafters to the wall plate or ridge. This calculator focuses on common rafters, which form the basic structure for most gable and shed roofs.

The Birdsmouth Cut Explained

The birdsmouth is a triangular notch cut into the underside of a rafter where it sits on the top plate of the wall. This notch consists of two cuts: the plumb cut (vertical face against the wall) and the seat cut (horizontal surface resting on the top plate). The birdsmouth provides a stable bearing surface and helps the rafter resist horizontal thrust. Building codes typically require at least 1.5 inches of bearing, but 3-4 inches is more common. The depth of the birdsmouth is critical - cutting too deep weakens the rafter, while insufficient depth reduces bearing area.

Rafter Length Calculation Method

Rafter length is calculated using the Pythagorean theorem, treating the roof as a right triangle. The run (horizontal distance) is half the building width minus half the ridge board thickness. The rise is calculated by multiplying the run by the pitch ratio (rise over 12). The rafter length is the hypotenuse: the square root of (run squared plus rise squared). To this theoretical length, you add the overhang length measured along the rafter slope, not horizontally. This calculator performs all these calculations automatically and accounts for the ridge board thickness.

Ridge Board Height and Wall Plates

The ridge board sits at the peak where opposing rafters meet. Its height above the wall plate equals the total rise calculated from the roof pitch and building span. This calculator computes the ridge board height from the top of the wall, which is crucial for setting up your ridge board supports during framing. When building, you'll typically set up a temporary support post at each end of the ridge board at the calculated height, then install rafters working from each end toward the middle. The wall height input helps you understand the total height from floor to ridge.

Overhang and Eave Details

The overhang (eave) is the portion of the rafter that extends beyond the exterior wall, providing weather protection for siding and windows while creating shadow lines that enhance architectural appearance. Common overhangs range from 12 to 24 inches. The overhang dimension can be measured horizontally or along the rafter slope - this calculator uses the horizontal measurement, which is how overhangs are typically specified. Remember that the overhang affects the total rafter length, and you may need to account for fascia board attachment and soffit framing in your final design.

Rafter Sizing and Span Tables

While this calculator computes cutting dimensions, it doesn't validate whether your selected rafter size is adequate for the span and load. Rafter sizing depends on span, on-center spacing, snow load, dead load, wood species, and lumber grade. A 2x6 rafter might span 10-12 feet at 16" OC in light snow areas, while a 2x10 can span 16-20 feet. Always consult span tables in your local building code or the American Wood Council's span tables to verify your rafter size is appropriate. In high snow load areas or for longer spans, you may need to use engineered lumber or reduce rafter spacing.