Common rafter calculator
Calculate rafter length, ridge height, birdsmouth, and cutting angles from building span and roof pitch.
Drop calc results into projects and quotes, invoice through your own Stripe, get paid. Built by a contractor.
Try TradeMaster Pro →How to Calculate Rafter Length
A common rafter runs from the ridge board down to the wall plate and beyond as the overhang. The total run is half the building span minus half the ridge thickness. The total rise is the run multiplied by the pitch ratio (e.g., 6/12 = 0.5).
The rafter length along the slope equals the run divided by the cosine of the roof angle. Add the overhang length (also measured along the slope) for the total rafter you need to cut.
The most common rafter calculation contractors run is for a common rafter on a gable roof: enter the building width, the roof pitch, and the desired overhang, and the calculator returns the rafter length, ridge height, and the cutting angles for the plumb cut, level cut, and birdsmouth seat. For roofs with unequal pitches, irregular hips, or valleys, additional inputs are needed for the secondary pitch and run length.
Birdsmouth Cut
The birdsmouth is a notch cut into the rafter where it sits on the wall plate. It has two parts: the seat cut (horizontal, bearing on the plate) and the plumb cut (vertical, against the wall). The HAP (height above plate) is the remaining rafter depth above the seat cut.
Common Roof Pitches
- 3/12 (14.0°) — Low slope, minimum for shingles
- 4/12 (18.4°) — Common for ranch-style homes
- 6/12 (26.6°) — Standard residential pitch
- 8/12 (33.7°) — Steeper, good for snow shedding
- 12/12 (45.0°) — 45-degree, steep residential
FAQ
How long should my rafters be?
Rafter length depends on the building width (run), roof pitch, and overhang. The basic formula is: rafter length = √(rise² + run²) + overhang. For a 24-foot wide building at 6/12 pitch and 12-inch overhang, the common rafter length comes out to about 13 feet 5 inches per side. Enter your dimensions in the calculator above for the exact number including birdsmouth depth and tail cut.
What roof pitch should I use?
Most residential homes use pitches between 4/12 and 9/12. Lower pitches (3/12 to 4/12) shed water but not snow well, and many shingle manufacturers void warranties below 4/12. Steeper pitches (8/12 and up) shed snow effectively and provide more attic space but cost more in materials and labor. In snow regions, 6/12 to 8/12 is the sweet spot.
How do I cut a birdsmouth?
The birdsmouth is the notch where the rafter sits on the top plate. It consists of two cuts: a horizontal seat cut (parallel to the top plate) and a vertical heel cut (plumb to the wall). The seat cut depth should not exceed 1/3 of the rafter's actual depth per IRC R802.7 — for a 2x10 (9.25" actual), that's a maximum 3" seat cut. The calculator above gives you the exact angles to set on your speed square.
What's the difference between common, hip, jack, and valley rafters?
Common rafters run from the wall plate to the ridge at 90° to both. Hip rafters run diagonally from the corner of the building to the ridge on hip roofs. Valley rafters run diagonally where two roof planes meet at an inside corner. Jack rafters are shorter rafters that run from the wall plate to a hip or valley rafter, getting progressively shorter as they move toward the corner.