What Is a Common Rafter?
A common rafter is the most fundamental structural member in a sloped roof system. It runs perpendicular to the ridge board at the top and seats on the wall plate at the bottom, spanning the full rise of the roof on one side. Common rafters are spaced evenly along the length of the building — typically 16 inches or 24 inches on center — and carry the roof sheathing, underlayment, and finished roofing material. Every other rafter type (hip, valley, jack) is derived from the common rafter geometry, making it the starting point for all roof framing calculations.
Unlike engineered trusses that arrive pre-fabricated, stick-framed roofs require each common rafter to be cut on site. The carpenter must calculate the correct length, mark the plumb cut at the ridge, cut the bird's mouth at the wall plate, and account for any overhang beyond the exterior wall. This calculator provides all four critical dimensions from two inputs: building span and roof pitch.
Span, Run, and Rise — Understanding the Three Dimensions
These three terms are the vocabulary of rafter layout, and confusing them is the most common source of framing errors. The span is the total horizontal distance from one outside wall to the other — measured from the outer face of the top plate on one side to the outer face on the opposite side. This is the number you measure with a tape measure across the building.
The run is exactly half the span for a symmetrical gable roof. It represents the horizontal distance one rafter must travel, from the wall plate to the centerline of the ridge. If your building is 24 feet wide, the run is 12 feet. This is the horizontal leg of the right triangle that defines your rafter.
The rise is the vertical height the roof gains over that run distance. It is calculated directly from the pitch: rise = run × (pitch value ÷ 12). For a 6:12 pitch over a 12-foot run, the rise is exactly 6 feet. The rise is the vertical leg of the same right triangle. Once you know the run and the rise, the rafter length is simply the hypotenuse.
How Rafter Length Is Calculated — the Pythagorean Theorem
Rafter length is the hypotenuse of a right triangle whose legs are the rise and the run. The formula is L = √(rise² + run²). For a 6:12 pitch with a 12-foot run: rise = 6 ft, so L = √(6² + 12²) = √(36 + 144) = √180 ≈ 13.42 feet. This is the structural rafter length from the ridge centerline to the outside face of the wall plate — before any ridge deduction or overhang addition.
This calculation is exact and repeatable for any pitch and run combination. The calculator performs it instantly and displays the result in feet and inches with fractional precision, which is the format carpenters use on the job site.
Ridge Board Deduction — Accounting for Lumber Thickness
When a ridge board is used (as opposed to a structural ridge beam), each rafter must be shortened by half the ridge board thickness before cutting. This is because the calculated length runs to the centerline of the ridge, but the actual rafter terminates at the face of the ridge board. For standard 2× dimensional lumber (actual thickness 1.5 inches), deduct 0.75 inches from each rafter. For 1× ridge boards (actual thickness 0.75 inches), deduct 0.375 inches.
This deduction sounds small, but omitting it means your ridge will be pushed up slightly and the opposing rafters will not seat correctly. The calculator's "to ridge face" output already incorporates this deduction when you enter a ridge board thickness.
Plumb Cut, Seat Cut, and the Bird's Mouth
Every common rafter requires two angled cuts: a plumb cut at the top where it meets the ridge, and a compound notch at the bottom called the bird's mouth. The plumb cut is a purely vertical cut — "plumb" meaning straight up-and-down — and its angle equals the roof pitch angle. For a 6:12 pitch, the plumb cut angle is 26.57°. Set your saw bevel to that angle and the cut face will be perfectly vertical when the rafter is in position, sitting flush against the ridge board.
The bird's mouth is the notch that allows the rafter to sit flat and securely on the wall plate. It consists of two cuts: a plumb cut (the same angle as the ridge cut, which forms the vertical face of the notch) and a seat cut (a horizontal cut perpendicular to the plumb cut). The seat cut angle is 90° minus the plumb angle — for 6:12 that is 63.43°. The depth of the seat cut is typically set so the rafter sits on the full width of the wall plate without cutting too deeply into the rafter, which would weaken it. A common rule of thumb is to limit the seat cut depth to no more than one-third of the rafter depth.
Overhang — Eave Extension Beyond the Wall
The overhang (also called the eave extension) is the portion of the rafter that extends beyond the exterior wall to form the soffit. It serves a critical function: directing rainwater away from the foundation and siding, shading windows from high summer sun, and protecting the wall from driving rain. Common residential overhangs range from 12 inches to 24 inches, though some architectural styles use overhangs of 36 inches or more.
The overhang adds directly to the total rafter board length you need to purchase, but it does not contribute to the structural span calculations. When ordering lumber, add the overhang length to the ridge-to-wall-plate length to get the total board length. A rafter that is 13.42 feet structurally with an 18-inch overhang requires a board that is at least 14.92 feet long — practically meaning you order 16-foot lumber.
Rafter Spacing — 16" OC vs 24" OC and Total Rafter Count
Rafters are spaced evenly along the building length, measured on center (OC) — from the center of one rafter to the center of the next. The two standard spacings in US residential construction are 16 inches OC and 24 inches OC. Sixteen-inch spacing is stronger, provides a stiffer deck, and is required in most jurisdictions for heavier roofing materials like tile and slate. Twenty-four-inch spacing uses fewer rafters and is acceptable for lighter materials like asphalt shingles on shorter spans.
To calculate the number of rafters needed per side: divide the building length (in inches) by the spacing (in inches) and add one rafter for the end. For a 40-foot building at 16 inches OC: (40 × 12 ÷ 16) + 1 = 31 rafters per side, or 62 total for both sides of a gable roof. The calculator performs this automatically when you enter building length and spacing.
Saw Bevel Settings for Common Pitches
Setting the circular saw bevel correctly before making plumb cuts saves time and prevents wasted lumber. The bevel angle equals the plumb cut angle, which is the arctangent of the pitch ratio. Most modern circular saws allow you to set the bevel from 0° (square cut) to 45° or 50°.
- 3:12 pitch → plumb cut 14.04° → set saw bevel to 14.0°
- 4:12 pitch → plumb cut 18.43° → set saw bevel to 18.4°
- 5:12 pitch → plumb cut 22.62° → set saw bevel to 22.6°
- 6:12 pitch → plumb cut 26.57° → set saw bevel to 26.6°
- 8:12 pitch → plumb cut 33.69° → set saw bevel to 33.7°
- 10:12 pitch → plumb cut 39.81° → set saw bevel to 39.8°
- 12:12 pitch → plumb cut 45.00° → set saw bevel to 45.0°
Common Rafter Measurement Mistakes to Avoid
Rafter framing errors typically fall into a handful of repeating categories. Understanding them before cutting saves material and rework time.
- Using span instead of run as the horizontal input — the run is half the span; entering the full span doubles your rise calculation and produces a rafter far too long
- Forgetting the ridge board deduction — the calculated length goes to the ridge centerline, not the ridge face; always subtract half the ridge board thickness
- Confusing overhang with structural rafter length — the overhang adds to board length but not to the structural triangle; keep them separate in your calculations
- Not accounting for the bird's mouth depth when specifying rafter size — a deep seat cut into a shallow rafter reduces structural capacity; check span tables for your lumber size, spacing, and pitch
- Measuring along the slope instead of horizontal for the run — always measure the horizontal projection, not the sloped distance