How to use this calculator
- Enter the span. Use the outside-wall to outside-wall building width for a centered gable roof.
- Set the roof pitch. Enter rise per 12 units of horizontal run, such as 6 for a 6/12 roof.
- Add overhang and ridge thickness. Enter horizontal eave overhang and the ridge board thickness, or use 0 for no ridge deduction.
- Enter length and spacing. Use building length along the ridge and the on-center rafter spacing for count and stock length.
- Read the rafter layout. Review run, rise, common length, tail length, pitch angle, rafter count and total stock length.
How it works
This rafter calculator is a roof-layout tool for a symmetric gable roof. It starts with the horizontal run from the outside wall to the side of the ridge:
run = span / 2 - ridge thickness / 2
Roof pitch is entered as rise per 12 units of run. The pitch angle and slope factor are:
theta = atan(pitch / 12)
slope factor = sqrt(12^2 + pitch^2) / 12
The common rafter length is the run multiplied by that slope factor. The eave tail uses the same slope factor from the horizontal overhang.
common rafter length = run x slope factor
cut length = (run + overhang) x slope factor
The count is a material takeoff estimate for common rafters: one pair at each end of the gable roof, plus each on-center spacing interval along the ridge.
Worked example
Verified against the live calculator
For a 24 ft span, 6/12 pitch,
12 in horizontal overhang, 1.5 in ridge board,
32 ft building length and 16 in spacing, the run to
the ridge is 11.94 ft. The slope factor is 1.118,
so the common rafter length is about 13.35 ft. The overhang tail
adds about 13.4 in, making the cut length about
14.46 ft. The roof needs 25 rafter pairs,
or 50 common rafters, before waste and layout changes.
Frequently asked questions
How do you calculate common rafter length?
Find the horizontal run first, then multiply by the slope factor. For a centered ridge, run = span / 2 - ridge thickness / 2. Slope factor = sqrt(12^2 + pitch^2) / 12. Common rafter length = run * slope factor.
Does this subtract the ridge board?
Yes. The calculator subtracts half the entered ridge board thickness from each side. Set ridge thickness to 0 if you want a pure half-span run with no ridge deduction.
Is this a rafter span calculator?
No. This is a layout calculator for common rafter geometry and material count. It does not choose 2x size, species, grade, spacing, snow load, wind load or code span limits.
What roof angle is a 6/12 pitch?
A 6/12 pitch has angle atan(6/12), which is about 26.6 degrees above level.
Does rafter count include both roof sides?
Yes. Rafter pairs are counted along the ridge, then multiplied by two for individual common rafters on both sides of a gable roof.
Does this include hip, valley or jack rafters?
No. It covers straight common rafters for a symmetric gable roof. Hip, valley, jack and dormer rafters need additional plan geometry.
Method & assumptions
- Assumes a symmetric gable roof with a centered ridge and common rafters only.
- Horizontal overhang is converted to sloped tail length with the same roof pitch.
- Angles are layout line angles: roof angle above level, plumb line angle to the rafter edge, and level seat line angle to the rafter edge.
- Rafter count assumes full-length common rafters at regular on-center spacing; openings, hips, valleys, rake details and blocking are not included.
- This is not a structural span table. Use approved rafter span tables or engineered design for species, grade, size, snow load, wind uplift, bracing, collar ties, bearing and code requirements.
- For material volume, use the board feet calculator. For beam-style checks, use the beam deflection calculator or LVL beam calculator.