Rafter Length Calculator
Calculate common rafter length from roof span, pitch, and overhang. Get exact rafter dimensions for hip, valley, and jack rafters with step-by-step cut calculations for any roof pitch.
📏 Framing🏗️ Rafter = (Run + Overhang) x Rafter Multiplier
Building width / span (ft)
Roof pitch (rise per 12-in run)
Overhang (inches)
Please enter valid values.
Formula & Reference
| Variable | Formula | Units |
|---|---|---|
| Rafter Length Calculator | Rafter = (Run + Overhang) x Rafter Multiplier | feet + inches |
Step-by-Step Examples
Example 1
6/12 Pitch, 28-ft Span
28 ft wide house, 6/12 pitch, 18 in overhang.
- Run = 28/2 = 14 ft
- Overhang = 18/12 = 1.5 ft
- Total run = 15.5 ft
- Multiplier = sqrt(1+(6/12)^2) = 1.1180
- Rafter = 15.5 x 1.1180 = 17.33 ft = 17 ft 4 in
✓ 17 ft 4 in rafter length
Example 2
4/12 Pitch, Ranch Home
24 ft span, 4/12 pitch, 12 in overhang.
- Run = 12 ft | Overhang = 1 ft | Total = 13 ft
- Multiplier = 1.0541
- Rafter = 13 x 1.0541 = 13.7 ft = 13 ft 8.4 in
✓ 13 ft 8 in rafter
Example 3
12/12 Pitch Steep Roof
30 ft span, 12/12 pitch, 24 in overhang.
- Run = 15 ft | Overhang = 2 ft | Total = 17 ft
- Multiplier = sqrt(2) = 1.4142
- Rafter = 17 x 1.4142 = 24.04 ft = 24 ft 0.5 in
✓ 24 ft rafter for steep 12/12 pitch
Real-World Applications
Roof Framing
Calculate rafter lengths before buying lumber to avoid waste.
Carpentry
Determine bird's mouth and tail cuts from rafter length and pitch.
Material Estimating
Know rafter length to order the right lumber length and quantity.
Trim Carpentry
Calculate fascia and soffit lengths from rafter projection dimensions.
Common Mistakes to Avoid
⚠️
Using total span instead of run
Run = half the total span. A 28-ft wide house has a 14-ft run. Using 28 ft as the run produces a rafter twice as long as needed.
⚠️
Forgetting the overhang in rafter length
The rafter extends past the wall to form the eave overhang. Always add the overhang to the run before applying the rafter multiplier.
⚠️
Cutting the bird's mouth too deep
The bird's mouth (notch where rafter sits on top plate) should not be cut deeper than 1/3 of the rafter depth. Too deep weakens the rafter significantly.
Frequently Asked Questions
What is a rafter multiplier? ▾
The rafter multiplier converts horizontal run to actual rafter length along the slope. Formula: sqrt(1 + (pitch/12)^2). For 6/12: sqrt(1.25) = 1.118.
Common rafter multipliers by pitch? ▾
4/12: 1.054. 5/12: 1.083. 6/12: 1.118. 7/12: 1.158. 8/12: 1.202. 10/12: 1.302. 12/12: 1.414.
What is the bird's mouth cut? ▾
A triangular notch cut near the bottom of the rafter that allows it to sit flat on the wall top plate. The horizontal cut (seat cut) is typically 3.5 inches wide. The depth should not exceed 1/3 of the rafter depth.
What is the difference between common, hip, and jack rafters? ▾
Common rafter: runs from ridge to plate perpendicular to the ridge. Hip rafter: runs from corner of building to ridge at 45 degrees. Jack rafter: shorter rafters that run from hip/valley to plate.
How far apart should rafters be spaced? ▾
Most residential roofs use 16-inch or 24-inch on-center spacing. 16 OC is more common in snow country. 24 OC is common in mild climates with lighter roof loads.