Rafter Length Calculator
Calculate the rafter length, ridge height, rise, and run for your roof. Enter the building width (span), select the roof pitch, and specify the overhang to get instant results.
Rafter Length Calculator
How to Calculate Rafter Length
- Measure the building span — the full width from the outside of one wall to the outside of the opposite wall, in feet or meters.
- Determine the run: divide the span by 2. For a 30 ft (9.14 m) building, the run is 15 ft (4.57 m).
- Find the rise: multiply the run by the pitch ratio. For a 6/12 pitch: 15 × (6/12) = 7.5 ft (2.29 m).
- Calculate the rafter length: use the Pythagorean theorem: √(run² + rise²). For our example: √(225 + 56.25) = √281.25 = 16.77 ft (5.11 m).
- Add overhang: for a 12″ (30 cm) overhang, add 1 ft (0.30 m): 16.77 + 1 = 17.77 ft (5.42 m). Round up to the next standard lumber length.
Common Rafter Lengths — 30 ft (9.14 m) Span
Rafter lengths for a 30-foot (9.14 m) building width at various pitches (run = 15 ft / 4.57 m, overhang = 12″ / 30 cm):
| Pitch | Rafter Length | Total w/ 12″ Overhang | Ridge Height |
|---|---|---|---|
| 2/12 | 15' 2" | 16' 2" | 2' 6" |
| 3/12 | 15' 6" | 16' 6" | 3' 9" |
| 4/12 | 15' 10" | 16' 10" | 5' 0" |
| 5/12 | 16' 3" | 17' 3" | 6' 3" |
| 6/12 | 16' 9" | 17' 9" | 7' 6" |
| 7/12 | 17' 4" | 18' 4" | 8' 9" |
| 8/12 | 18' 0" | 19' 0" | 10' 0" |
| 9/12 | 18' 9" | 19' 9" | 11' 3" |
| 10/12 | 19' 6" | 20' 6" | 12' 6" |
| 12/12 | 21' 3" | 22' 3" | 15' 0" |
Tip: Always round up to the next standard lumber length (8, 10, 12, 14, 16, 18, or 20 ft) when purchasing rafter material.
Roof Pitch Guide
- Low slope (1/12–3/12): Requires built-up, membrane, or metal roofing. Not suitable for standard asphalt shingles. Common on commercial buildings and modern flat-roof homes.
- Conventional (4/12–9/12): Works with all standard roofing materials including asphalt shingles, metal, tile, and slate. The 6/12 pitch is the most common residential choice.
- Steep (10/12–12/12): Excellent for snow shedding and dramatic aesthetics. Requires additional safety precautions during installation. May need extra fastening per local codes.
Frequently Asked Questions
How do I measure rafter length?
Measure the building width (span) from the outside of one wall to the outside of the opposite wall. Divide by 2 to get the run. Determine your roof pitch (rise per 12 inches of run). Calculate: rise = run × (pitch ÷ 12). Rafter length = √(run² + rise²). Add the overhang (eave) length — typically 12″ to 24″ — for total rafter length.
What roof pitch is most common for residential homes?
The most common residential roof pitch is 6/12 (26.57°), which provides good water drainage, adequate attic space, and works with most roofing materials. Pitches between 4/12 and 9/12 are standard for homes. Low-slope roofs (below 3/12) require special membrane roofing, while steep pitches (above 9/12) add cost but improve snow shedding.
Do I need to add overhang to the rafter length?
Yes. The overhang (also called the eave or soffit projection) extends the rafter beyond the exterior wall plate. Standard overhang is 12″ to 24″. It protects siding, windows, and the foundation from rain. When ordering lumber, always add the overhang to your calculated rafter length, then round up to the next standard lumber length (8, 10, 12, 14, 16, 18, or 20 ft).
Related Calculators
- Roof Pitch Calculator — calculate pitch from rise and run measurements
- Shingle Calculator — estimate shingles and bundles needed for your roof
- Roof Area Calculator — calculate total roof surface area from dimensions
- Board Foot Calculator — price the lumber for your rafters
- 2D Area & Perimeter Calculator — Pythagorean triangle math for rafter runs
- All Construction Calculators
Rafter length is calculated using the Pythagorean theorem: rafter = √(run² + rise²). Run = span ÷ 2. Rise = run × (pitch ÷ 12). These calculations assume a standard symmetrical gable roof. Always consult local building codes and a structural engineer for load-bearing requirements.