# Rafter Length Calculator

Calculate rafter length for roof framing from building span, pitch, and overhang. Get ridge height, roof area, plumb cut angle, and seat cut angle.

## What this calculates

Accurate rafter length is fundamental to roof framing. Too short and the rafter won't reach the ridge; too long means waste and recutting. This calculator determines the total rafter length from building span, roof pitch, and eave overhang, accounting for the ridge board thickness.

## Inputs

- **Building Span** (ft) — min 0 — Total width of the building from wall to wall
- **Roof Pitch** (:12) — min 0 — Pitch as rise per 12 inches of run (e.g., 6 for a 6:12 pitch)
- **Eave Overhang** (in) — min 0 — Horizontal distance the rafter extends past the wall (typical: 12-24 inches / 30-60 cm)
- **Ridge Board Thickness** (in) — min 0 — Thickness of the ridge board (standard 2x = 1.5 inches)

## Outputs

- **Rafter Length** (ft) — Total rafter length including overhang (order this length)
- **Rafter Length** — formatted as text — Rafter length in friendly format
- **Ridge Height** (ft) — Height of the ridge above the top plate
- **Roof Area per Side** (sq ft/ft) — Roof surface area per linear foot of building length (one side)
- **Plumb Cut Angle** (°) — Angle for plumb cuts (ridge and tail) from vertical
- **Seat Cut Angle** (°) — Angle for the seat (birdsmouth) cut from horizontal

## Details

The rafter forms the hypotenuse of a right triangle where the run (horizontal distance from wall to ridge) and rise (vertical height gained) are the two legs. The formula is: Rafter Length = √(run² + rise²) + overhang rafter.

The run is half the building span minus half the ridge board thickness (the ridge board sits between opposing rafters). The rise equals the run multiplied by the pitch factor (pitch ÷ 12). The overhang extends the rafter past the wall, calculated the same way using the overhang distance as additional run.

The calculator also provides the plumb cut angle (for cuts at the ridge and tail) and the seat cut angle (for the birdsmouth where the rafter sits on the wall plate). These angles are complements; they add up to 90 degrees. A speed square or framing square set to the pitch ratio produces these same angles.

## Frequently Asked Questions

**Q: How do I account for the ridge board in rafter length?**

A: The ridge board sits between opposing rafters, so each rafter's run is shortened by half the ridge board thickness. For a standard 2x ridge board (1.5" actual), subtract 0.75 inches from each rafter's run. This calculator handles this automatically when you enter the ridge board thickness.

**Q: What size lumber should I use for rafters?**

A: Rafter size depends on span, spacing, load, and species. Common sizes: 2x6 for spans up to 10 ft, 2x8 for spans up to 14 ft, 2x10 for spans up to 18 ft, 2x12 for spans up to 22 ft (at 16" on-center with 30 PSF live load). Always check span tables for your specific species, grade, and loading conditions.

**Q: What is a birdsmouth cut?**

A: A birdsmouth is a notch cut into the bottom of the rafter where it sits on the wall's top plate. It consists of a plumb cut (vertical) and a seat cut (horizontal). The seat cut should be at least 1.5 inches to bear on a 2x4 wall plate. The plumb cut depth should not exceed 1/3 of the rafter depth to maintain structural integrity.

**Q: How much overhang should I include?**

A: Typical eave overhangs are 12-24 inches. A 12-inch overhang provides basic protection. An 18-24 inch overhang better protects siding and windows from rain. In areas with heavy rain or snow, consider 24+ inches. Rake overhangs (gable ends) are typically 6-12 inches. Local building codes may limit maximum overhang.

**Q: How many rafters do I need?**

A: Divide the building length by the rafter spacing (16" or 24" on-center), then add 1 for the starting rafter. For a 30-foot building at 16" OC: (30 x 12 / 16) + 1 = 23.5 → 24 rafters per side. Double this for both sides of a gable roof: 48 rafters. Add extras for hip/valley rafters and waste.

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Source: https://vastcalc.com/calculators/construction/rafter-length
Category: Construction
Last updated: 2026-04-21