# Rafter Calculator

Plan a rafter takeoff. Calculates rafter count, total lumber length, board feet, cut angles, and 10% waste. Covers both sides of a gable roof for framing orders.

## What this calculates

This roof rafter calculator plans a complete rafter order for a gable roof. Enter building length, span, pitch, and spacing; the calculator returns rafter count per side, total rafters, lumber length to order, total linear feet with 10% waste, board feet, and the plumb and seat cut angles. For single-rafter length only, use the separate Rafter Length Calculator.

## Inputs

- **Building Length** (ft) — min 0 — Length of the building along the ridge
- **Building Span** (ft) — min 0 — Total span across the building (wall to wall, perpendicular to ridge)
- **Roof Pitch** (:12) — min 0, max 24 — Rise per 12 inches of run (6 = 6:12 pitch)
- **Rafter Spacing** — options: 12" OC, 16" OC (standard), 24" OC
- **Eave Overhang** (in) — min 0 — Horizontal overhang past the wall (typical: 12-24 in)
- **Lumber Length** — options: Auto (nearest standard size), 10 ft, 12 ft, 14 ft, 16 ft, 20 ft
- **Rafter Lumber Size** — options: 2x6, 2x8, 2x10, 2x12 — Used for the board feet estimate

## Outputs

- **Rafters per Side**
- **Total Rafters (Both Sides)**
- **Rafter Length** (ft) — Total rafter length including overhang
- **Lumber Length to Order** (ft) — Nearest standard lumber length that accommodates the rafter
- **Total Linear Feet (with 10% waste)** (lin ft)
- **Total Board Feet** (BF)
- **Plumb Cut Angle** (°)
- **Seat Cut Angle** (°)

## Details

## How Rafter Counts Are Calculated

**Rafters per side** = ceil(building length / rafter spacing in feet) + 1 (for the starting rafter).

For a 40-foot building at 16-inch spacing: 40 / 1.333 = 30, plus 1 = **31 rafters per side**. A gable roof needs these on both sides, so **total rafters = 62**.

## Rafter Length Formula

Each rafter is the hypotenuse of a right triangle:

**Rafter length = sqrt(run² + rise²) + overhang rafter**

Where:
- **Run** = half the building span
- **Rise** = run x (pitch / 12)
- **Overhang rafter** is the extra length past the wall, calculated the same way

For a 28-foot span with a 6:12 pitch and 12-inch overhang:
- Run = 14 ft, Rise = 14 x (6/12) = 7 ft
- Main rafter = sqrt(14² + 7²) = sqrt(245) = 15.65 ft
- Overhang rafter = sqrt(1² + 0.5²) = 1.12 ft
- **Total rafter length = 16.77 ft, order 18 ft lumber**

## Rafter Length Table (common pitches, per foot of run)

| Pitch | Length per foot of run |
|-------|------------------------|
| 3:12 | 1.031 ft |
| 4:12 | 1.054 ft |
| 5:12 | 1.083 ft |
| 6:12 | 1.118 ft |
| 8:12 | 1.202 ft |
| 10:12 | 1.302 ft |
| 12:12 | 1.414 ft |

Multiply run (in feet) by this factor to get rafter length (main rafter only, add overhang separately).

## Lumber Quantity and Board Feet

The rafter calculator rounds to standard lumber lengths (8, 10, 12, 14, 16, 20 ft) and applies a 10% waste factor to cover miscuts, knots, and breakage.

Board feet per linear foot of common rafter sizes:

- 2x6 = 1.0 BF/ft
- 2x8 = 1.33 BF/ft
- 2x10 = 1.67 BF/ft
- 2x12 = 2.0 BF/ft

## Cut Angles

**Plumb cut angle** (vertical cuts at the ridge and rafter tail) equals the roof angle: arctan(pitch / 12). For a 6:12 pitch, that's 26.57°.

**Seat cut angle** (horizontal cut of the birdsmouth) is the complement: 90° - plumb angle. For a 6:12 pitch, the seat cut is 63.43°.

## When to Use This Calculator vs Rafter Length Calculator

Use this **rafter calculator** when you need a complete lumber takeoff for a roofing order (count, total linear feet, board feet). Use the **rafter length calculator** when you need the precise length of a single rafter including ridge board adjustment for actual cutting.

## Frequently Asked Questions

**Q: How do I calculate the number of rafters I need?**

A: Divide the building length in feet by the rafter spacing in feet (16" = 1.33 ft), round up, and add 1 for the starting rafter. Then double for both sides of a gable roof. A 40-foot building at 16" OC needs 31 rafters per side, or 62 total.

**Q: What length rafter do I need for a 24-foot span?**

A: With a 6:12 pitch and 12" overhang, a 24-foot span needs about 14.5-foot rafters (12 ft run + 6 ft rise + 1 ft overhang rafter). Order 16-foot lumber and trim. With an 8:12 pitch, rafter length grows to about 15.4 feet so you would still order 16-foot stock.

**Q: What is the rafter length for a 12:12 pitch?**

A: A 12:12 pitch has rafter length of 1.414 feet per foot of run. For a 28-foot span (14-foot run): 14 x 1.414 = 19.8 feet. Add overhang (about 1.4 ft for a 12" horizontal overhang on a 12:12) to get 21.2 feet total. Order 24-foot lumber.

**Q: How much waste should I add for rafters?**

A: 10% is standard for straight-cut rafters on a simple gable. For complex roofs with hips, valleys, or dormers, use 15-20%. For very small jobs with expensive lumber, measuring each rafter individually and using 5% waste can save money.

**Q: What is the plumb cut angle for a 6:12 roof?**

A: The plumb cut angle for a 6:12 pitch is 26.57°, measured from horizontal. The seat cut is the complement: 63.43°. For framing, set your speed square to 6 on the common scale (or 26.5 degrees) for the plumb cut at the ridge and tail.

**Q: How many board feet are in a typical rafter?**

A: A 16-foot 2x8 rafter has 1.33 BF per linear foot x 16 ft = 21.3 board feet. A 16-foot 2x10 has 26.7 BF. A 16-foot 2x12 has 32 BF. Multiply by total rafter count and add 10% waste to get the total board feet for your order.

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Source: https://vastcalc.com/calculators/construction/rafter
Category: Construction
Last updated: 2026-04-08