# Roof Length Calculator

Roof length calculator finds roof slope length, rafter length, rise, and total roof area from building span and pitch. Replaces the traditional roof rafter length table.

## What this calculates

The roof length calculator determines the straight-line distance from eave to ridge for any pitched roof. This number is what you need when ordering metal roof panels, underlayment rolls, or shingle courses by run. It also returns total rafter length (including overhang), ridge height above the wall plate, and full roof surface area in both square feet and roofing squares.

## Inputs

- **Building Span (width)** (ft) — min 0 — Width of the building from eave to eave (across the roof)
- **Building Length (long dimension)** (ft) — min 0 — Length along the ridge (used for roof area)
- **Roof Pitch** (:12) — min 0, max 24 — Rise per 12 inches of run (6:12 pitch = 6 rise, 12 run)
- **Eave Overhang** (in) — min 0 — Horizontal eave overhang past the wall (typical 12-24 in)

## Outputs

- **Roof Slope Length (eave to ridge)** (ft) — Straight-line distance up the roof from eave to ridge
- **Rafter Length (incl. overhang)** (ft) — Total rafter length including overhang
- **Rafter Length (ft / in)** — formatted as text
- **Rise (ridge height above plate)** (ft)
- **Roof Surface Area (both sides)** (sq ft)
- **Roof Squares** — 1 roofing square = 100 sq ft

## Details

## What Is Roof Length?

Roof length has two meanings in residential construction:

1. **Roof slope length:** the straight-line distance from eave to ridge measured up the slope of the roof. This is the number you order metal panels and underlayment by.
2. **Rafter length:** the full length of a rafter from ridge to the tip of the overhang. This is what you cut lumber to.

This roof length calculator returns both.

## Roof Length Formula

Roof length uses the Pythagorean theorem applied to the roof triangle:

**Slope length = sqrt(run^2 + rise^2)**

Where:
- **Run** = half the building span (half the width wall to wall)
- **Rise** = run x pitch/12 (for a 6:12 pitch, rise = run x 0.5)

For a 32 ft wide building with a 6:12 pitch:
- Run = 32 / 2 = 16 ft
- Rise = 16 x (6/12) = 8 ft
- Slope length = sqrt(16^2 + 8^2) = sqrt(256 + 64) = sqrt(320) = **17.89 ft**

## Roof Rafter Length Calculator

The **roof rafter length calculator** adds the overhang to the slope length. For a 12-inch eave overhang on a 6:12 pitch:
- Overhang run = 1 ft
- Overhang rise = 0.5 ft
- Overhang slope = sqrt(1 + 0.25) = 1.118 ft

Total rafter length = 17.89 + 1.12 = **19.01 ft** or roughly 19 ft 0 in.

## Roof Rafter Length Table (Quick Reference)

This table replaces the old printed roof rafter length table found in framing guides. Slope length per foot of run:

| Pitch | Slope 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 |
| 7:12 | 1.158 ft |
| 8:12 | 1.202 ft |
| 9:12 | 1.250 ft |
| 10:12 | 1.302 ft |
| 12:12 | 1.414 ft |

To find rafter length: multiply run (half the span) in feet by the factor for your pitch. Example: 16 ft run at 8:12 pitch = 16 x 1.202 = 19.23 ft.

## Roof Surface Area

For a simple gable roof, roof area = 2 x (rafter length x building length). Roofing materials are sold by the "square" (100 sq ft), so divide by 100 to get squares.

Example: 32 ft x 40 ft building with 6:12 pitch and 12 in overhang:
- Rafter length = 19.01 ft
- Roof area = 2 x 19.01 x 40 = 1,521 sq ft
- Roofing squares = 15.21

Add 10 percent for waste on standard shingles, 15 percent for architectural patterns, and 20 percent on complex roofs with valleys.

## Using Roof Length for Metal Panels

Metal roof panels come in lengths up to 40 ft. Measure roof slope length first. If your slope length is 19 ft, order 19 ft panels cut to length, or 20 ft panels and trim to fit. Never splice a single slope run with multiple shorter panels unless using a floating seam system.

## Frequently Asked Questions

**Q: How does a roof length calculator work?**

A: A roof length calculator uses the Pythagorean theorem: slope length = sqrt(run² + rise²). Run is half the building span. Rise is run multiplied by pitch/12. For a 32 ft building at 6:12 pitch: run 16, rise 8, slope length 17.89 ft. Add the overhang's slope length for total rafter length.

**Q: What is the roof rafter length calculator input?**

A: A roof rafter length calculator needs four inputs: building span (wall-to-wall width), roof pitch (as a rise:12 ratio), eave overhang (usually 12-24 inches), and building length (for total roof area). Some calculators also accept ridge board thickness for precise rafter cut-to-length.

**Q: Can I use a roof rafter length table instead?**

A: Yes, a roof rafter length table gives slope length per foot of run at each pitch (3:12 = 1.031 ft, 6:12 = 1.118, 12:12 = 1.414). Multiply your run in feet by the factor for your pitch to get slope length. A calculator is faster and handles any pitch, plus overhang, without interpolation.

**Q: What pitch gives the steepest roof?**

A: Residential roofs usually range from 3:12 (low slope) to 12:12 (45 degrees, steep). Anything above 9:12 is considered steep and needs special safety harnesses for roofers. A 12:12 roof has slope length 1.414 x run, so a 16 ft run gives a 22.6 ft rafter, versus 17.89 ft at 6:12.

**Q: How do I calculate roof area from roof length?**

A: For a simple gable roof: area = 2 x (rafter length x building length). Example: 32 x 40 ft building at 6:12 pitch with 12 in overhang gives rafter length 19 ft, so area = 2 x 19 x 40 = 1,520 sq ft or 15.2 roofing squares. Add 10-20% for waste and trim.

**Q: Is roof length the same as rafter length?**

A: No. Roof slope length is eave to ridge only. Rafter length adds the overhang past the wall. For a 32 ft building at 6:12 pitch with 12 in overhang: slope length = 17.89 ft, rafter length = 19.01 ft. Use slope length for ordering metal panels or shingle coverage. Use rafter length for cutting lumber.

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