How to Measure a Roof

Safety Checklist (read this first)

• The safest measurement is the one taken from the ground or the attic — the methods below are ordered accordingly.
• If you go up: harness anchored to something structural, dry conditions, soft-soled shoes, a partner on site, and no working a roof steeper than 8/12.
• Ladders: extend 3 ft above the eave, secure the base at a 4:1 angle, and never stand on the top two rungs.

Step 1 — Measure the Footprint (from the ground)

Measure the building's outside walls: length × width, in feet. For simple rectangular homes this is the footprint. Add overhangs if you plan to shingle them — typically 1–1.5 ft per side, so add twice that to each dimension. For L-shaped or T-shaped homes, split the plan into rectangles, measure each, and add the areas.

Step 2 — Find the Pitch (from the attic)

Hold a 12-inch level against a rafter, center the bubble, and measure the vertical rise at the level's end. Rise in inches over 12 inches of run is your pitch — 6 inches means 6/12. Detailed instructions for this and three other methods are in How to Measure Roof Pitch.

Step 3 — Convert Footprint to Roof Surface Area

Multiply the footprint by the pitch multiplier — √(1 + (rise/run)²) — to get the true sloped surface area. Common multipliers: 4/12 → 1.054, 6/12 → 1.118, 8/12 → 1.202, 12/12 → 1.414. The Roof Area Calculator does this automatically if you enter length, width and pitch.

Step 4 — Convert to Roofing Squares and Order

Roofing material is priced by the square (100 sq ft of roof area). Divide surface area by 100, then add a waste allowance (10% for simple gable roofs, 15% for hips and valleys, more for cut-up roofs) and round up to whole packages. The Roofing Squares Calculator handles the waste math, and the Roofing Material Calculator turns the result into a full order list.

Worked Example

1. Ground measurement: 50 ft × 30 ft walls, 1 ft overhang on all sides.
2. Footprint with overhang: 52 × 32 = 1,664 sq ft.
3. Attic pitch measurement: 6 in rise per 12 in run → 6/12, multiplier 1.118.
4. Roof surface area: 1,664 × 1.118 ≈ 1,860 sq ft (18.6 squares).
5. With 10% waste: 20.5 squares → order 21 squares (≈ 62 bundles of 3-bundle shingles).

When to Hire a Professional Measurer

Steep or multi-story roofs, complex hip-and-valley geometry, or homes where the quote depends on precise measurements — a roofing contractor will measure for free as part of an estimate, and drone measurement services produce detailed reports without anyone climbing. Use your own measurements to sanity-check quotes, not necessarily to place the material order.