Roofing is sold by the "square" — 100 square feet — and every miscalculation is a round trip to the supply house plus a half day of lost crew time. Getting this right before you place the order is worth the extra hour. Here's how to measure a roof from the ground, from the attic, or up on it.
The "square" in roofing
One roofing square = 100 sq ft. A standard bundle of 3-tab shingles covers ~33.3 sq ft (3 bundles per square). Architectural/dimensional shingles vary — always check the bundle coverage printed on the wrapper.
Method 1: Measure from the ground (footprint method)
For homes without complex features, measure the footprint of the house, then multiply by the pitch factor.
- Measure the length and width of the house footprint (include overhangs)
- Multiply to get footprint area
- Multiply by the pitch factor from the table below
Pitch multipliers
| Roof pitch | Angle | Multiplier |
|---|---|---|
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 7/12 | 30.3° | 1.158 |
| 8/12 | 33.7° | 1.202 |
| 9/12 | 36.9° | 1.250 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45° | 1.414 |
Example: 40 × 30 ft rectangular house, 6/12 pitch.
- Footprint: 1,200 sq ft
- Roof area: 1,200 × 1.118 = 1,342 sq ft
- Squares: 1,342 ÷ 100 = 13.4 squares
Method 2: Measure on the roof
If you can safely walk the roof, measure each plane (each side of the roof) individually:
- Measure length from ridge to eave
- Measure horizontal length along the ridge
- Multiply for each plane
- Sum all planes
This is the most accurate method for hip roofs, valleys, and dormers where the footprint math gets messy.
Determining pitch
Pitch is rise over run in inches per 12 inches. From inside the attic:
- Place a level against a rafter, pointing horizontal
- Mark 12 inches from the bottom of the rafter along the level
- Measure vertically from that mark to the rafter
- That vertical measurement = pitch (e.g., 6 inches = 6/12 pitch)
From outside: hold a level against a gable and measure rise 12" along the horizontal.
Waste factor
- 10% — simple gable, no valleys
- 15% — standard for hip roofs, dormers (default)
- 20% — multiple valleys, complex geometry, architectural shingles
- 5% minimum — leftover pieces for future repairs and wind damage
Materials you'll need per square
| Material | Per square |
|---|---|
| 3-tab shingles | 3 bundles |
| Architectural shingles | 3–4 bundles (varies) |
| 15 lb felt underlayment | 1/4 roll (400 sq ft/roll) |
| Synthetic underlayment | 1/10 roll (1,000 sq ft/roll) |
| Starter strip | Linear ft of eaves |
| Ridge cap shingles | Linear ft of ridges + hips |
| Drip edge | Linear ft of eaves + rakes |
| Ice & water shield | First 2 feet of eaves (cold climates) |
| Roofing nails (1-1/4" or 1-3/4") | ~320 nails per square |
Ventilation and flashing
Don't forget:
- Ridge vent: linear feet of ridge (typically 20–40 ft)
- Soffit vent: linear feet of eaves
- Plumbing boot flashings: one per vent pipe
- Step flashing: around chimneys, walls (~10 pieces)
- Valley metal: linear feet of all valleys
Complete roofing estimate
For our 13.4-square example roof:
- Roof area: 1,342 sq ft
- With 15% waste: 1,543 sq ft = 15.43 squares
- Order 16 squares of shingles (~48 bundles architectural)
- Underlayment: 4 rolls of 15 lb felt (or 1-2 rolls synthetic)
- Starter strip: ~80 linear ft
- Ridge cap: ~40 linear ft
- Drip edge: ~140 linear ft
Handling complex roof shapes
Real roofs rarely are simple rectangles. For accurate measurement on complex shapes:
- L-shaped or T-shaped homes: break the footprint into two rectangles, compute each, then add. Don't forget the inside corner where the two wings meet — it creates a valley that needs valley metal.
- Hip roofs: the pitch multiplier method on the full footprint works well since hip roofs don't have gable overhangs.
- Dormers: measure each face of each dormer separately and add to the main roof. Small shed dormers add 20–40 sq ft; large gabled dormers 60–120 sq ft.
- Porches and attached sheds: measure separately. A wrap-around porch can add 300+ sq ft of low-pitch roof that's often a different material.
Safety when measuring
Most accidents happen on perfectly normal single-story roofs at 6/12 pitch. If you go up:
- Use a proper extension ladder extending 3 ft above the eave
- Wear soft-soled shoes (not boots) for grip
- Never measure in wet or frosty conditions
- Consider a roof harness anchor for 8/12 pitch or steeper — or don't go up at all
Modern alternative: many roofers use satellite measurement services (EagleView, RoofSnap) that deliver a full measurement report from aerial imagery for $20–$40. For a one-off homeowner job, that's often worth it vs. climbing up.
Cross-checking with the square-footage shortcut
A quick sanity check: roof area is typically 1.05×–1.40× living square footage, depending on pitch and overhangs. A 2,000 sq ft one-story home with 6/12 pitch should calculate to roughly 2,300–2,400 sq ft of roof. If your measured number is way off this ratio, re-check — you probably missed a plane or got the pitch wrong.
Estimate your roof
Our roofing calculator handles all the math — squares, bundles, underlayment, and trim — from your footprint and pitch. It'll save you a second supply-house trip.