How to use this roofing calculator
- Measure the outside length and width of the house (the footprint).
- Enter the eave overhang; 12 inches is common.
- Pick the roof pitch. A 6/12 roof rises 6 inches for every 12 inches of horizontal run.
- Use 10% waste for simple gable roofs and 15% or more for hips, valleys and dormers.
Example
A 50 × 30 ft house with 12 inch overhangs and a 6/12 roof has 2,046 sq ft (190.1 m²) of roof including 10% waste. That is 20.5 squares, or 62 bundles of shingles.
What to know before you buy
Understanding roof pitch
Pitch is the rise in inches for every 12 inches of horizontal run. Low-slope roofs (below 4/12) need special underlayment or different materials, as standard shingles are generally not allowed below 2/12. Roofs of 8/12 and steeper are harder and more dangerous to walk on, which raises labor costs.
What else goes on a roof
Besides shingles, a re-roof usually needs underlayment, drip edge, starter strips, ridge cap, ice-and-water shield in valleys and at eaves in cold climates, flashing around chimneys and vents, and roofing nails (about 320 per square).
Roof squares for a 40 × 30 ft house (12 in overhang, 10% waste)
| Roof pitch | Roofing squares | Shingle bundles (3 per square) | Pitch multiplier |
|---|---|---|---|
| 3/12 | 15.2 squares | 46 | 1.031 |
| 4/12 | 15.6 squares | 47 | 1.054 |
| 6/12 | 16.5 squares | 50 | 1.118 |
| 8/12 | 17.8 squares | 54 | 1.202 |
| 10/12 | 19.2 squares | 58 | 1.302 |
| 12/12 | 20.9 squares | 63 | 1.414 |
Formula
Roof area = (length + 2 × overhang) × (width + 2 × overhang) × √(1 + (pitch ÷ 12)²). One roofing square = 100 sq ft; most shingles come 3 bundles per square.
Tips from the pros
- Buy starter strips for the eaves and rakes and ridge cap shingles separately; they are measured in linear feet.
- Use 15% waste for roofs with hips, valleys and dormers.
- Check local code for ice-and-water shield at the eaves in cold climates.
Common mistakes to avoid
- Using the house's floor area instead of the sloped roof area: a 6/12 roof is about 12% larger than its footprint.
- Forgetting the overhangs, which can add 10% or more to small roofs.
Frequently asked questions
What is a roofing square?
A square is 100 square feet of roof surface. Roofers price materials and labor per square.
How many bundles of shingles per square?
Most 3-tab and architectural shingles need 3 bundles per square; some heavy designer shingles need 4 or 5.
How do I find my roof pitch?
Hold a level horizontally against the rafter or roof, measure 12 inches along it and measure the vertical distance to the roof. That number is the rise per 12.
How many squares is a 2,000 sq ft house roof?
It depends on the footprint and pitch. A single-story 2,000 sq ft house with a 6/12 roof and overhangs is typically about 25–27 squares including waste.
How many nails per square of shingles?
About 320 nails per square with 4 nails per shingle, or more in high-wind areas.