What the roofing calculator does
A roof's surface is larger than the footprint it covers, because it slopes. Enter the building's footprint length and width and the roof pitch, and the calculator returns the actual roof area — and the number of roofing squares (100 ft² each), which is how shingles and roofing labour are quoted in North America. Use it for a materials estimate, to sanity-check a contractor's quote, or to size gutters and underlay. Runs in your browser.
The calculation
Pitch factor = √(1 + (rise ÷ run)²) for a pitch given as rise per 12 in of run
Roof area = footprint area × pitch factor
Squares = roof area ÷ 100 ft²
Footprint 40 × 30 ft, pitch 6:12:
factor = √(1 + (6/12)²) = √1.25 = 1.118
area = 1,200 ft² × 1.118 = 1,342 ft² → 13.4 squares, order 15 with wasteThe footprint should include the overhangs (eaves and rakes) — measure to the outside of the fascia, not the walls. A 1 ft overhang on a 40 × 30 house adds about 150 ft² of roof.
Pitch factors
| Pitch (rise:12) | Angle | Factor | Notes |
|---|---|---|---|
| 2:12 | 9.5° | 1.014 | Low slope — needs membrane or special shingles |
| 3:12 | 14° | 1.031 | Minimum for standard shingles with extra underlay |
| 4:12 | 18.4° | 1.054 | Common on ranch-style homes |
| 5:12 | 22.6° | 1.083 | |
| 6:12 | 26.6° | 1.118 | Most common residential pitch |
| 8:12 | 33.7° | 1.202 | Walkable with care |
| 10:12 | 39.8° | 1.302 | Steep — staging required |
| 12:12 | 45° | 1.414 | Very steep; Victorian, chalet styles |
In degrees: factor = 1 ÷ cos(angle). A 30° roof (about 7:12) has a factor of 1.155.
From area to materials
- Shingles: three bundles per square for standard three-tab and most architectural shingles. Add 10% waste for a simple gable roof, 15% for hips and valleys, and cap shingles for ridges and hips (about one bundle per 35 ft).
- Underlay: synthetic rolls cover 10 squares; felt 2–4 squares, depending on weight.
- Metal roofing: panel coverage is by width × length; use the roof area and the rake length to work out panel count.
- Tiles (clay/concrete): the manufacturer gives tiles per m² at the recommended gauge, typically 10–16 for interlocking tiles and 55–60 for plain tiles.
- Nails, starter strip, drip edge, flashing: based on perimeter and penetrations.
Complex roofs
The calculator assumes the whole footprint is under one pitch — a gable or a hip roof of uniform slope. For a roof with several sections at different pitches, dormers or an L-shaped plan, split the footprint into rectangles, calculate each with its own pitch, and add them. Hip roofs have the same area as a gable of the same pitch and footprint; they just need more cuts, hence the higher waste figure.