The biggest error in roof estimating is using the plan area. A roof is sloped, so its true surface area is always bigger than its footprint, and the steeper the pitch, the bigger the difference. On a steep roof, pricing the footprint can leave you a third short on materials. Start with the right area and the rest of the takeoff follows.
The pitch factor
Pitch factor = √(1 + (rise ÷ run)²). Multiply the plan area, including the overhangs, by this factor to get the true roof area.
| Pitch | Degrees | Factor |
|---|---|---|
| 4/12 | 18.4° | 1.054 |
| 6/12 | 26.6° | 1.118 |
| 8/12 | 33.7° | 1.202 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45° | 1.414 |
For a hip roof where every slope has the same pitch, the same factor applies. A hip roof over a given footprint has the same area as a gable. Only the edges differ: hips instead of rakes. Roofs with different pitches on different faces need each face worked out separately.
Measuring the roof
Take the building's length and width from the wall faces, then add the overhang at the eaves and rakes. On complex roofs, split the plan into rectangles, one for each roof plane or wing, and work out each one. Satellite and drone measurement services are a good starting point, but check them against at least one site measurement.
Squares and bundles
Roofers count in squares: 100 sq ft of roof. Most shingles come three bundles to a square, though some heavy designer shingles are four or five. Add 10% waste for simple gables and 15% or more for hips, valleys and dormers, where every edge course is cut.
The edges
Measure eaves, rakes, ridge, hips and valleys separately. They drive many material lines:
- Starter strip along the eaves and rakes
- Drip edge
- Hip and ridge cap
- Ice and water shield at eaves and valleys
- Ventilation at eaves and ridge
- Flashings at walls, chimneys and roof windows
Hip length = (half the building width) × hip factor, where hip factor = √(2 + (rise ÷ run)²). Valleys use the same factor.
Underlayment
Synthetic underlayment comes in rolls covering about 10 squares; #15 felt covers about 4 squares per roll. Below 4/12 most shingle makers require double coverage, and the building code requires eave protection (ice and water membrane) from the roof edge to at least 900 mm up the slope and 300 mm inside the inner face of the wall in most of Canada.
Fasteners and sundries
Roofing nails at about 320 per square (four per shingle), plus cap nails or staples for underlayment, sealant and pipe boots.
Check the minimum pitch
Asphalt shingles need at least 2/12. Below that, use a metal or membrane system. Below the minimum the roof will leak no matter how well it's laid.
Don't forget the rest of the job
- Tear-off and disposal of the old roof (price by square, and by layer)
- Decking repairs, priced as an allowance
- Scaffolding or edge protection
- Gutters, if they're being replaced
- Dumpsters
The roofing calculator works all of this out for gable, hip and mono-pitch roofs, and the roof pitch calculator converts between every pitch format.
A worked example
A 40 × 28 ft gable house with 12" overhangs and a 6/12 pitch: plan area 42 × 30 = 1,260 sq ft. True roof area 1,260 × 1.118 = 1,409 sq ft = 14.1 squares. With 10% waste, 15.5 squares, or 47 bundles of shingles. Eaves 84 ft, rakes about 67 ft and ridge 42 ft drive the starter, drip edge and ridge cap lines.
Pricing checklist
- Area from the plan × pitch factor, not from the footprint
- Waste set for the roof's complexity
- Every edge measured and its materials listed
- Underlay, ventilation and flashings included
- Tear-off, disposal and repairs priced or excluded in writing
- Access, edge protection and skips included
Reading the roof on site
Before you price, walk the roof or inspect it from a ladder or drone. Note anything that adds work: soft or sagging areas, which point to rotten decking; old layers that must come off; chimneys that need new flashing or repointing; skylights and vents; and how close the roof is to power lines, trees and neighbours. Photograph everything. The photos back up your price, and they help if there's a dispute later about what was there before you started.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.