How the estimate works
Start with the area the roof covers seen from above, overhangs included. A sloped roof has more surface than its plan, so multiply by the pitch factor. Divide by 100 to get squares, the unit shingles are sold in, then add waste for cuts. Starter, ridge cap and drip edge are counted from the lengths of the edges they run along, not from the area.
Formula
- Pitch factor
- √(12² + rise²) ÷ 12
- Roof area
- plan area × pitch factor
- Squares
- roof area ÷ 100 × (1 + waste)
- Bundles
- ⌈squares × bundles per square⌉
- Starter
- ⌈(eaves + rakes) ÷ lf per bundle⌉
- Ridge cap
- ⌈(ridges + hips) ÷ lf per bundle⌉
- Drip edge
- ⌈(eaves + rakes) ÷ (piece − 2″ lap)⌉
⌈ ⌉ means round up to the next whole bundle, roll or piece.
Worked example
A 40 × 28 ft house with 12″ overhangs all round covers 42.0 × 30.0 = 1,260 sq ft in plan. At 6/12 the pitch factor is 1.118, so the roof itself is 1,408.7 sq ft, or 14.09 squares. Add 10% waste for 15.50 squares, and at 3 bundles a square you buy 47 bundles.
The eaves are 84.0 ft and the four rakes 67.1 ft, so drip edge covers 151.1 ft: 16 ten-foot pieces once each joint laps 2″. The ice barrier has to reach 24″ inside the wall: 12″ of overhang plus 24″, measured level, is 3′-4 1/4″ up the slope, so it takes 2 courses of 36″ membrane along the eaves.
Measuring plane by plane
Hip roofs, L-shapes and roofs with dormers don't fit a simple footprint. Measure each roof plane on the roof surface instead: its width along the eave, its width along the top if it narrows to a shorter ridge, and its length up the slope. Rectangles cover gable sides, trapezoids the long sides of a hip roof, and triangles the hip ends. Then add up the edges: eaves and rakes for starter and drip edge, ridges and hips for cap shingles, valleys for the valley liner.
Assumptions
- The footprint mode is a simple gable: two equal planes, no dormers, valleys or skylights.
- Waste goes on the field shingles only, 10% for a gable and 15% plane by plane. Coverage figures are typical of the major shingle makers; check the wrapper or spec sheet for yours.
- Underlayment covers the whole roof; the per-roll figure is after side laps. From 2/12 up to 4/12 the IRC calls for two layers, so the roll count doubles. Where wind design is required, the 2021 IRC calls for two layers at any slope.
- Ice & water barrier comes in 36″ rolls lapped 3″ at the sides and 6″ at the ends. At 8/12 and steeper it also has to run at least 36″ up the slope. Valleys get one 36″ strip centred on the valley.
- Starter runs along the eaves and, by default, the rakes. Drip edge runs along both.
The IRC requires an ice barrier where the local building department has marked a history of ice damming. If you're not sure, ask them; you can turn the ice barrier off in the assumptions.
References
- ICC, International Residential Code 2021: R905.1.1 and Table R905.1.1(2) underlayment, R905.1.2 ice barriers, R905.2.2 slope, R905.2.8.2 valleys, R905.2.8.5 drip edge.
- CertainTeed, Master Craftsman Shingle Applicator's Manual, chapter 3, estimating.
- Data sheets and installation instructions from GAF, Owens Corning, CertainTeed, IKO, TAMKO and Malarkey for bundle, starter, ridge cap, underlayment and ice barrier coverage. The instructions for the shingle you buy govern starter, nailing and exposure.