A 40 × 30 ft house with 12 in overhangs and a 6/12 gable roof has 1,503 sq ft of roof, 15.03 squares. With 10% waste that is 16.53 squares, or 50 bundles of architectural shingles at 3 bundles per square, plus 2 rolls of synthetic underlayment, 4 rolls of ice and water membrane, 1 bundle of starter, 2 bundles of ridge cap and 16 pieces of drip edge. Change the house, the pitch or the products and the list follows.
How roof square footage is calculated
Measure the house on the ground, where it is safe, and add the overhangs: the eave overhang on the two long sides and the rake overhang on the gable ends. That gives the plan area, the roof as seen from above. Multiply by the pitch multiplier to get the sloped area, then divide by 100 for roofing squares. The multiplier works for any gable, hip or shed roof where every plane has the same pitch.
Roof area = (length + 2 × rake overhang) × (width + 2 × eave overhang) × √(1 + (pitch ÷ 12)²)
For the default house: (40 + 2) × (30 + 2) = 1,344 sq ft of plan, × 1.1180 at 6/12 = 1,502.6 sq ft, which is 15.03 squares. The roof pitch calculator gives the multiplier for any pitch if you want to check it by hand.
If your roof has dormers, a garage wing or planes at different pitches, switch to "Roof planes" and enter each rectangle as measured on the roof: its width along the eave and its length from eave to ridge. Count a triangular hip end as half its eave length.
Waste by roof type
Shingles are cut at every rake, hip, valley and wall, and the offcuts rarely fit elsewhere. The calculator starts at 10% for a gable or shed roof and switches to 15% when you pick a hip roof; raise it further for a roof with several valleys or dormers. On the same 40 × 30 house a hip roof has the same 15.03 squares but needs 52 bundles at 15%, and its ridge and four hips add up to 106 ft, which is 4 bundles of ridge cap instead of 2.
What goes on the roof besides shingles
Underlayment. Every asphalt shingle roof gets underlayment (IRC R905.1.1). From 4/12 up it is one layer lapped 2 inches; from 2/12 up to 4/12 the code asks for two layers, starting with a 19 in strip and 36 in sheets lapped 19 in, so the calculator doubles the rolls. At 3/12 the same house needs 4 rolls of synthetic instead of 2. TAMKO's Synthetic Guard roll covers 905.67 sq ft after laps; its No. 30 felt roll covers 204 sq ft.
Ice barrier. Where your building department's climate table shows a history of ice damming, the IRC (R905.1.2) requires an ice barrier from the eave edge to at least 24 inches inside the exterior wall, and on 8/12 and steeper also at least 36 inches up the slope. With a 12 in overhang at 6/12 that is (12 + 24) × 1.118 = 40¼ in up the slope, just over one 39⅜ in course of TAMKO TW, so it takes two courses along 84 ft of eave: 4 rolls. At 9/12 it is 45 in.
Drip edge. Required at eaves and rakes of a shingle roof, with pieces lapped at least 2 inches and nailed every 12 inches (R905.2.8.5). Enter the piece length your supplier stocks.
Starter and ridge cap. Starter runs along the eaves, and along the rakes if your shingle's wind warranty asks for it. TAMKO's Shingle Starter covers 102 ft a bundle, CertainTeed SwiftStart 116 ft. Hip and ridge cap covers about 33⅓ ft a bundle: TAMKO lists 3 bundles per 100 ft for its 12¼ × 12 in cap, and three-tab shingles cut into three 12 in pieces at a 5 in exposure cover the same.
Nails. At least four per strip shingle (R905.2.6), 12 gauge with a ⅜ in head, long enough to go ¾ in into the deck or through it (R905.2.5). Architectural shingles run 64 to a square, so 16.53 squares take 1,058 shingles and 4,232 nails.
| Item | Default house (6/12 gable) | Rule used |
|---|---|---|
| Shingles | 50 bundles | 16.53 squares × 3 |
| Underlayment | 2 rolls | 1,503 sq ft × 1.1 ÷ 906 |
| Ice and water membrane | 4 rolls | 2 courses × 84 ft |
| Starter | 1 bundle | 84 ft ÷ 102 ft |
| Ridge cap | 2 bundles | 42 ft ÷ 33⅓ ft |
| Drip edge | 16 pieces | 155.6 ft ÷ 9 ft 10 in |
Calculators for each part of the job
- Not sure of the pitch? Measure it with the roof pitch calculator.
- Already know the squares? The shingle calculator compares 3-tab, architectural and designer bundles and nails.
- Going with steel: the metal roof calculator lays out panels, screws, trim and closures.
- New framing: size and cut rafters in the rafter calculator, or count trusses in the roof truss calculator.
- Hanging new gutters with the roof: the gutter slope calculator marks every hanger.
- Pricing it: the roof replacement cost calculator puts a range on the job for your state, and the dumpster size calculator sizes the container for the tear-off.