A 40 × 28 ft house with a 6/12 gable roof and 12 in overhangs takes 47 bundles of architectural shingles: 1,409 ft² of roof, 15.5 squares with 10 % waste, three bundles a square. On top of the shingles it needs about 4,000 roofing nails, two rolls of synthetic underlayment, two bundles of starter, two of ridge cap and sixteen 10 ft pieces of drip edge. The calculator above builds this list for any gable, hip, shed or gambrel roof.
How the calculator works
Enter the roof shape, the building size, the pitch and the overhangs, choose the covering and the waste. The calculator finds the roof area by the slope factor, turns it into squares and bundles, and adds everything that goes under and around the shingles. Switch the covering to roll roofing or modified bitumen for low slopes.
Bundles and shingles
Both 3-tab and architectural shingles come three bundles to the square of 100 ft². A square holds 80 three-tab shingles (12 × 36 in at 5 in exposure) or about 65 laminated ones (13¼ × 39⅜ in at 5⅝ in). Each shingle takes four nails (IRC R905.2.6), six where the manufacturer asks for high-wind nailing.
Underlayment by slope
Asphalt shingles need a slope of at least 2/12 (R905.2.2). From 4/12 up, one layer of underlayment lapped 2 in is enough. Between 2/12 and 4/12 the code asks for two layers: a 19 in starter strip, then full sheets lapped 19 in (R905.1.1). The calculator switches to double coverage on its own, which roughly doubles the underlayment.
Ice barrier
Where ice dams are common, an ice barrier runs from the eave edge to a point at least 24 in inside the exterior wall (R905.1.2). With a 12 in overhang at 6/12 that is 3.35 ft up the slope — two courses of a 36 in roll along every eave. It replaces the underlayment in that strip.
Starter, cap and drip edge
| Part | Where | On the example roof |
|---|---|---|
| Starter | eaves, usually rakes too | 151 ft → 2 bundles |
| Hip and ridge cap | ridge and hips | 42 ft → 2 bundles |
| Drip edge | eaves and rakes (R905.2.8.5) | 151 ft → 16 pieces of 10 ft |
The same house with a hip roof has the same area but needs 15 % waste — 49 bundles — and 102 ft of cap for the ridge and four hips.
Roll roofing and modified bitumen
Below 2/12 shingles are not allowed. Mineral-surfaced roll roofing goes down to 1/12 (R905.5.2) and modified bitumen to ¼/12 (R905.11.1). For rolls the calculator divides the area by what is left of a roll after the side and end laps, multiplies by the layers and the reserve, and adds the adhesive between layers and the primer.
Common mistakes
- Bundles from the floor area. Use the roof area, slope and overhangs included.
- Single underlayment on a low slope. Under 4/12 it must be doubled.
- Forgetting the trim. Starter, cap and drip edge are separate products on most architectural lines, not cut from field shingles.
For the area alone, use the Roofing Calculator; for gutters, the Gutter Calculator.