How to measure a roof from the ground
You don't need to climb up. Measure the outside walls of the house, add the overhang on each side, and multiply length by width — that's the roof's footprint. Then multiply by the pitch factor below to account for the slope. For a roof where every plane has the same pitch, this works for hip roofs as well as gables.
Example: a 40' x 28' house with 1-foot overhangs has a 42' x 30' footprint, or 1,260 sq ft. At a 6:12 pitch the factor is 1.118, so the roof is about 1,409 sq ft — 14.1 squares. With 10% waste that's 15.5 squares, or 47 bundles of three-bundle-per-square shingles.
Pitch factors
| Pitch | Angle | Multiply footprint by |
|---|---|---|
| 3:12 | 14.0° | 1.031 |
| 4:12 | 18.4° | 1.054 |
| 5:12 | 22.6° | 1.083 |
| 6:12 | 26.6° | 1.118 |
| 8:12 | 33.7° | 1.202 |
| 10:12 | 39.8° | 1.302 |
| 12:12 | 45.0° | 1.414 |
The factor is √(1 + (rise ÷ 12)²). To find your pitch, hold a level against a rafter in the attic (or the roof edge), mark 12 inches along it, and measure straight up to the underside of the roof.
Squares and bundles
Roofing is sold by the square, which is 100 sq ft. Standard 3-tab and most architectural shingles are packed three bundles to a square; heavier designer shingles are often four. Add about 10% waste for a simple gable and 15% for a hip roof, since every hip edge is cut. Roofs with valleys, dormers and many penetrations can need 15–20%.
Starter strip, ridge caps and drip edge
- Starter strip goes along every eave, and along the rakes where the shingle maker calls for it. A bundle covers roughly 100–120 linear feet.
- Hip and ridge caps cover the ridge and every hip. Coverage ranges from about 20 linear feet per bundle for thick high-profile caps to about 33 for standard ones.
- Drip edge is required at eaves and rakes by the residential code, lapped at least 2 inches; it comes in 10-foot lengths.
Underlayment and ice barrier
Synthetic underlayment rolls usually cover 10 squares; #15 felt about 4 and #30 felt about 2. Roofs between 2:12 and 4:12 need two layers of underlayment, and asphalt shingles aren't allowed below 2:12.
Where ice dams are common, the code requires an ice barrier from the lowest edge of the roof to at least 24 inches inside the exterior wall line. Because it's measured up the slope, it's more than one 36-inch course on most houses: with a 12-inch overhang and a 6-inch wall on a 6:12 roof, the barrier has to reach about 47 inches up from the drip edge.
Nails
Shingles are nailed with four nails each, or six in high-wind areas, which works out to roughly 320 nails per square (480 in high wind). Coil roofing nails come 7,200 to a box.
Planning a deck while the crew is there? Try the deck board calculator.
Sources
- Ice-Dam Protection: How Far Up the Roof Slope? (IRC R905.1.2) — Fine Homebuilding
- How Many Shingles in a Bundle — IKO
- Asphalt Shingle Installation code summary — City of Meriden, CT Building Department