Pitch finder
Measured the rise over some other run? Enter both in the same unit.
Pitch: 6:12
How we calculate this
Every plane of a simple roof has the same pitch, so the roof surface is the footprint (measured to the outside of the overhangs) times a slope factor:
- Gable: two eaves the length of the roof, four rakes, one ridge.
- Hip: eaves all the way around, four hips and a shorter ridge (length minus width). We assume all four sides share one pitch.
- Shed: one eave, two rakes, no ridge.
Coverage figures come from GAF’s spec sheets for Timberline HDZ (3 bundles and 256 nails per square), Pro-Start starter (120.33 ft per bundle) and Seal-A-Ridge ridge cap (4 bundles per 100 ft). The minimum slope, double underlayment and drip edge rules are from the 2024 IRC. Underlayment uses GAF Tiger Paw rolls (48 in × 250 ft): on slopes of 4:12 and up each course is lapped 3 in, so it covers 45 in of roof; from 2:12 to under 4:12 GAF’s double-coverage method exposes 22.5 in per course. End laps are 6 in. Other brands lap differently, so check your roll.
Worked example
A 40 ft × 28 ft gable roof at 6:12 (26.6°):
- Slope factor: √(1 + (6 ÷ 12)²) = 1.118.
- Area: 1,120 sq ft × 1.118 = 1,252 sq ft, or 12.52 squares.
- Shingles: 12.52 × 1.1 = 13.77 squares × 3 = 42 bundles.
- Starter: 80 ft of eaves + 62.6 ft of rakes = 142.6 ft → 2 bundles.
- Ridge cap: 40 ft of ridge → 2 bundles. As a hip roof, the same footprint would have a 12 ft ridge and 84 ft of hips.
When to call a pro
Roofing means working on a slope, high off the ground. Hire a licensed roofer if the roof is steep, two stories or higher, has soft or rotted decking, or if you aren’t set up with a harness and anchor. Also call one if the calculator says your pitch is too low for shingles.
Numbers this calculator uses
Every value comes from the data files behind this page. Anything marked PLACEHOLDER hasn’t been checked against a source yet, and you can override it in the calculator where it matters.
| Number | Value | Source |
|---|---|---|
| Square feet in one roofing square | 100 sq ft | Unit definition |
| GAF Timberline HDZ bundles per square | 3 | GAFchecked 2026-10-30 |
| GAF Timberline HDZ nails per square (4-nail pattern) | 256 | GAFchecked 2026-10-30 |
| GAF Pro-Start starter strip per bundle | 120.33 ft | GAFchecked 2026-10-30 |
| GAF Seal-A-Ridge ridge cap per bundle4 bundles cover 100 linear feet. | 25 ft | GAFchecked 2026-10-30 |
| Lowest slope allowed for asphalt shinglesR905.2.2. | 2 in 12 | 2024 IRCchecked 2026-10-30 |
| Double underlayment required belowR905.2.2: from 2:12 up to 4:12. | 4 in 12 | 2024 IRCchecked 2026-10-30 |
| Drip edge overlap at jointsR905.2.8.5: at least 2 in. | 2 in | 2024 IRCchecked 2026-10-30 |
| Drip edge piece lengthGibraltar and Amerimax drip edge are sold in 10 ft pieces. | 10 ft | The Home Depotchecked 2026-10-30 |
| Synthetic underlayment roll width (GAF Tiger Paw)A 10-square roll, 48 in x 250 ft. The 1,000 sq ft on the label excludes laps. | 48 in | GAFchecked 2026-10-30 |
| Synthetic underlayment roll length | 250 ft | GAFchecked 2026-10-30 |
| Underlayment side lap (4:12 and steeper) | 3 in | GAFchecked 2026-10-30 |
| Underlayment end lap | 6 in | GAFchecked 2026-10-30 |
| Underlayment exposure for double coverage (2:12 to under 4:12) | 22 1/2 in | GAFchecked 2026-10-30 |
| Roofing nails per box (coil)Grip-Rite 1-1/4 in coil roofing nails. Match this to the box you price. | 7200 | The Home Depotchecked 2026-10-30 |
| Waste factor for roofing (default; you can change it)Not yet backed by a source. For reference, Fine Homebuilding roofing writer Mike Guertin figures about 1% on a simple roof and 5% or more on a complex one; our higher default leaves room for DIY mistakes. Raise it for hips, valleys and dormers. | 10% | PLACEHOLDER |
Sources
| Source | What we use from it | Last checked |
|---|---|---|
| GAF Timberline HDZ shingles: specificationsGAF · manufacturer spec | 3 bundles per square, 64 pieces per square, 256 nails per square (4-nail pattern), 5-5/8 in exposure. | 2026-10-30 |
| GAF Pro-Start Starter Strip ShinglesGAF · manufacturer spec | About 120.33 linear feet of coverage per bundle. | 2026-10-30 |
| GAF Seal-A-Ridge Ridge Cap Shingles sell sheet (RESHR112-1124)GAF · manufacturer spec | 4 bundles cover 100 linear feet (25 ft per bundle) at 6-2/3 in exposure. | 2026-10-30 |
| GAF Tiger Paw Premium Roof Deck Protection data sheet (RESUL179-1224)GAF · manufacturer spec | Roll size 10 squares (1,000 sq ft, laps excluded), 48 in wide by 250 ft long. | 2026-10-30 |
| GAF Tiger Paw installation instructionsGAF · manufacturer guide | Slopes 4:12 and up: 3 in side laps and 6 in end laps. 2:12 to under 4:12: double coverage with a 22.5 in exposure. | 2026-10-30 |
| 2024 International Residential Code, Section R905.2 (Asphalt shingles)International Code Council · building code | Minimum slope 2:12 and double underlayment from 2:12 to 4:12 (R905.2.2); drip edge at eaves and rakes, lapped at least 2 in (R905.2.8.5). | 2026-10-30 |
| Gibraltar Building Products galvanized steel drip edge, 10 ft (model 08331), Home Depot listingThe Home Depot · store listing | Drip edge sold in 10 ft pieces. | 2026-10-30 |
| Grip-Rite 1-1/4 in 15° coil roofing nails, 7,200 per box (GRCR3DADE), Home Depot listingThe Home Depot · store listing | Coil roofing nails sold 7,200 per box. | 2026-10-30 |
Spotted a number that’s out of date? Tell us and we’ll recheck it and note the fix in the change log.
Frequently asked questions
How many bundles of shingles are in a square?
A roofing square is 100 sq ft. For GAF Timberline HDZ, one of the most common architectural shingles, it’s 3 bundles per square (GAF spec sheet). Other shingles can differ, so check the bundle wrapper and pick “Another shingle” in the calculator to enter its coverage.
How do I find my roof’s pitch?
Pitch is inches of rise per 12 in of level run. Hold a level horizontally against the roof (or a rafter in the attic), mark 12 in along it, and measure straight up to the roof surface. If you measured a different run, the pitch finder on this page scales it to 12 for you.
What is the lowest roof pitch for asphalt shingles?
2:12. Between 2:12 and 4:12 the code requires two layers of underlayment (2024 IRC R905.2.2), which the calculator adds automatically. Flatter roofs need a low-slope roofing system instead.
How much extra should I order for waste?
Our default is 10%. We haven’t found a manufacturer figure, so it’s still marked as a placeholder. For comparison, Fine Homebuilding roofing writer Mike Guertin figures about 1% for a simple roof and 5% or more for a complex one with open valleys; our higher default leaves room for DIY mistakes. Starter and ridge cap are counted from the edge lengths, and underlayment laps are counted exactly, so the waste factor only covers cutting and mistakes.
How many nails per square of shingles?
GAF lists 256 nails per square for Timberline HDZ, based on its standard 4-nail pattern. Some high-wind areas and steep roofs call for more nails per shingle; follow the shingle maker’s instructions for your roof.
Do I need drip edge?
Yes. The 2024 IRC requires drip edge at the eaves and rakes of shingle roofs, with joints lapped at least 2 in (R905.2.8.5). The calculator counts both edges and allows for the laps.
Save this plan
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