Estimate roof surface area, squares, shingle bundles, underlayment rolls, drip edge and ridge cap from footprint and pitch.
Inches accept fractions: 12 5/8, 12-5/8, or 12.625
| Item | Qty | Notes |
|---|---|---|
| Shingle bundles | 45 | 3/sq |
| Underlayment rolls | 4 | 4 sq/roll |
| Drip edge | 140 | linear ft |
| Starter strip | 60 | linear ft, eaves |
| Ridge cap | 30 | linear ft |
| Roofing nails | 15 | lb (1 lb/sq) |
Advanced · bundles/square, underlayment, nails
Pitch multiplier reference table
| Pitch | Angle | Slope factor |
|---|---|---|
| 2/12 | 9.5° | 1.014 |
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 7/12 | 30.3° | 1.158 |
| 8/12 | 33.7° | 1.202 |
| 9/12 | 36.9° | 1.250 |
| 10/12 | 39.8° | 1.302 |
| 11/12 | 42.5° | 1.357 |
| 12/12 | 45.0° | 1.414 |
Slope factor = √(rise² + run²) / run. Multiply footprint sq ft by this to get true roof surface area.
FAQ
- Why is a "square" 100 square feet?
- The square is the trade unit US asphalt-shingle manufacturers have used since at least the 1930s — one square covers 100 ft² of roof surface, and most shingle products are sold and rated per square. Underlayment, ice-and-water shield and ridge-cap are also priced per square at the supplier counter.
- How accurate is "3 bundles per square"?
- True for standard 3-tab and most dimensional asphalt (GAF Timberline, Owens Corning Duration, IKO Cambridge). Premium-thickness lines (CertainTeed Presidential, GAF Grand Sequoia) ship at 4 bundles/sq, and slate-look luxury at 5. Switch the bundle rate in Advanced if you are pricing a non-3-tab line.
- Should I add waste on top of slope factor?
- Yes. Slope factor only converts footprint to true surface; it does not cover trim cuts, valley overlap, starter strip, hip-and-ridge overlap, or installer error. 10% for a simple gable, 12% for hip, 15% for complex roofs with dormers and multiple valleys. Tear-off adds 2-3% on top — broken decking is the usual cause.
- Where does the drip edge length come from?
- This calc uses a rectangular-footprint approximation: drip edge runs the full perimeter (2 × length + 2 × width). Starter strip runs only the two eaves (≈ 2 × length for a simple gable). Ridge cap equals the ridge length, which is the longer footprint dimension for a simple gable, and longer perimeter-fraction for a hip roof. Complex roofs with dormers or T-shapes will exceed this — measure the roof plan to confirm.
From footprint to bundles — the three-stage conversion
A roof is sloped, so the square footage you walk under is not what the supplier sells. The conversion has three stages:
- Footprint:
length × widthin feet. - Slope factor stretches the footprint into the true sloped surface:
√(rise² + run²) / run. A 6/12 pitch gives√180 / 12 = 1.118. - Squares — the trade unit since the 1930s. 1 square = 100 ft². Divide sloped area by 100.
Waste is then added: 10% for a simple gable, 12% for a hip, 15% for complex roofs with dormers or multiple valleys. Tear-off of an existing layer adds another 2-3% — broken decking and rotted nailers come out with the old material.
Worked example — 30×40 ranch at 6/12 pitch
- Footprint:
30 × 40 = 1,200 ft² - Slope factor at 6/12:
1.118→ true roof surface1,342 ft² - Base squares:
1,342 / 100 = 13.42 - With 10% waste:
13.42 × 1.10 = 14.76 squares to buy - Bundles at 3/sq:
ceil(14.76 × 3) = 45 bundles - Underlayment 4 sq/roll synthetic:
4 rolls; drip edge perimeter140 LF; ridge cap (gable = length)30 LF; starter (2 eaves)60 LF; nails at 1 lb/sq:15 lb
Switch to 8/12 (slope 1.202) and bundles climb to 48 — pitch matters more than people expect. A 12/12 roof needs 1.414× the bundles of the flat footprint, plus more waste because you cannot walk it. Chimney pads sit outside this estimator — use the concrete footing calculator for that.
Below 2/12 pitch, asphalt shingles are not the right product — use a low-slope membrane (TPO, EPDM, modified bitumen). IRC R905.1.1 limits asphalt shingles to ≥ 2/12, and requires double underlayment from 2/12 to 4/12. The Advanced bundle rate covers premium lines: 3 bundles/sq for standard dimensional (GAF Timberline, OC Duration, IKO Cambridge, CertainTeed Landmark), 4 for premium (CertainTeed Presidential, Grand Sequoia), 5 for slate-look luxury — the bundle count jumps roughly 65% from 3 to 5.
Common questions
- What waste percentage for a tear-off?
- Tear-off stacks 2-3% on top of complexity waste — broken decking, rotted nailers and irregular substrate cause shingle damage on install. A simple gable tear-off lands at 12-13%, not 10%.
- Do I include overhang in the footprint or add it separately?
- Add it separately. Plans give wall-to-wall footprint; the roof typically overhangs 12-24″ on every side. The calc takes overhang per-side and extends both length and width by 2× that figure before applying slope. A 30×40 house with 12″ overhangs becomes a 32×42 roof footprint — 8% more material before any waste.
- Metal panels or clay tile?
- Slope factor and squares math still apply — pure geometry. Bundles and underlayment rolls do not. Compute roof area here, then convert to your product’s unit. Tile-specific lookups are in the clay vs concrete roof tile calculator.
- Is the hip-roof ridge length right?
- Footprint and squares are identical to a gable. The hip preset bumps waste to 12% (every face has trim cuts) and approximates ridge-cap LF as
|L−W| + 4 × √(2×(W/2)²)— central ridge plus four hip lines. For a 40×30 hip that lands near 95 LF total.
Estimator only. Roofing involves fall risk, IRC R905 code compliance, and substrate inspection this calc cannot do. Use the numbers to size supplier delivery and the dumpster — confirm tear-off layers, deck thickness, and ice-shield zone with a licensed roofer before ordering. For attic ventilation and condensation checks, the dew point calculator handles the moisture side.
