Estimate the sloped area of a roof from the house footprint, pitch or angle, and eave overhang, then convert that area into US shingle, roll-roofing, and felt counts. Use the materials tab when you already know the roof surface.
Typical slope correction factors
| Pitch | Angle | Multiply by | Pitch | Angle | Multiply by |
|---|---|---|---|---|---|
| 1/12 | 4.8° | 1.003 | 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 |
| 13/12 | 47.3° | 1.474 | 14/12 | 49.4° | 1.537 |
| 15/12 | 51.3° | 1.601 | 16/12 | 53.1° | 1.667 |
| 17/12 | 54.8° | 1.734 | 18/12 | 56.3° | 1.803 |
| 19/12 | 57.7° | 1.873 | 20/12 | 59.0° | 1.944 |
| 21/12 | 60.3° | 2.016 | 22/12 | 61.4° | 2.088 |
| 23/12 | 62.4° | 2.162 | 24/12 | 63.4° | 2.236 |
Examples
Default 2,000 sq ft house, 6/12, 1 ft eaves
House base 2,000 sq ft, pitch 6/12, eaves 1 ft. Estimated roof area is 2,461 square feet (228.6 m², 273.4 yd²). With a 10% buffer that is 28 roof squares, 82 bundles of composition shingles, 25 rolls of roll roofing, 7 rolls of #15 felt, and 13 rolls of #30 felt. At $3 per square foot the material price is $7,383.
Same house, no eaves
Pitch 6/12 and eaves 0 on 2,000 sq ft is 2,236 square feet (207.7 m²). That is 25 squares and 74 shingle bundles.
Known 2,000 sq ft roof surface
On the materials tab, 2,000 sq ft of roof (already sloped) needs 22 squares with buffer, 66 shingle bundles, 20 rolls of roll roofing, 5 rolls of #15 felt, and 10 rolls of #30 felt.
Formula
Lengths use the international foot (1 ft = 0.3048 m). The house footprint is treated as a rectangle whose perimeter is 10% longer than a square of the same area, then expanded by the eave overhang:
plan = A + 4.4 × eaves × (√A + eaves)
pitch = √(1 + (rise/12)²)
angle = 1 / cos(θ)
roof = plan × pitch (or angle)
A 6/12 pitch is rise 6, so √(1 + 0.5²) = 1.118. A 25° roof uses 1 / cos(25°) ≈ 1.103. Materials are counted on the sloped area in square feet:
squares (10% extra) = ceil(roof_ft² × 1.1 / 100)
shingle bundles = ceil(roof_ft² × 1.1 × 3 / 100)
roll roofing = ceil(roof_ft² / 100)
#15 felt = ceil(roof_ft² / 400)
#30 felt = ceil(roof_ft² / 200)
Purchasing assumptions and limits
A roof should be measured plane by plane when possible. The footprint mode's eave calculation estimates perimeter from area and can be inaccurate for long, L-shaped, hip, valley, dormer, or multi-level roofs. The 10% allowance is a planning convention, not a guarantee that it covers the cuts and starter, ridge, hip, and repair needs of a particular roof.
The GAF shingle measurement guide confirms that a roofing square is 100 ft² and that many standard products use three bundles per square, while product specifications can differ. Roll roofing and felt results use the stated coverage constants without the shingle waste factor. Confirm installed coverage, laps, slope limits, underlayment, fasteners, flashing, ventilation, and local requirements for the selected system.