Roof Pitch & Shingle Calculator
Actual roof area, rafter length, and shingle/underlayment material estimate from your plan dimensions and pitch — standard roofing-trade formulas, not a building code lookup.
How this is calculated
A roof's actual sloped surface is bigger than its flat "plan" footprint — the steeper the pitch, the bigger the difference. The standard roofing-trade pitch factor captures that: factor = √(1 + (rise/12)²). A 6/12 pitch, for example, has a factor of about 1.118 — the actual roof area (and the rafter length per foot of horizontal run) is about 11.8% more than the flat measurement.
One roofing square is 100 sq ft — the standard unit the trade prices and orders material in. Standard 3-tab and most architectural shingles cover a square in 3 bundles (~33.3 sq ft/bundle); some heavier premium architectural shingles need 4 bundles per square instead.
Waste allowance covers cuts, starter courses, ridge caps, and mistakes — simple roofs need less, roofs with more hips, valleys, or dormers need more. Underlayment roll coverage varies by product; the figures here (felt ≈4 squares/roll, synthetic ≈10 squares/roll) are typical, not a fixed spec — check the actual product's coverage rating before ordering.
How this calculator works
Pitch factor √(1 + (rise ÷ 12)²) converts plan area to roof area; 100 sq ft per square; bundles per square and waste allowance chosen by the user.
Worked example
A 40 × 30 ft simple gable roof at 6/12 pitch, 15 ft run, 10% waste, 3-bundle shingles and synthetic underlayment.
Pitch factor = √(1 + (6/12)²) = 1.118. Actual area = 1,200 × 1.118 = 1,342 sq ft. With 10% waste that is 14.76 squares, so 45 bundles of shingles and 2 rolls of synthetic underlayment.
Each rafter, from wall to ridge, is 15 × 1.118 = 16.8 ft before overhang.
Frequently asked questions
What is a roofing square?
A square is 100 sq ft of roof surface. Shingles and most roofing materials are priced and ordered by the square.
How many bundles are in a square?
Most 3-tab and standard architectural shingles take 3 bundles per square. Some heavy premium architectural shingles take 4.
How much waste should I add?
About 10% for a simple gable, 15% with hips and valleys, and 20% for complex roofs with dormers and multiple valleys.
How do I find my roof pitch?
Measure how many inches the roof rises over 12 inches of horizontal run. A level and tape measure on a rafter or the roof surface works, or measure from the attic.
Common mistakes
- Measuring the plan area as roof area. A sloped roof is larger than its footprint. Multiply the plan area by the pitch factor (1.118 for 6/12).
- Shingles on a low slope. Asphalt shingles may only be used on slopes of 2:12 or more, and from 2:12 to 4:12 double underlayment is required (IRC R905.2).
- Too little waste on cut-up roofs. Hips, valleys and dormers waste more material; choose a higher waste allowance for complex roofs.
- Assuming three bundles per square. Bundles per square depend on the product; check the wrapper or the manufacturer's data.
What it does not cover
Roofs with several pitches (add each plane separately); hips, valleys and dormers only through the waste allowance; manufacturer-specific coverage.
Sources
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