How this roofing estimate works
Start with the building footprint: length times width. That is the plan area under the roof, not the sloped surface. Multiply by a pitch multiplier for the rise/12 you select. For example, a 6/12 roof uses 1.118, so a 40×30 ft footprint becomes about 1,342 sq ft of roof surface before waste.
Divide roof area by 100 to get squares. Common 3-tab and architectural shingles are often sold as three bundles per square; laminate lines can differ, so read the bundle coverage. Waste is applied to squares and bundles. Underlayment is shown in the same squares-with-waste figure.
Pitch multipliers we use
0/12 = 1.000, 1/12 = 1.003, 2/12 = 1.014, 3/12 = 1.031, 4/12 = 1.054, 5/12 = 1.083, 6/12 = 1.118, 7/12 = 1.158, 8/12 = 1.202, 9/12 = 1.250, 10/12 = 1.302, 11/12 = 1.357, 12/12 = 1.414.
Simple vs complex roofs
A simple gable over a rectangle is what the footprint × multiplier model describes. Hips, valleys, dormers, and cut-up plans add surface and cut waste that this page cannot see. Mark Complex to default waste to 15%, and treat the result as a planning number — not a bid.
Ridge cap is not guessed from the footprint. Enter ridge length only if you measured it (or know the ridge run). For floor area of rooms under the roof, use the Square Footage Calculator. For driveway stone, see the Gravel Calculator.
FAQ
What is roof pitch?
Pitch is rise over run, written as rise/12. A 6/12 roof rises 6 inches for every 12 inches of horizontal run. Steeper pitches use a larger area multiplier.
How many bundles are in a square?
Common 3-tab and many architectural shingles are packed so that three bundles cover about one square (100 sq ft). Laminate and specialty products can differ — check the wrapper.
How much waste should I add?
About 10% is typical for a simple gable. Mark the roof complex (hips, valleys, dormers) to default to 15%. Unusual cut-up roofs may need more; a roofer’s takeoff beats any online estimate.
Does this work for hip roofs and dormers?
The footprint × pitch multiplier model assumes a simple gable over a rectangle. Hips, valleys, dormers, and cut-up plans need real surface measurements or a professional takeoff.