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General Roofing Calculator

Estimate roof surface area, materials, and squares. Includes pitch logic and waste factor calculations for contractors.

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About

Accurate material estimation is the difference between profit and loss in roofing projects. Measuring the footprint of a house is simple but accounting for the roof's slope and complexity requires specific multipliers. A steep roof has significantly more surface area than a flat one covering the same ground dimensions. Furthermore professional estimators must always include a waste factor to account for cutting, overlap, and errors.

This calculator determines the true surface area based on the roof pitch. It automatically converts these dimensions into 'Squares' (the industry standard unit of 100 sq. ft.) and estimates the required plywood sheets and underlayment rolls. It is designed for Gable and Hip style roofs where pitch affects the hypotenuse length.

construction roofing home improvement estimator building materials

Formulas

The actual roof area Aroof is the base area Abase multiplied by the pitch factor M.

M = √1 + (RiseRun)2

Total squares S is calculated by dividing the total area (including waste) by 100.

Reference Data

Roof Pitch (Rise/12)Angle (°)Area MultiplierNotes
Flat (0/12)0.0°1.000Built-up / Membrane
Low (3/12)14.0°1.031Minimum for shingles
Standard (4/12)18.4°1.054Walkable
Moderate (6/12)26.6°1.118Common residential
Steep (9/12)36.9°1.250Difficult to walk
Very Steep (12/12)45.0°1.414Harness required
Mansard (18/12)56.3°1.803Near vertical

Frequently Asked Questions

In the roofing industry a 'Square' is a unit of area equal to 100 square feet (10' x 10'). Materials like shingles and underlayment are typically sold by the square or by the bundle where 3 bundles equal 1 square.
For simple gable roofs 5-10% waste is standard. For hip roofs or roofs with many valleys, dormers, and chimneys, 10-15% is safer. If you are an amateur DIYer, calculate 15-20% to account for cutting mistakes.
Yes. A 12/12 pitch roof has 41% more surface area than a flat roof covering the same house footprint. Ignoring pitch will result in a severe shortage of materials.
Standard sheathing comes in 4x8 foot sheets, covering 32 square feet. This tool calculates the number of whole sheets needed based on the total roof surface area.