Roofing Calculator

What this calculator does

The Roofing Calculator estimates the sloped area of a roof from the horizontal area the house covers and the roof pitch. It converts that area into roofing squares, applies a waste allowance, and estimates how many bundles of composition shingles are needed.

Roofing materials are usually sold by the square. One square covers 100 square feet of roof surface. Shingle bundles are commonly rated at about one-third of a square, or roughly 33.3 square feet per bundle. The calculator uses that standard coverage and rounds the bundle count up to a whole number.

How to use this calculator

1. Enter the house base area in square feet. This is the horizontal footprint, not the sloped roof area.
2. Enter the roof pitch as inches of rise per 12 inches of run. A 6/12 roof, for example, rises 6 inches over every 12 horizontal inches.
3. Enter a waste percentage. Ten percent is a common starting point for hips, valleys, and cut pieces.
4. Optionally change the bundle coverage to match the product you plan to buy.
5. Click Calculate.

The default example uses a 2,000-square-foot base area, a 6/12 pitch, 10 percent waste, and 33.3 square feet per bundle. It returns about 2,236 square feet of roof area, 22.36 squares, and 74 bundles.

Roofing formulas

pitch multiplier = √(12² + pitch²) ÷ 12
roof area        = base area × pitch multiplier
roof squares     = roof area ÷ 100
squares + waste  = roof squares × (1 + waste % ÷ 100)
bundles          = roof area × (1 + waste % ÷ 100) ÷ bundle coverage
roof angle       = atan(pitch ÷ 12)

For a 6/12 pitch, the multiplier is about 1.118. A 2,000-square-foot footprint therefore has about 2,236 square feet of sloped roof. The same pitch has a roof angle of about 26.57 degrees.

Assumptions and limitations

• The base area is the horizontal footprint of the building, not the sloped roof surface.
• The calculation assumes a simple gable or shed roof with one consistent pitch over the entire footprint.
• Eaves, overhangs, dormers, hips, valleys, and multiple roof planes are not added automatically. Measure each plane separately for a complex roof and add the results.
• The pitch must be a whole number from 1/12 through 24/12.
• Bundle coverage varies by shingle style, manufacturer, and installation method. Confirm the coverage printed on the bundle.
• The estimate does not include underlayment, drip edge, flashing, ridge vent, nails, ice-and-water shield, or labor.
• Waste is applied only to the shingle area; it is not a structural allowance or a substitute for careful measurement.

Frequently asked questions

What is a roofing square?

A roofing square is 100 square feet of roof surface. It is the unit most roofing suppliers use when quoting shingles, underlayment, and labor.

How many shingle bundles are in a square?

Three bundles usually cover one square, because each bundle covers about 33.3 square feet. Some architectural shingles have different coverage, so check the label before ordering.

Why is the roof area larger than the house footprint?

A sloped roof covers more surface than the horizontal area beneath it. The steeper the pitch, the larger the multiplier. At 24/12, the roof area is about 2.236 times the footprint.

Source

Reference: Calculator.net — Roofing Calculator.

This calculator provides a material estimate. Have a roofing contractor verify measurements and quantities before ordering.