Roofing Calculator

How to measure a roof without climbing on it

You do not need to get on the roof for a materials estimate. Measure the house footprint from the ground, including the roof overhangs (eaves and rakes), in feet, then multiply length by width. For houses with attached garages or wings, measure each section and add them. Then find the pitch.

Finding the roof pitch

Pitch is the number of inches a roof rises for every 12 inches of horizontal run, written like 6/12. To measure it, hold a level horizontally against a rafter or the underside of the roof edge, mark 12 in along the level, and measure straight up from that mark to the roof. That measurement is the rise. Pitch apps and inexpensive pitch gauges also work. If you have plans for the house, the pitch is often marked on the elevation drawings.

The formula

A sloped roof has more surface area than its footprint. The pitch factor = √(1 + (pitch ÷ 12)²). For example, a 6/12 roof has a factor of about 1.118 and a 12/12 roof about 1.414. Roof area = footprint × pitch factor, plus waste. Roofing is sold in squares, where one square = 100 sq ft.

Worked example

A house with an 1,800 sq ft footprint including overhangs, a 6/12 pitch, 10% waste and 3 bundles per square:

• 1,800 × 1.118 = 2,012 sq ft of roof
• Add 10%: 2,214 sq ft = 22.1 squares
• 22.1 × 3 = 66.4, rounded up to 67 bundles

Waste factors

Simple gable roofs with long straight runs are often figured with about 10% waste. Hip roofs, valleys, dormers and many penetrations create more cutting, so 15% or more is common. Ridge cap and starter strip shingles are usually bought separately; measure the total length of ridges and hips and the eaves, and check the coverage per bundle on the product.

Common mistakes

• Forgetting overhangs. Measuring the walls instead of the roof edges can undercount by a lot on houses with deep eaves.
• Using one pitch for a complex roof. If different sections have different pitches, calculate each section separately.
• Assuming every shingle is 3 bundles per square. Many common shingles are, but some heavier products need more. Read the wrapper.
• Ignoring underlayment, drip edge and flashing. These are separate line items.
• Underestimating safety. Roof work involves serious fall risk. Use proper fall protection, or hire a licensed roofer.

Buying tips

• Shingles are sold by the bundle and priced by the square. Check the wrapper for the number of bundles per square and enter it.
• Underlayment comes in rolls, and coverage is printed on each roll. Ice and water shield is often required along eaves and in valleys in cold climates; check local code.
• Roofing nails are sold by the box or coil. The shingle maker's instructions specify nail length and nails per shingle, which affects warranty coverage.
• Many suppliers will deliver bundles straight onto the roof. Ask when you order.