Roof Area Calculator

A pitched roof is always larger than the building underneath it, and ordering material by the floor plan is how roofs come up short. This calculator applies the pitch to the footprint to give the real surface area, converts it into roofing squares, and adds a wastage allowance for hips, valleys and offcuts.

How to use it

1. Enter the building length and width in feet.
2. Enter the pitch as rise per 12 — a 6-in-12 roof rises 6 inches per foot.
3. Read the actual roof area, the material to buy, and the number of squares.

Examples

• A 40 x 30 ft building with a 6-in-12 pitch: 1,342 sq ft of roof, not the 1,200 of the footprint.
• With 10% wastage that is about 14.8 roofing squares.

Frequently asked questions

Why is my roof bigger than my floor plan?

Because a sloped surface covering a given width is longer than that width. The multiplier is the square root of one plus the pitch ratio squared, so a 6-in-12 roof is 1.12 times its footprint and a 12-in-12 is 1.41 times. On a 1,200 sq ft footprint that is 140 to 500 extra square feet of material.

How do I find my roof pitch?

Hold a level horizontally against the roof, measure 12 inches along it, then measure straight down to the roof surface. That drop is the rise per 12. A 6-in-12 pitch is common on houses; a low-slope roof might be 2 or 3, and steep roofs run 10 to 12.

What is a roofing square?

One hundred square feet of roof surface. Roofing materials and labour are commonly quoted per square rather than per square foot, so the calculator gives both — a 1,476 sq ft order is 14.8 squares.

How much wastage should I allow for roofing?

Ten per cent for a simple gable roof. Fifteen or more for a hipped roof or one with valleys, dormers and skylights, because every hip and valley means cutting sheets at an angle and discarding the offcut. Starter and ridge courses also consume extra material.