How to measure a roof
Measuring a roof comes down to two things: the footprint of each plane and its pitch. Get those two right and every material count follows automatically. This guide walks through doing it accurately without guesswork.
Start by breaking the roof into planes
Almost no roof is a single shape, but almost every roof can be split into simple ones. Look at your roof and identify each flat plane: the two sides of a gable, the four faces of a hip roof, a dormer, a porch overhang. Each of these is a rectangle, a triangle, or a trapezoid. Measure them one at a time rather than trying to treat the whole roof as a single number — a gable is two rectangles, a hip roof is two trapezoids plus two triangles. This plane-by-plane approach is the single biggest difference between a rough guess and an accurate estimate, and it is exactly how the full calculator is built to work.
Measure the footprint of each plane
The footprint is the flat, ground-level dimension of each plane — its length and width as if you were looking straight down. For a rectangle, measure length and width. For a triangle, measure the base and the height. For a trapezoid, measure both parallel sides and the depth between them. You can often take these measurements from the ground, from the building's plans, or from a satellite measurement service, which is safer than climbing up. Record each plane separately; do not add them into one number yet, because each plane may have a different pitch.
Find the pitch of each plane
Pitch is the rise of the roof over a 12-inch horizontal run, written as rise:12. The simplest way to measure it is to hold a level horizontally against the underside of a rafter or against the roof surface, mark 12 inches along the level, and measure the vertical distance from that 12-inch mark down to the roof. That vertical measurement in inches is your rise. A reading of 6 inches means a 6/12 pitch. If you prefer degrees or percent, the pitch calculator converts between all three, and each pitch reference page shows the slope diagram.
Convert footprint to true surface area
Footprint area always understates a pitched roof, because the sloped surface is longer than its shadow on the ground. To correct for this, multiply each plane's footprint by its pitch multiplier. A 4/12 roof multiplies by about 1.054, a 6/12 roof by about 1.118, and an 8/12 roof by about 1.202 — the steeper the roof, the larger the correction. So a plane with a 1,000 sq ft footprint at 6/12 is really about 1,118 sq ft of roof surface. Apply the right multiplier to each plane, then add the true areas together for the whole roof.
Add waste, then convert to squares
Once you have the total true area, add a waste allowance for cuts, valleys, and mistakes — 10% for a simple gable, around 15% for a moderate roof, and 20–25% for a complex hip roof with dormers and valleys. Then divide by 100 to get roofing squares, the unit the trade orders in. A 2,000 sq ft roof plus 15% waste is 2,300 sq ft, or 23 squares. See what is a roofing square for why this unit matters and waste factor explained for choosing the right percentage.
Sanity-check before you order
A quick reality check catches most mistakes. Compare your roof-square count against a rough estimate from your home's footprint — a single-story ranch's roof is usually a bit larger than its floor area, not three times larger. If your number looks far off, re-check a plane's dimensions or its pitch before placing an order. Measuring twice costs minutes; an order that is short or massively over costs money and a second trip to the supplier.