Roofing calculator

The slope factor

The single thing that matters here is that roof area is not footprint area. A pitched roof covers the same ground but tilts, so its surface is longer along the slope. The multiplier is:

slope factor = 1 / cos(pitch angle)

For a 6:12 roof, the angle is 26.57° and the factor is 1.118 — the roof is about 12% larger than the footprint. At 12:12 (45°) the factor is 1.414, so the roof is 41% larger. Ordering materials from footprint alone is the most expensive mistake in roof estimating.

Reading pitch notation

Roof pitch is traditionally written as rise over a run of 12: a "6 in 12" or 6:12 roof rises 6 units for every 12 horizontal. Converting to degrees is arctan(rise/12).

• 4:12 — 18.4°, slope factor 1.054
• 6:12 — 26.6°, slope factor 1.118
• 9:12 — 36.9°, slope factor 1.250
• 12:12 — 45.0°, slope factor 1.414

Below about 4:12, most asphalt shingles are outside their rated pitch and need a full underlayment or a different covering entirely. Above roughly 8:12, work becomes slower and more expensive because staging and fall protection are required.

Squares and bundles

Roofing is sold and quoted by the square, which is 100 square feet — about 9.29 square metres. Asphalt shingles typically come three bundles to a square, though heavier architectural shingles sometimes run four.

This unit predates metric measurement in the trade and has stubbornly survived, so a quote in squares is normal even in metric countries. The calculator gives both.

Waste, hips and valleys

A simple gable roof wastes little — 10% is usually enough. Every hip and valley adds cut waste, because courses have to be trimmed at an angle along the whole length. Complex roofs with dormers and multiple intersections can waste 20% or more.

The default of 12% suits a straightforward roof with a couple of features. Increase it if you can see more than two changes of plane from the ground.

What this does not cover

The calculation assumes a simple gable — two planes meeting at a ridge. Hip roofs have the same total area for a given footprint and pitch, so the area figure still holds, but the rafter and ridge figures do not apply to the hip rafters, which run diagonally and are longer.

Nothing here covers structural sizing. Rafter dimensions, spacing and load capacity are governed by building regulations and depend on span, timber grade, and snow and wind loading. That is a job for span tables or an engineer, not an area calculator.

Common questions