Three areas that are easy to confuse
| Measurement | What it includes |
|---|---|
| Building footprint | The horizontal building outline. It may exclude roof overhangs. |
| Roof projection | The roof’s horizontal plan area, including any eaves you measured. |
| Roof surface | The actual sloping plane area. This is the starting area for roofing-square and shingle calculations. |
A pitch multiplier converts a horizontal roof projection into a sloping surface area. It does not add omitted eaves or recognise dormers and openings.
Convert a projection to surface area
Here p is rise per 12 horizontal units. At 6:12, the multiplier is approximately 1.118034.
A 40 × 30 ft roof projection covers 1,200 sq ft horizontally. At a uniform 6:12 pitch, the surface is approximately 1,341.6408 sq ft.
If that full projection includes both sides of a gable, do not multiply by two again.
Different pitches need separate sections
For two nonoverlapping projections of 600 sq ft each, one at 6:12 and one at 12:12, calculate 600 × √1.25 + 600 × √2. The total is approximately 1,519.3485 sq ft. Applying a simple average pitch produces a different answer.
The Roof Area Calculator accepts rectangle dimensions or a known projected area for each section. The latter is useful when you have already measured a triangular or irregular plan area. It does not infer the shape from a name.
A roofing square is an area unit
Divide square feet by 100 to obtain roofing squares. Thus 1,341.6408 sq ft is approximately 13.4164 squares. The fractional part is meaningful; a square is not a package you must buy whole.
Use the Roofing Square Calculator for direct conversions, including square metres.
Keep waste separate
Measured area describes the geometry. A chosen waste allowance increases the planned material coverage. Whole-bundle rounding can increase purchased coverage again. These are three different quantities.
For 2,000 sq ft and a chosen 10% allowance, target coverage is 2,200 sq ft. When you transfer a result to the Shingle Calculator, RoofFolio transfers the measured area only. Select the allowance there to avoid adding it twice.
No automatic adjustment is made for openings, valleys, cutting patterns or product requirements. Verify the measurements and material-planning approach for your project.