Roof Area Calculator
Convert a building footprint into the true sloped roof surface area you actually have to cover.
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Formula and method
Half the building width plus the overhang gives the run, and the rise gives the geometry of one rafter. The slope length is multiplied by the roof length including overhangs to give the surface area, which the waste allowance then increases.
Roof area is not floor area
The covered footprint is always smaller than the roof surface. At 30° the slope adds about 15 %, at 45° about 41 %, because the factor is one over the cosine of the pitch. Buying covering for the footprint leaves you 15 to 30 % short on an ordinary gable roof, which is an expensive way to learn the difference.
The calculator takes the building dimensions plus the overhang, works out the footprint and multiplies by the pitch factor. Pitch can be entered either as a rise or as a percentage; both produce the same factor, so use whichever your drawing shows.
Overhangs, valleys and what is excluded
The overhang enlarges both dimensions and therefore every edge you buy. A 40 cm overhang on a 10 × 8 m building adds nearly 8 % of area, and it is added on all four sides even though only two of them are usually described as eaves.
Valleys, dormers, chimneys and ridge details are not included. On a plain cold roof the error is small; on a roof with two dormers and a chimney, 5 to 10 % of the surface is in cuts and flashings that the area figure does not describe.
From area to covering
The roof area is the basis for insulation, underlay, battens and covering, but each layer has its own waste. Tiles and shingles need 5 to 10 % depending on hips and valleys, membranes need 10 % for laps, and sheet coverings need the effective width rather than the nominal width.
Take the area from here into the roofing sheet calculator to translate it into panels. That page subtracts the side lap from the nominal width, which is the step where a plain area figure turns into the number of sheets actually delivered.
How to measure the inputs
- Measure the building length and width at the wall line, then add the overhang you plan.
- Measure the rise or the pitch from the eaves to the ridge.
- Record the overhang at the eaves and the gable ends separately if they differ.
Worked example
Building length 32.81 ft; Building width 26.25 ft; Roof rise (optional) 6.56 ft; Roof overhang (optional) 11.81 in; Waste 5 % → Roof area with waste: 1,136.29 ft²
Reference data
| Angle, ° | Slope, % | Area multiplier |
|---|---|---|
| 20 | 36 | 1.06 |
| 30 | 58 | 1.15 |
| 35 | 70 | 1.22 |
| 45 | 100 | 1.41 |
Multiplier is 1 / cos(angle); it turns the plan area into the real roof area.
Practical tips
- Measure the roof rather than the ceiling wherever it is safe to do so.
- Add dormers, rooflights and valleys as separate areas.
- Order roofing materials after the roof structure is complete and checked.
Sources
- СП 17.13330.2017 — Кровли
Checked and updated October 22, 2026