Roofing
Roof Area Calculator
Turn building footprint and pitch into actual roof area, which is what material and labour are priced from.
Result
This assumes a simple gable. Hips, valleys, dormers and multiple ridge lines all add area and, more importantly, labour, which is why the complexity allowance matters more on cut-up roofs.
Get a Free Business AuditWhat moves the number
- Pitch multiplier, since a steep roof has far more surface than its footprint.
- Overhangs and eaves, which are roof and are frequently left out of a footprint measurement.
- Dormers, valleys and hips, which add area and add far more labour than area.
Where this one goes wrong
Pricing from the building footprint. A moderately pitched roof with overhangs has substantially more surface than the floor plan suggests, and quoting from the footprint underprices every job.
Questions
About this calculator
Why is roof area bigger than the house footprint?
Because of pitch and overhang. The slope stretches the surface over the same plan area, and the eaves extend beyond the walls. Both are roof and both need covering.
Do valleys and hips need extra material?
Yes, for cutting waste and for the valley detail itself. They also take disproportionately more labour than their area suggests, which is the part more often underpriced.
Can I measure a roof from satellite imagery?
For planning and for a first conversation, often. For ordering, the pitch and the details still need confirming, and imagery does not show condition at all.
The formula
roof area = footprint × slope multiplier × complexity factor
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How to use this calculator
- Building length (ft) (ft). Enter your own figure; the page starts at 48 ft.
- Building width (ft) (ft). Enter your own figure; the page starts at 30 ft.
- Roof pitch (rise per 12). Enter your own figure; the page starts at 6/12 (common).
- Eave overhang (inches) (in). Enter your own figure; the page starts at 18 in.
- Complexity allowance (%). Dormers, valleys, hips and penetrations.
The result updates as you type, so you can try a figure, see what it does to roof surface area, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Building length (ft) | 48 ft |
| Building width (ft) | 30 ft |
| Roof pitch (rise per 12) | 6/12 (common) |
| Eave overhang (inches) | 18 in |
| Complexity allowance | 8% |
What comes out
| Result | Value |
|---|---|
| Roof surface area main result | 2,032.2 |
| Roofing squares | 20.3 |
| Footprint including overhang | 1,683.0 |
| Slope multiplier | 1.118 |
A 48 × 30 building at 6/12 has about 2,032.2 sq ft of roof, 20.3 squares including overhang and complexity.
How sensitive the answer is
Holding everything else steady and changing only building length (ft), this is what happens to roof surface area. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Building length (ft) (ft) | Roof surface area |
|---|---|
| 24 ft | 1,075.9 |
| 36 ft | 1,554.0 |
| 48 ft example above | 2,032.2 |
| 72 ft | 2,988.5 |
| 96 ft | 3,944.8 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Building length (ft) (ft)
- Measured in ft, starting at 48 ft.
- Building width (ft) (ft)
- Measured in ft, starting at 30 ft.
- Roof pitch (rise per 12)
- Choose from 1/12 (very low), 2/12, 3/12, 4/12, 5/12, 6/12 (common), 7/12, 8/12, 9/12, 10/12, 12/12 (45°), 14/12 (steep).
- Eave overhang (inches) (in)
- Measured in in, between 0 and 60, starting at 18 in.
- Complexity allowance (%)
- Dormers, valleys, hips and penetrations.
What the results tell you
| Result | What it is |
|---|---|
| Roof surface area | The headline result. A decimal figure. |
| Roofing squares | A decimal figure. |
| Footprint including overhang | A decimal figure. |
| Slope multiplier | A number. |
What this cannot see
Material quantities assume a clean rectangular plane and the waste factor you enter. Valleys, hips, dormers, penetrations and a steep pitch all raise real waste, and a tear-off frequently uncovers decking that was not in the quote.
Keep going
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Open calculatorRoof Square Calculator
Convert roof area into squares, the unit roofing material and labour are actually quoted in.
Open calculatorEstimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.