How the calculation works
For a roof with one even pitch, every roof plane rises at the same angle, so the whole surface is the plan area scaled by the same factor: 1 ÷ cos(pitch). The plan area is the footprint of the roof seen from above, which includes the eaves. Hips, valleys and gables change the shape of the planes and the length of ridge and hip capping, but not the total surface area.
The calculator does not account for roofs with several different pitches, upper storeys with their own roofs, dormers, skillion verandahs or carports. Add those sections separately if you want a closer figure.
Pitch factors at a glance
The last column shows the roof surface for an example plan area of 200 m².
| Roof pitch | Area factor | 200 m² plan becomes |
|---|---|---|
| 5° | 1.004 | 201 m² |
| 10° | 1.015 | 203 m² |
| 15° | 1.035 | 207 m² |
| 20° | 1.064 | 213 m² |
| 22.5° | 1.082 | 216 m² |
| 25° | 1.103 | 221 m² |
| 30° | 1.155 | 231 m² |
| 35° | 1.221 | 244 m² |
| 40° | 1.305 | 261 m² |
Using the result when you compare quotes
Restoration and replacement quotes are often based on the roof surface area. If one quote uses a much larger or smaller area than the others, ask how it was measured and whether eaves, verandahs or garages are included. Area is only one input: the condition of the roof, the repairs found, height, access and the products used also change the price. See roof restoration costs and roof replacement costs, and our guide to comparing roofing quotes.
Measure from the ground or from plans. Do not climb onto the roof or use a ladder to measure it.
Frequently asked questions
Why is my roof area bigger than my floor area?
Because the roof slopes. A sloping surface covering the same footprint is longer than the flat plan, and the roof usually extends past the walls as eaves. The steeper the pitch, the bigger the difference.
Does a hip roof have more area than a gable roof?
Not if the footprint and pitch are the same. For a roof with one even pitch, the surface area equals the plan area divided by the cosine of the pitch, whatever the shape of the hips, gables and valleys. Hips and valleys add detail work and ridge length, not area.
How can I find my roof pitch without going on the roof?
Check the house plans or building approval documents, which often state the pitch, or ask the contractor who inspects the roof. Do not climb onto the roof to measure it.
Why might two quotes use different areas for the same roof?
Contractors measure differently: some include eaves and verandahs, some measure on site and some work from plans or aerial images, and figures are often rounded. Ask each contractor what area their price is based on and how it was measured.