How it is calculated
Add overhangs, valleys, and hips on their own. The angle is to the horizontal, in degrees.
Area = length × width / cos(angle)
Where the numbers come from
- Plan length — unit in the field: m
- Plan width — unit in the field: m
- Slope — unit in the field: °
Example
A house of 10×8 m at a 30° slope gives an area of about 92,4 m².
Questions
A gable roof?
When both slopes match and the plan is already the whole building, the formula gives the two slopes together only if the width is the full span and there is a single angle. Two matching slopes over a rectangle are exactly that case: the plan area is divided by the cosine.
Slope as a percent?
Convert to degrees: angle = arctan(percent / 100).
A flat roof?
An angle of 0 leaves the plan area.