Formula
Both slopes are the same:
area = 2 × building length × rafter length
The rafter length is measured along the slope. The pitch page instead multiplies a flat footprint by a rise-and-run factor.
Assumptions
- The roof has two equal rectangular slopes.
- The rafter length includes the overhang you want covered.
- Length and rafter use the same unit, shown here in feet.
Worked example
A 40 foot building with 16 foot rafters has 1,280 ft² of roof.
Input definitions
- Rafter length
- The distance from eave to ridge along the slope.
- Building length
- The length of the ridge, including the overhang along that direction.
Limitations
Hips, dormers, and a different slope on the second side are not added.
Questions
Why not use rise and run?
Those make a slope factor for a footprint. This page already has the sloped rafter length.
Does the result include both sides?
Yes. The 2 in the formula is the pair of slopes.