Formula
The slope factor is the rafter length divided by the horizontal run:
factor = sqrt(rise² + run²) / run
Sloped area is the plan area times that factor, rounded to the nearest whole unit. A rise of zero keeps the plan area unchanged.
Assumptions
- Rise and run use the same length unit. The usual expression is inches of rise per 12 inches of run.
- The plan area is the horizontal footprint, not a measured rafter length.
- One pitch covers the whole footprint.
Worked example
A 6-in-12 pitch over a 1,000 square foot footprint has a sloped area of 1,118.
Input definitions
- Rise
- How far the roof climbs over the run. Use 0 for a flat roof.
- Run
- The horizontal distance that goes with the rise. 12 is the usual reference.
- Plan area
- Footprint measured on the flat. The result stays in that area unit.
Limitations
Hips, valleys, overhangs, and starter courses are not added. The shingle page turns squares into bundles.
Questions
Is a 6-in-12 pitch the same in meters?
The factor is a ratio, so 6 and 12 can be any matching length unit. The plan area unit is what the result uses.
Does this order shingles?
No. It stops at sloped area. The roofing page turns that area into squares, and the shingle page turns squares into bundles.