Formula
Each direction rounds up before the two counts multiply:
panels = ceil(width / coverage) × ceil(slope / sheet length)
Coverage is the exposed width after the side lap. This is one rectangular slope, not a square-foot yield.
Assumptions
- Panels run up the slope.
- The coverage width already excludes the lap.
- A partial panel in either direction is a whole panel.
Worked example
A 30 foot width at 3 feet of coverage, and a 20 foot slope with 12 foot sheets, needs 20 panels.
Input definitions
- Coverage
- How much roof one panel covers sideways after it overlaps the next panel.
- Sheet length
- The length of one panel. A slope longer than that needs more than one panel up the roof.
Limitations
End laps, gable trim, and a second slope are not added. Cut panels are still counted whole.
Questions
Why not divide area by panel area?
A leftover strip at the edge still needs a full panel. Rounding each direction catches that strip.
What if the sheet is longer than the slope?
The up-slope count is 1. You still cut the sheet to the slope, and the extra length is not reused here.