Roof Panel Calculator (Sloped Coverage)
Calculate the number of roof sandwich panels needed from roof length, width, slope percentage, panel dimensions, and wastage allowance.
How this works
The footprint area is scaled up by a slope factor to give the actual sloped surface:
slopeFactor = √(1 + (slopePct / 100)²)
actualRoofArea = roofLength × roofWidth × slopeFactor
panelArea = panelWidth × panelLength
nPanels = ceil((actualRoofArea / panelArea) × (1 + wastagePct / 100))
coveredArea = nPanels × panelAreaSlope factor ≥ 1 for any non-negative slope, so a sloped roof always needs at least as much panel area as the equivalent flat roof.
Worked example
A 12 m × 8 m footprint at a 30% slope, with 1 m × 6 m panels and a 5% wastage allowance:
- Slope factor =
√(1 + 0.30²)≈ 1.044 - Actual roof area =
96 × 1.044≈ 100.23 m² - Panel area =
1 × 6= 6 m² - Raw count =
(100.23 / 6) × 1.05 ≈ 17.54 - Round up = 18 panels
Sources
- Standard sloped-roof coverage rule (footprint × √(1 + s²) × wastage)
FAQ
How is the sloped roof area different from the footprint?
The footprint is what you see on a plan view — length × width as projected onto the ground. A sloped roof always covers more area than its footprint because the slope adds vertical run for every horizontal step. We multiply the footprint by a slope factor of √(1 + (slope/100)²). For a flat roof (0%), this factor is 1.0; for a 30% slope it is ≈ 1.044; for a 50% slope it is ≈ 1.118.
Do I enter the slope as a percentage, ratio, or angle?
As a percentage. A 30% slope means 30 mm of vertical rise for every 100 mm of horizontal run, which is roughly equivalent to a 16.7° angle or a 1:3.33 ratio. To convert from degrees, use slopePct = tan(angle) × 100. To convert from ratio, use slopePct = (rise/run) × 100. Most South Asian residential roofs are between 15% and 50% slope.
Should I include eaves, ridge laps, and gable overhangs in the dimensions?
Yes — enter the actual length and width of the roof you want to cover, including any eave overhang past the wall line. The wastage allowance handles small overlaps at the ridge and at lap joints (typically 100–150 mm), but it is not a substitute for entering the right footprint dimensions. For complex hip or pyramidal roofs, sum the projected area of each plane and run the calculator once per plane.
What wastage allowance is appropriate for roof panels?
5 percent works for simple gable or shed roofs. Hip roofs with multiple cuts at the corners and intersecting valleys need 8–12 percent. Skylights, dormers, and chimneys add another 2–3 percent each. Roof panels are also more easily damaged in transit than wall panels because of their length, so adding 1–2 percent for shipping breakage is sensible.
Why does my answer round up to the next whole panel?
Roof sandwich panels are sold as whole units. Even if you only need 9.2 panels' worth of cover, you must order 10 — the 0.8-panel offcut becomes part of your wastage allowance and can be repurposed for short cap pieces or accessories. The `coveredArea` field shows you exactly how much actual roof surface the ordered panels can cover.
Does this calculator work for arched or curved roofs?
No — this calculator assumes a single flat plane (or two symmetric planes for a gable, where you would run it twice). For barrel-vault or arched roofs, calculate the developed length of the arc separately, multiply by the building length, and treat that as the actual roof area. Use a structural engineer's section to size the panel thickness for curved spans.