Roof Slope Layer Calculator
Average thickness and volume from span and slope.
Inputs
Result
Formula
| Average = min thickness + span × slope ÷ 2 | slope in percent |
| Maximum = min + span × slope | at the gutter |
| Volume = area × average thickness | divide mm by 1000 |
| Added dead load = thickness × density |
12 m span at 2% with a 30 mm minimum over 100 m² of cement perlite: rise 240 mm, average thickness 150 mm, 270 mm at the gutter; 15.00 m³ of material weighing 12.00 t, adding 1.20 kN/m² of dead load.
FAQ
Why average thickness instead of the minimum?
The layer is a triangular wedge growing from the minimum to the gutter. Using the minimum understates material and weight by half, the maximum overstates by nearly double. The average (area of a triangle) is correct - and it is one of the most disputed items in roof settlement, especially on wide, steep roofs.
Structural fall or screed fall?
Structural fall slopes the slab itself - no screed, less weight, faster, but complex formwork and a sloping soffit, which suits long-span industrial and fair-faced work. Screed fall is simple and gives a level ceiling but adds 1-2 kN/m² of dead load. Residential and public buildings usually use screed fall for ceiling and level tolerances.
What goes wrong with a thick fall layer?
Three things: weight (150 mm of perlite is 1.2 kN/m², 250 mm is 2.0, possibly beyond the design allowance); moisture (perlite absorbs water, loses insulation value and blisters the membrane); and shrinkage cracking, which tears the waterproofing above. Long spans should prefer structural fall or add a dividing ridge to shorten the slope run.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.