# Roof Slope Layer Calculator

Average thickness and volume from span and slope.

- Page: https://jianzhujisuan.com/en/roof-slope-layer
- Category: Temporary Works (https://jianzhujisuan.com/en/c/temporary-works)
- Chinese version: https://jianzhujisuan.com/zh/roof-slope-layer.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Slope span to gutter 12 m, Design slope 2%, Minimum thickness 30 mm, Roof area 100 m², Material Cement perlite (800 kg/m³)): **Average thickness 150 mm**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Slope span to gutter | number | 12 | m |  |
| Design slope | number | 2 | % |  |
| Minimum thickness | number | 30 | mm |  |
| Roof area | number | 100 | m² |  |
| Material | select | Cement perlite (800 kg/m³) | — |  |

## 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**: 

## Worked example

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.

## Questions

### 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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Generated by BuildCalc. All calculations run in your browser; no data is uploaded. Formulas follow national standards and common trade practice; local codes govern.