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Roof Slope Layer Calculator

Average thickness and volume from span and slope.

Quick answer

With the default inputs: Slope span to gutter 12 · Design slope 2% · Minimum thickness 30 etc.Average thickness 150 mm

Edit the inputs below to recalculate.
Slope span to gutt mDesign slope %
Mean thickness = minimum thickness + span × slope / 2. Using the minimum loses about half the material — the classic error on slope layers.

Inputs

%
m²

Result

Average thickness
150mm
Rise over span240mm
Maximum thickness at gutter270mm
Volume15.00m³
Bulk density800kg/m³
Total mass12.00t
Added dead load1.18kN/m²

Using half the triangular section - minimum thickness plus span × slope ÷ 2 - is the standard take-off method. Two things are easily missed: the self-weight is significant (150 mm average perlite mortar is about 1.2 kN/m² and must be added to dead load), and the thickness at the gutter is nearly twice the average, so local slab loading and edge formwork must be checked against the maximum. Design slopes are typically 2-3% for structural falls.

Formula

Average = min thickness + span × slope ÷ 2slope in percent
Maximum = min + span × slopeat the gutter
Volume = area × average thicknessdivide 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.

How to work it out

  1. Measure the slope span: the horizontal distance from the ridge or high point to the gutter or outlet, not the sloped length. Where the gutter falls both ways, use one side.
  2. Fall = span × gradient. A 2% fall over 12 m gives 240 mm of rise.
  3. Average thickness = minimum thickness + fall ÷ 2. This is the core formula, because thickness varies linearly and the mean sits at mid-span.
  4. Volume = roof area × average thickness, with the area taken on plan to match the span convention.
  5. Convert to weight by density: cement mortar 20 kN/m³, expanded clay concrete 8-12, aerated concrete chips 6-8.

Where it goes wrong

Material and cost

Cement mortar slope with labour 30-55 CNY/m² at 50 mm average, expanded clay concrete 45-80, lightweight aggregate 40-70. Cement 400-500 CNY/t, expanded clay 200-350 CNY/m³.

On site

Basis

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.

Cite this page

Use this format in reports, method statements or coursework. The link is permanent.

Building Calculator. Roof Slope Layer Calculator. 2026-10-25. https://jianzhujisuan.com/en/roof-slope-layer

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.