Waterproofing Membrane Quantity Calculator

How are waterproofing quantities worked out?

First find the total area to waterproof. Add to the horizontal area the strip where the waterproofing turns up at walls and parapets: total area = horizontal area + edge length × upturn height. Then multiply by the number of layers and the waste allowance.

Sheet membrane

Membrane rolls are laid with overlaps, so one roll covers less than its own area: net area = (width − side lap) × (length − end lap). A 1 m × 10 m roll with a 10 cm side lap and a 15 cm end lap covers 8.865 m². The number of rolls is the area to cover divided by this net area, rounded up.

Liquid applied membrane

For liquid products the manufacturer gives consumption per coat in kg/m². The calculation is mass = area × coats × consumption × (1 + waste), and the result is divided by the pack weight.

Example

A 120 m² roof terrace with a 30 cm upturn along 46 m of parapet has a total area of 133.8 m². Two layers with 5 percent waste mean 280.98 m² of membrane, which is 32 rolls. The same area with a liquid membrane at 1 kg/m² per coat needs 281 kg, or 15 packs of 20 kg.

Limits

Lap sizes and consumption depend on the product, and the values here are approximate starting points. Corner reinforcement, drain details, primer and protection layers are calculated separately.