Waterproofing Area and Membrane Calculator
Total area with upstands, membrane rolls and primer.
Inputs
Result
Formula
| Upstand area | perimeter × height |
| Total area | flat area + upstands |
| With laps | total × (1 + lap %) |
| Per roll | width × length |
| Rolls | ceil(quantity ÷ per roll) |
| Details | total × 10% |
100 m² flat with 40 m of upstand at 0.3 m, 12% laps and 1 × 10 m rolls: 12.00 m² of upstand, 112.00 m² total, 125.44 m² with laps, 13 rolls and 33.6 kg of primer.
FAQ
Why does waterproofing always come out larger than expected?
Because upstands and details get forgotten. A 100 m² wet room adds a dozen square metres at 300 mm wall bases, doubling where shower walls go to 1.8 m, and corner, collar and outlet details add more — typically 30-50% over the flat area. Quoting from floor area alone loses money.
How much lap is needed?
By material: SBS membrane needs at least 100 mm along the length and 150 mm across; self-adhesive can be less but not under 80 mm; liquid membranes have no laps but need an extra coat. Laps are where leaks start, so codes also require them staggered — end laps of adjacent rolls at least 500 mm apart, never forming a continuous cross joint.
How is waterproofing inspected?
With a flood or spray test. Wet rooms are flooded 20 mm deep for 24 hours, 48 under some codes, and checked from below for leaks; roofs get a two-hour spray or 24-hour flood. Critically, test after the membrane and before the finish — once tiling is done the leak path is hidden and repair means breaking tiles.
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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.