# Waterproofing Area and Membrane Calculator

Total area with upstands, membrane rolls and primer.

- Page: https://jianzhujisuan.com/en/waterproof
- Category: Construction (https://jianzhujisuan.com/en/c/construction)
- Chinese version: https://jianzhujisuan.com/zh/waterproof.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Flat area 100 m², Upstand perimeter 40 m, Upstand height 0.3 m, Lap and waste 12%, Roll width 1.0 m, Roll length 10 m): **Total waterproofing area 112.00 m²**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Flat area | number | 100 | m² |  |
| Upstand perimeter | number | 40 | m | Wall bases, pipe collars, parapet upstands |
| Upstand height | number | 0.3 | m | 0.3 wet rooms, 1.8 shower walls, 0.25+ parapets |
| Lap and waste | number | 12 | % | 10 cm laps minimum, 15% on complex details |
| Roll width | number | 1.0 | m |  |
| Roll length | number | 10 | m |  |

## 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%

## Worked example

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.

## Questions

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