Waterproofing Area and Membrane Calculator
Total area with upstands, membrane rolls and primer.
Quick answer
With the default inputs: Flat area 100 m² · Upstand perimeter 40 · Upstand height 0.3 etc.Total waterproofing area 112.00 m²
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Result
Total waterproofing area
112.00m²
| Flat area | 100.00m² |
| Upstand area | 12.00m² |
| Upstand share | 10.7% |
| Membrane with laps | 125.44m² |
| Area per roll | 10.00m² |
| Rolls to order | 13rolls |
| Primer at 0.3 kg/m² | 33.6kg |
| Detail membrane (10%) | 11.20m² |
| Total with details | 137.98m² |
Upstand heights vary by location: 300 mm minimum at wet-room wall bases, 1800 mm in showers, and 250 mm or more at roof parapets, tucked into a groove. Laps are not optional — 100 mm along the length and 150 mm across, which works out at roughly 8-12%, so 12% is a fair allowance. Add detail layers at internal and external corners, pipe collars and outlets.
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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.
How to work it out
- Establish the flat waterproofing area, then add every upstand: perimeter of each upstand × its height.
- Upstand heights follow the location: 1.8 m in shower areas, 0.3 m on other walls, at least 250 mm up parapets on roofs, and above external ground level on basement walls.
- Total = flat area + upstand area. Add 10-15% for reinforcement layers at corners, pipe penetrations and outlets, which are the most labour-intensive parts.
- Rolls = total × (1 + lap allowance) ÷ roll area. Laps run 80-100 mm on long edges and 100-150 mm on short edges, giving 10-15% overall.
- Primer at 0.2-0.3 kg/m², or for liquid membranes thickness × rate at about 1.5-1.8 kg/m² per millimetre.
Where it goes wrong
- Ignoring upstands — a 3.6 m² bathroom with 1.8 m walls is over 15 m² of membrane, so leaving out the upstand undercounts threefold and the job runs out.
- Allowing 5% for laps — laps are 80-150 mm, which on a 1 m roll is 10%, before any cutting at corners.
- Ignoring reinforcement layers — corners, pipe collars and outlets each need 300-500 mm wide reinforcement, and a single bathroom has over a dozen such details.
- Counting one layer where two are specified — roofs commonly get two layers of SBS, and the quantity must double.
Material and cost
3 mm SBS membrane 25-45 CNY/m², self-adhesive 30-55, polyurethane liquid at 1.5 mm 25-50, JS cementitious 15-30. Supply and apply with preparation and detailing 45-90 CNY/m².
On site
- The substrate must be dry (below 9% moisture), flat (5 mm under a 2 m straightedge) and dust free — poor substrate causes most waterproofing failures.
- Terminate membrane edges with a clamped batten and sealant; lifting edges are the commonest leak path.
- Flood test every job: 24 hours of ponding at least 20 mm deep in bathrooms, 24 hours ponding or 2 hours hosing on roofs, before acceptance.
Basis
- GB 50345-2012 Technical code for roof engineering, Chapter 5 (waterproofing)
- GB 50207-2012 Code for acceptance of roof works, Chapters 5 and 6
- GB 50208-2011 Code for acceptance of underground waterproofing works, Chapter 4
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Waterproofing Area and Membrane Calculator. 2026-10-25. https://jianzhujisuan.com/en/waterproof
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.