Waterproofing
Quick answer: A 2.5 × 2 m bathroom with a 1.8 × 2 m shower zone needs 9.56 m² of waterproofing: about 15.8 kg of liquid membrane (0.5 mm dry + 25 %, with a 10 % reserve), 11.2 m of sealing tape and 4 corner pieces.
Coatings and membranes for bathrooms, balconies, foundations, roofs, shafts and pools
Next step
How the calculation works
Choose the application (bathroom, balcony, foundation slab, foundation walls, basement from inside, flat roof, shaft, pool or a manual area) and the system type. Enter the dimensions – the calculator works out the area by parts, primer, the main material from the minimum dry thickness or number of coats, sealing tape, corner pieces, collars, reinforcing mesh and protection, plus packs or rolls.
Formula and assumptions
Coatings specified by thickness: quantity = area × consumption per mm of dry thickness × planned thickness × (1 + reserve), where planned thickness = minimum dry thickness × (1 + allowance for unevenness, usually 25 %). Crystalline and cold bitumen coatings: area × consumption per coat × number of coats. Membranes: rolls = area × layers × (1 + reserve) ÷ useful roll area, where useful area = (1 m − side lap) × (roll length − end lap).
Worked example
Bathroom 2.5 × 2 m, shower zone 1.8 × 2 m, door 0.8 m: floor 5 m², shower zone 3.6 m² and a strip along the floor outside the shower 0.96 m² = 9.56 m². Liquid membrane: at least 0.5 mm dry + 25 % = 0.625 mm × 2.4 kg/m² per mm = 1.5 kg/m²; with a 10 % reserve 15.77 kg (one 20 kg pail). Primer 1.58 kg, sealing tape 11.22 m (2 rolls of 10 m), 4 internal corner pieces, 2 pipe collars and 1 drain collar.
More worked examples
- Foundation walls, perimeter 36 m, 2.5 m below ground and 0.3 m above = 100.8 m². PMBC for W1-E: 3 mm + 25 % = 3.75 mm × 1.3 L/m² per mm = 4.875 L/m², with a 10 % reserve 540.54 L (18 × 30 L + 1 × 10 L); bitumen primer 22.18 L, dimpled membrane 99 m² and top strip 39.6 m.
- Flat roof 100 m² with 2 % slope and a 40 m × 0.15 m parapet upstand = 106.02 m². Two layers of bitumen membrane with 10 and 15 cm laps: a 1 × 10 m roll covers 8.865 m², so 233.24 m² with a 10 % reserve needs 27 rolls.
- Round shaft, 1 m diameter and 2 m deep, inside with base: walls 6.28 m² + base 0.79 m² = 7.07 m². 2-component cementitious coating 2 mm + 25 % = 2.5 mm × 1.7 = 4.25 kg/m², with reserve 33.05 kg.
Consumption per m² from the minimum dry thickness (with 25 % allowance, no reserve)
| System and class | Minimum dry (mm) | Planned (mm) | Consumption per m² |
|---|---|---|---|
| Liquid membrane, W1-I | 0.5 | 0.625 | 1.5 kg |
| Cementitious 2C, W1-I | 2 | 2.5 | 4.25 kg |
| Polyurethane membrane | 1.5 | 1.875 | 2.81 kg |
| PMBC, W1-E | 3 | 3.75 | 4.88 L |
| PMBC with mesh, W2.1-E | 4 | 5 | 6.5 L |
| Cementitious 2C, W2.1-E | 3 | 3.75 | 6.38 kg |
What affects real consumption
- Roughness, absorbency and unevenness of the substrate (hence the thickness allowance).
- Application method (trowel, roller, spray) and mixing losses.
- Number of corners, penetrations and joints that need tape, collars and extra coats.
Common mistakes
- Calculating with the minimum dry thickness only, without an allowance for unevenness.
- Forgetting tape at the floor–wall joint and in corners, and collars around pipes and drains.
- Bitumen coating on a wet or frozen substrate, and backfilling before it has dried.
- Dispersion membrane or bitumen on the inside of a basement (negative pressure).
Frequently asked questions
How many kilograms of waterproofing do I need per m²?
It depends on the system and thickness: a liquid membrane for 0.5 mm dry takes about 1.2–1.5 kg/m² in two coats, a 2-component cementitious coating for 2 mm dry about 3.4 kg/m² (about 4.25 kg/m² with a 25 % allowance), and a thick bitumen coating for 3 mm dry about 4–5 L/m².
How many coats of waterproofing are needed?
Coatings go on in at least two coats; the second only once the first has dried enough. The calculator sets the number of coats from the thickness, and for membranes two layers with staggered joints are usual on roofs.
How much should bitumen membranes overlap?
Usually 8–10 cm at the sides and 10–15 cm at the ends. Laps cut the useful area of a 10 m² roll to about 8.9 m², so you need more rolls than the area alone suggests.
Can a basement be waterproofed from inside?
From inside, water pushes the coating off the substrate (negative pressure), so only mineral systems are suitable – crystalline or cementitious coating on a sound cement substrate. It is a repair; the lasting solution is waterproofing from outside.
What are water exposure classes?
Standards for wet rooms (W0-I to W3-I) and building parts in contact with the ground (W1-E to W4-E) grade the water load. The calculator uses them only as guidance for the minimum thickness; water under pressure needs a design.
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Publisher: Art-Visum d.o.o. · Content reviewed:
Calculations use common consumption rates and manufacturers' technical data sheets; results are estimates. Methodology · Standards · About