What is waterproofing?
Waterproofing is the set of materials and measures that makes a building element impermeable to water, even when that water is under pressure. Typical examples are basements below the water table, flat roofs, wet rooms and water tanks. Waterproofing goes further than damp-proofing: a damp-proof element resists moisture, a waterproof element also resists standing or pressurised water.
Systems
- Bituminous membranes: APP- or SBS-modified sheets on roofs and against basement walls.
- Synthetic membranes: EPDM, PVC and similar sheets, loose-laid, bonded or mechanically fixed.
- Cementitious tanking: thick mortar or slurry coatings applied to concrete or masonry.
- Watertight concrete: a structure that is itself watertight thanks to a low water-cement ratio, good compaction, crack-width control reinforcement and sealed construction joints.
- Liquid-applied membranes: brush- or roller-applied coatings, widely used under tiles in bathrooms and on balconies.
- Injection: sealing cracks or joints afterwards with injection resin or injection hoses.
Which side to seal?
The best position is the water side (positive side): for a basement, that means the outside. Water pressure then pushes the membrane against the structure. A membrane on the inside (negative side) must resist the water pressure on its own and can debond; it is mainly a repair option. In the UK, BS 8102 sets out the design approach for below-ground waterproofing.
Common mistakes
- Forgetting the details: leaks almost always occur at penetrations, corners, construction joints and material transitions, not in the field.
- Poor substrate preparation: dust, wet surfaces or sharp protrusions cause debonding or punctures.
- Relying on silicone in a shower: sealant is maintenance, not waterproofing. Fit a tanking system under the tiles, with reinforcing tape in corners and collars around pipes.
- Mixing systems: not every material is compatible. Use one system from one manufacturer.
Related terms
- Bitumen
- Damp-proof course
- Injection hose
- Groundwater level
- Hydrophobizing
- Water-cement ratio
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