Build-up and materials
The waterproofing sits on the side the water comes from: outside, between soil and structure. Under the floor slab it is laid before the concrete is poured; on the walls after the formwork comes off. DIN 18533 covers the materials in separate parts:
- Part 2 – sheet materials: bitumen and polymer-bitumen membranes, often torch-applied, plus plastic and elastomer sheets.
- Part 3 – liquid-applied materials: including polymer-modified bitumen thick coatings (formerly KMB, now PMBC) and mineral sealing slurries.
Before the excavation is backfilled, a protective layer goes on top, often perimeter insulation or a dimpled membrane. It stops stones from puncturing the seal during compaction.
Which waterproofing for which water load
How elaborate the black tank has to be depends on how water acts on the structure. DIN 18533 distinguishes four classes; since the 2026 edition they are called water stress classes (Wasserbeanspruchungsklassen) instead of water exposure classes:
- W1-E: soil moisture and non-pressing water
- W2-E: pressing water, split into W2.1-E (up to 3 m immersion depth) and W2.2-E (more than 3 m)
- W3-E: non-pressing water on soil-covered slabs
- W4-E: splash water and soil moisture at the wall base, plus capillary water in and under walls
Which class applies follows from the subsoil and the design water level – ideally from a geotechnical report. With pressing water, the requirements for material, layer thickness and connections rise sharply. Waterproofing planned for W1-E is not designed for pressing groundwater.
Strengths and weak points
Strengths
- Works on masonry too – so for basements not built from watertight concrete
- Crack-bridging materials stay sealed when fine cracks form in the substrate
- Can be applied to complex floor plans and retrofitted to existing buildings
Weak points
- Invisible once backfilled; damage only shows on the inside
- Stones during backfilling, roots or settlement can damage it
- Defects are hard to locate, because water can travel between seal and wall and often comes out somewhere other than where it gets in
- Transitions are critical: wall base, pipe penetrations, light wells, the joint between slab and wall
Black or white tank?
In new builds with pressing groundwater, a white tank of watertight concrete is often planned, because structure and waterproofing are one. A black tank is the obvious choice for masonry basements, existing buildings and rooms meant for high-quality use where an extra sealing layer is wanted. Combinations are common too, such as watertight concrete with additional external waterproofing. Which solution fits should be decided by a structural engineer or expert on the basis of the geotechnical report – not by price alone.
What you can check as an owner
- Damp patches at the base of walls, in corners and around pipe penetrations – often first after long spells of rain.
- Salt efflorescence and flaking plaster show water travelling through the wall.
- Light wells and wall base: are the wells sealed tightly to the wall, and does rainwater run away from the house rather than towards it?
- Paperwork: building file, waterproofing plan and the assumed water class. Without them it is hard to judge whether the waterproofing matches the actual water level.
If you find wet spots, a moisture survey by an expert clarifies where the water comes from: water penetrating from outside, rising damp or condensation. The repair depends on it – more under basement waterproofing.
Insurance and monitoring
Water that gets in through damaged waterproofing is usually not insured. According to the GDV, natural hazard insurance does not cover rising groundwater that penetrates the masonry; groundwater is only covered if it rises to the surface and enters the house from there. The Verbraucherzentrale likewise points out that owners are liable themselves for damp damage caused by high groundwater.
If there is a drainage system with a pump sump in front of the wall, Drainbee records level, pump cycles and inflow there. If noticeably more water flows in after comparable rainfall than it used to, that is a reason to look more closely. Drainbee does not measure whether the waterproofing itself is intact – it does not detect damp in the wall or water on the basement floor.