Published
Most complaints about damp basements come from choosing the wrong face, not the wrong product. On a below-grade retaining wall, water pushes from outside in. Applied to the face the pressure comes from, the waterproofing is pressed onto the substrate; applied to the inner face, that same pressure works to peel it off.
The positive side: the preferred solution
The positive side is the face water reaches first — the outer face of the retaining wall, against the soil. In new build, while the excavation is still open, that face is reachable and a two-component cement-based system can be applied.
The advantage is mechanical: water pressure holds the layer against the concrete. Even with a small weakness, the system keeps doing its job for a while longer.
The negative side: the only option in an existing building
In an existing building the outer face is usually unreachable for reasons of excavation, services and cost. Waterproofing then has to be done from the inside, and film-forming products such as liquid membranes fall short there: water pressure eventually separates the film from the surface.
This is where capillary crystalline products come in. Rather than forming a layer on the surface, they penetrate the concrete's capillary voids and crystallise in the presence of water. Those crystals block the voids and stop water inside the section. The waterproofing is now in the concrete itself, not on it — so pressure cannot peel it away.
With an active leak the order changes
If water is actively running from the wall, no permanent system will hold. The flow is first stopped with a rapid-set plug, which hardens within seconds against pressurised water.
Only then is the surface prepared and the permanent system applied. Skipping that order and starting straight in with the permanent product achieves nothing except washing the material away.
In short
Waterproof from the positive side where you can. Where you cannot, work with capillary crystalline products that act inside the concrete rather than with film-forming ones. In both cases surface preparation matters more than the product you picked.