Identify the source first
Water entering a basement has four typical sources, and each calls for a different intervention. Work carried out on the wrong diagnosis spends the money without solving the problem.
1. Groundwater
When the water table rises above foundation level it applies pressure to the retaining wall. The sign: damp that is seasonal and continuous, independent of rainfall. It concentrates at the lower levels of the wall, and seepage appears at the floor to wall junction. The solution is external drainage and waterproofing.
2. Surface water
Rainwater pooling around the building and soaking into the ground. The sign: wetness seen only after heavy rain that disappears within a few days. The fix is usually simple: grading the ground away from the building, extending downpipes, and perimeter drainage.
3. Plumbing leak
The sign: wetness at the same spot all year round, independent of season and rainfall. An unexplained rise in the water bill is a strong clue. It is located with a thermal camera or a pressure test.
4. Condensation
Warm, humid air condensing on a cold wall surface. The sign: droplets on the wall surface while the wall itself is dry. It increases in summer. The solution is not waterproofing but ventilation and thermal insulation. Applying a membrane without making this distinction produces no result.
Diagnostic methods
- Moisture meter: tells you whether the surface or the body of the wall is wet
- Thermal camera: makes plumbing leaks and cold bridges visible
- Geotechnical survey report: gives the groundwater level
- Foil test: if the inner face of plastic sheeting taped to the wall sweats, it is condensation; if the outer face gets wet, the water is coming through the wall
Inside or outside?
External application is always more durable because it stops water before it enters the structure. Where excavation is impossible, negative side waterproofing or polyurethane injection is applied from inside. An internal solution leaves the water inside the wall, so the risk of reinforcement corrosion continues and shorter inspection intervals are needed.
The cost of delay
Persistent damp does more than ruin plaster and paint. Once it reaches the reinforcing steel, corrosion begins and the steel cross section is reduced. Past that point the problem stops being a waterproofing job and becomes a structural strengthening job, and the cost multiplies.