Why liquid PU waterproofing fails: the common causes

01Before: the substrate

• MoistureWater in the concrete turns to vapour as the surface warms and pushes up through the curing film: bubbles, pinholes and a poor bond. Moisture is checked before priming — for example with the plastic-sheet method, ASTM D4263 — against the limits in the TDS.
• Laitance and contaminationA weak cement skin, dust, oil or curing compound under the primer means the membrane bonds to the contaminant, not to the concrete.
• Wrong or missing primerThe primer is chosen for the substrate — concrete, old tiles or metal — and its moisture; skipping it or swapping it is a common cause of delamination.
• Sharp corners and cracksUntreated cracks and sharp internal corners concentrate strain on the membrane; fillets and crack treatment come first.

02During application

• Too thinSpreading the material too far gives a thin film that tears, wears and lets water through. Wet-film thickness is measured during the work and consumption is tracked per area.
• One heavy coatA single heavy coat traps gases and cures badly; the thickness is built up in the number of coats the system specifies.
• Missing the recoat windowToo early and the coats mix; too late and the next one does not bond. The window comes from the TDS and the weather.
• WeatherHigh humidity, dew, heat and direct sun on a hot substrate change the cure; work stops when conditions fall outside the TDS limits.
• Unreinforced detailsDrains, upstands, corners and penetrations need reinforcing fabric and extra build-up — that is where failures cluster.

03After: in service

• No UV protectionAromatic polyurethane chalks and degrades in sunlight; exposed systems need the aliphatic top coat their system specifies.
• Traffic and later worksFoot traffic, AC installation and fixings puncture an unprotected membrane; maintenance routes and later works need protection and detailing.
• PondingStanding water on poor falls keeps the membrane wet and dirty and finds every weak point.
• Incompatible overlaysTiles, adhesives and coatings over the membrane must be compatible with it.

04Reading the symptoms

05What prevents it

1. Moisture readings recorded before priming.
2. A primer matched to the substrate, per the TDS.
3. Wet-film gauge readings and consumption recorded per area.
4. Reinforced details and fillets at corners.
5. A top coat wherever the system stays exposed.
6. A flood test before protection or tiling (ASTM D5957 is the usual guide).
7. Protection of the finished membrane from other trades.