PU injection grouting and surface waterproofing are often presented as alternatives. They are usually complementary. Each method addresses a different defect class. Picking the wrong one — or picking only one when both are needed — is a common reason a "repaired" leak comes back.
What PU injection does
PU injection treats a specific defect: an active leak through a crack, joint or pipe penetration. Resin is pumped into the substrate through mechanical packers, reacts with water, expands and seals the crack from inside. It does not waterproof a wall area — it stops a leak at a point.
What surface waterproofing does
Surface waterproofing — cementitious, acrylic, PU or bituminous — coats the wall or slab to keep water off the substrate from outside (or the dry side, in negative-side tanking). It protects a substrate; it does not seal a structural defect through which water is already flowing.
When to use each
- Active leak through a single visible crack: PU injection
- General fatigue across an external wall, hairline cracking, paint chalking: surface waterproofing
- Roof slab with no visible cracks but background dampness below: surface waterproofing + crystalline
- Basement wall with active drip and damp wall around it: PU injection at the leak + cementitious tanking afterwards
- Construction joint between pours showing seepage: PU injection at the joint, hydrophilic strip in new joints
Why surface coating alone fails on an active leak
A surface coating cures by reacting with air, by drying or by chemical cross-link. Applying it over a wet, leaking crack interrupts the cure — the coating either does not bond, or bonds but is then blown off by hydrostatic pressure. The leak shows back through the coating within days or weeks.
Why PU injection alone is not enough on a fatigued wall
Injection stops one crack. If the wall is generally fatigued — multiple cracks, failed paint, weathered substrate — sealing one crack pushes the leak to the next one. A surface waterproofing system is needed to control the broader exposure.
Combined approach
Most real jobs combine both. Inject the active points first to stop flow. Repair substrate where needed. Then apply the surface system to control the wider exposure. The sequence matters — surface waterproofing applied before injection traps water behind the coating.