Basement & Carpark Leak Detection

Two completely different problems wear the same stain.

Water on a carpark wall or soffit comes from one of two places, and telling them apart decides everything that follows.

From the ground around it. Groundwater under hydrostatic pressure, pushing through the wall from outside. The waterproofing is on the external face — buried, inaccessible, and installed before the backfill went in. This is a positive-side system failing, repaired from the negative side.

From the structure above it. A podium deck, planter, driveway or landscaped area on top of the carpark, where the membrane has failed and water is arriving from overhead rather than sideways. This looks identical on the ceiling and is an entirely different repair.

Efflorescence and stalactites on a soffit are the classic sign of the second, and the classic thing to be blamed on the first.

Where below-ground structures leak

• Construction joints — the horizontal line where one pour met the next. Where the waterstop was displaced, damaged or omitted, the joint is a continuous path and the leak runs along it.
• Form tie holes — every tie through a wall left a hole. Where they were not properly plugged, each one weeps individually — a line of small, evenly spaced damp spots is diagnostic.
• Shrinkage and structural cracks — concrete cracks as it cures and as it moves. Below ground, a crack under head pressure is an active leak rather than a cosmetic one.
• Service penetrations — pipes, conduits and drainage entering the wall, each one a break in the waterproofing.
• The podium or driveway membrane above — failed membrane, blocked drainage cell, or a planter box that has been leaking into the slab for years.
• Drainage that has stopped working — subsoil drains silted up, agricultural drains crushed, sumps or pumps failed. Water that used to be carried away is now standing against the wall.
• Lift pits and stair cores — the lowest points of the structure, and therefore where water collects. Also the most expensive place to discover it late.

How we work out which it is

Repairs from the inside

You cannot re-waterproof the outside of a basement wall without excavating to it, which on a completed building is rarely feasible. So below-ground repairs are made from the negative side — sealing the entry point against the pressure trying to push water through it.

In practice that means resin injection into cracks and joints, hydrophilic and hydrophobic grouts selected to suit whether the leak is active, cementitious crystalline systems that grow into the concrete matrix, and re-establishing the drainage that should have been carrying the water away in the first place.

Where the source turns out to be above rather than beside, the repair is a conventional membrane job on the podium or planter — done properly, once, rather than injected repeatedly from underneath for years.

Why leaving it is expensive

Water moving through concrete carries dissolved salts out with it, which is what the white staining is. The same water reaches the reinforcement, and chloride-laden water in a coastal environment corrodes it. Corroding steel expands, and expanding steel spalls the concrete off the face of the slab.

At that point the problem has changed from waterproofing to concrete repair, and the cost has changed by an order of magnitude. A carpark soffit that is weeping now is cheaper than the same soffit in five years, by a wide margin.

Why the stain is rarely under the defect →