Foundation waterproofing has two fundamentally different scenarios, and confusing them is expensive. On a construction site, while the pit is open, water is stopped from outside — before it reaches the concrete. In an existing building there is no longer any access from outside, and the work has to be done from inside, against the water pressure. We go through both cases: what is done, why it works and where the limits are.
Where the water in a basement comes from
The soil around a foundation is a sponge. After rain and snowmelt, water collects at the walls and presses on them; if the groundwater level is high, it presses constantly. Concrete is not watertight: it has capillaries, cold joints between pours, shrinkage cracks, pipe entries. That is where the water comes through: first as damp and white efflorescence low on the walls, then as puddles on the floor of the lowest level.
Scenario 1. Construction site: from outside, before backfill
This is the only moment when the outer face of the wall is accessible. What we do:
Preparation. The concrete must have gained strength; the surface must be dry, free of voids, fins and sharp edges. Corners and wall-to-slab transitions are rounded with a fillet.
Primer. A primer on the concrete for membrane adhesion.
Spray-applied bitumen-rubber membrane ('liquid rubber'). Applied by cold airless spraying in two coats over the walls and under the slab, carried up above ground level. The result is a seamless elastic layer that closes capillary cracks and stretches with the concrete. Where water presses and at cold joints — reinforcement with geotextile or glass-fibre mesh.
Swelling waterstops. Laid in the wall-to-slab cold joints and between pour sections before concreting. On contact with water the material expands and seals the joint from inside the concrete.
Drainage membrane. A dimpled sheet over the waterproofing: protects it from damage during backfill and leads water down to the drain.
Perimeter drain. A pipe in a gravel surround at the foundation footing, discharging to a manhole or the storm sewer. It takes the water pressure off the structure — the waterproofing works as intended instead of holding back a column of water.
Only after that — backfill. If backfilling was done before the waterproofing, the only way back to the wall is to dig it out again, and in dense urban development that is often impossible.
Scenario 2. Existing building: from inside
If the basement is already wet and there is no access from outside, we work from the inner side. The task is harder: the material has to hold back water pressing from the far side of the wall — so-called negative-side pressure. Ordinary coatings and paints are not designed for this: the water simply pulls them off the concrete.
Stop active leaks. Where water flows in a stream or seeps through — a fast-setting water plug. Cracks and joints — injection of polyurethane or acrylate compounds that fill the cavity and expand inside it.
Crystalline compounds. Applied to damp concrete; the active components penetrate the capillaries and form crystals in them that block the path of the water. It works within the body of the concrete, not on the surface, so pressure is no threat to it.
Flexible cementitious compounds. Where flexibility is needed — at transitions, around entries, over cracks after injection.
Service entries. Every pipe entry is a separate detail: cleaning out, a swelling waterstop or injection around the perimeter, sealing.
What cannot be solved from inside
Honestly: working from inside is a repair, not a replacement for external waterproofing.
- Moving cracks and expansion joints. No compound on its own will bridge a joint that moves. That needs a separate detail with injection and a flexible tape.
- The water pressure remains. There is no drain outside, and the water keeps pressing on the structure. The waterproofing holds, but the concrete works under harsher conditions.
- Finishes over damp walls. Plaster, plasterboard and paint applied before the water is stopped will come off. Waterproofing first, finishes after.
Typical mistakes
- Bitumen mastic on the basement floor from inside: the water pulls it off within a season.
- Backfilling straight after the walls are cast — 'we'll do the waterproofing later'.
- Waterproofing without drainage where the groundwater level is high.
- The outer layer left unprotected: without a drainage membrane or protection board, crushed stone punctures it during backfill.
- Pipe entries left 'as they are' — these leak most often of all.
When to call an engineer
On a construction site — before backfill: the engineer will look at the pit, the walls and the joints and say what needs to be done from outside while it is still possible. In an existing building — as soon as efflorescence or damp appears low on the walls: the earlier, the smaller the scope. Inspection — 100 ₾, deducted from the estimate if you order the work.