A wall-floor joint is the junction between the floor and the rising wall — in effect the joint where those two elements meet. It is one of the most common leak points in a building, because two elements never meet completely seamlessly and small voids always remain.
Where water stands under pressure on one side, that shows up straight away: in a basement groundwater pushes in through the joint, and in a bathroom splash water runs behind the tiles at the joint.
Where the joint causes trouble
| Situation | What happens |
|---|---|
| Basement | Groundwater is forced in through the joint between slab and wall |
| Bathroom and shower | Splash and leak water runs behind the tiles and into the substrate |
| Crawl space and garage | Damp and rising water at the wall foot, with mould and salt efflorescence |
| Concrete work | The construction joint between slab and wall is never monolithic and stays a weak line |
How the joint is sealed
In new concrete work — the seal is cast in: a metal waterstop, a swelling strip or an injection hose is built into the construction joint.
In a bathroom — the substrate is cleaned and primed, then a waterproof membrane (liquid-applied or with sealing tape) is taken over the joint, running continuously from floor to wall. Only then are the tiles fixed. Sealant along the top is not waterproofing: sealant is the final visible joint, not the water barrier.
In an existing leaking basement — the joint is raked out and closed with a rapid-setting waterproof mortar, or injected from inside: drill into the joint, fill with injection resin or gel, then finish with a waterproof mortar.
Points to watch
- Carry the detail through — treat vertical joints and service penetrations the same way; water finds the weakest spot
- Look for the cause — in a basement a high water table or a failed drain is often the real problem rather than the joint itself
- Sealing from inside works against the water pressure — it can be done, but only with products tested for it; stay within one supplier’s system and do not mix brands
- Under the tiles, not on them — the membrane belongs directly on the substrate, behind the tile build-up
Related terms
- Levelling course
- Metal waterstop
- Kicker formwork
- Basement
- Sealant
- Damp-proof course
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