Leakage along a building expansion joint
An expansion joint is a deliberate gap that lets the structure move. It leaks when the sealing system has failed, or — very commonly — when someone has packed it with rigid mortar, which both defeats the joint and cracks the structure either side of it.
Not an emergency, but it will get worse and more expensive with every monsoon it survives.
Probable causes, ranked
Ordered by how often we find each one to be the actual source. The 'tell' is what distinguishes it from the others.
- 01most common
Sealant has aged, torn or debonded
The tell — A visible split along one face of the joint.
Expansion Joint Treatment → - 02common
Joint filled with rigid mortar during a previous repair
The tell — The joint is solid to probe, with fresh cracking in the structure beside it.
Expansion Joint Treatment → - 03common
Sealant movement capability under-specified for the joint
The tell — Failure recurs each year within a season of resealing.
Expansion Joint Treatment → - 04less common
Backer rod omitted, causing three-sided adhesion
The tell — Sealant has torn down its centre rather than at an edge.
SBS Membrane Waterproofing →
Sixty seconds, before you call anyone
- Photograph it while it is actively wet. A dried stain tells a surveyor far less than a running one.
- Note whether it worsens with rain or with usage. Rain-linked points to the building envelope; usage-linked points to plumbing. This single distinction settles most arguments about responsibility.
- Check the area directly above and beside the damage, not only below it. Water travels along a slab before it drops.
- Look specifically for the tell of the most likely cause: a visible split along one face of the joint.
- Do not chip into saturated plaster or paint over a damp patch. The first risks bringing material down; the second destroys the evidence needed to trace the path.
What it becomes if you leave it
A leaking joint runs the full height or length of a building, so water is delivered to many floors at once. Rigid filling is worse still: it transfers movement into the structure and cracks the concrete on both sides.
Often seen alongside this
- Visible crack in an RCC slab or beam
A crack in structural concrete is either dormant or live, and the difference decides everything.
- Podium leaking into the parking below
A podium deck carries landscaping and vehicles directly above occupied space, and its waterproofing sits buried under soil, paving and services.
Leakage along a building expansion joint — questions we get asked
What is the most likely cause of leakage along a building expansion joint?
Sealant has aged, torn or debonded. The sign that points to it is: a visible split along one face of the joint. That said, the ranking here is a starting point — the actual source is confirmed by survey, because more than one of these can be true at once.
What happens if I leave it?
A leaking joint runs the full height or length of a building, so water is delivered to many floors at once. Rigid filling is worse still: it transfers movement into the structure and cracks the concrete on both sides.
Can I diagnose this myself before calling anyone?
Partly, and it is worth doing. Photograph it while it is actively wet, note whether it worsens with rain or with usage, and check the area directly above and beside the damage. That single rain-versus-usage distinction narrows the cause more than anything else you can do in sixty seconds.
How urgent is this?
Act this season. Not an emergency, but it will get worse and more expensive with every monsoon it survives.
Send us a photograph and we will tell you what we think it is
WhatsApp a photo taken while it is wet, with a note on whether it tracks with rain or with usage. That is usually enough for a first opinion, and the site inspection that follows is free.
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