Concrete Slab Cracks Leaking During Heavy Rain — Causes, Fixes & Cost in KL
Water appears on the ceiling only during heavy monsoon rains, tracing a specific crack line in the slab above.
Estimated professional fix: RM 650–RM 3,800+ depending on crack length
A ceiling stain that only appears during heavy monsoon rain, tracing a straight line in the slab above, is the classic signature of a leaking concrete slab crack. The crack is usually a shrinkage or movement crack in the concrete — small, often under 3 mm, and structurally unremarkable — but it is a direct channel from the roof or the slab surface to the ceiling below, and surface waterproofing that has aged out over decades cannot close it. The reason it only leaks in heavy rain is that light rain soaks into the concrete's pores and dries out, while a sustained downpour fills the crack channel faster than the surrounding slab can absorb it. The modern fix does not require accessing the roof: PU foam is injected under pressure from below through small drilled ports, expanding to fill the crack completely and flexing with the slab for the next 8–15 years.
Common causes
Professional solutions
Safe DIY checks first
- 1During a heavy rain, mark the drip line with masking tape — the straightness of the line is what separates a slab crack from a random pipe leak
- 2Check the roof or the slab surface above (if accessible) for the same crack line — a visible crack plus the drip line confirms the diagnosis
- 3Note the width of any accessible crack: under 3 mm is typical injection territory; wider structural cracks need a structural engineer first
- 4Check whether the leak has worsened year on year — a growing stain means the crack channel is widening or the surface seal is failing
When to call a pro
What affects the cost
PU grouting of a slab crack is priced by crack length and access, and a typical monsoon leak runs RM650–RM1,800 for a single line, rising toward RM3,800+ for multiple lines or difficult access. The job takes 2–4 hours, is done from below with no roof access needed, cures the same day, and carries a written warranty of up to 5 years. Where the surface above is accessible, we can also reseal it with an acrylic membrane in the same visit for longer-term cover. We confirm the crack line by tracing the drip before quoting.
How to prevent it
Related service
This problem usually connects to Waterproofing & Leak Repair Services. Review service scope, pricing, warranty, and process before booking.
View Waterproofing & Leak Repair ServicesFAQs
Is PU grouting a permanent fix for slab cracks?
PU foam is flexible and lasts 8 to 15 years typically — it flexes with the slab so it doesn't crack again. We provide up to 5-year written warranty.
Can you grout without accessing the top of the slab?
Yes. Injection is done from below, which is why PU grouting is popular for inter-floor leaks between condo units.
How long does PU grouting take?
Most concrete slab crack projects finish in 2 to 4 hours — same day, no mess, immediate curing.