Question: Water appears on my ceiling only during heavy monsoon rain, tracing a straight line under the slab above. Traditional waterproofing already failed. What actually seals a slab crack?
Answer: A rain-only drip along a straight line is usually a shrinkage or movement crack in the concrete slab, not a bathroom membrane and not a roof tile. High-pressure PU grouting from below fills the crack and stays flexible as the slab moves. Published guidance is that PU foam typically lasts 8 to 15 years and carries up to a 5-year written warranty. Most crack projects finish in two to four hours. A top-side acrylic membrane is added only if that surface is accessible.
This is a linear, rain-only slab crack. A bathroom that drips when someone showers is leaking bathroom. A whole condo balcony deck is balcony leak condo.
Why Slabs Crack and Then Leak
- Shrinkage or structural movement in the slab.
- Movement between building components at a construction joint.
- Loss of surface waterproofing over decades.
- Poor drainage so water ponds on the slab during a downpour.
How to Recognise This Fault
| Sign | Points to a slab crack |
|---|---|
| Drip only during rain | External water on the slab, not a pipe |
| Stain follows a straight line | Crack path, not a splash zone |
| No one is using the bathroom above | Not a wet-area membrane |
| Previous “paint-on waterproofing” failed | The crack is still open |
What PU Grouting Involves
- Locate the crack path from below (moisture meter / thermal imaging where useful).
- Drill injection ports along the crack.
- Inject high-pressure PU until the crack is filled.
- Reseal the top surface with an acrylic membrane if that side is accessible.
Injection is done from below, which is why the method is used for inter-floor leaks between condo units. Waterproofing carries the published Up to 5-Year Written Leak-Free Guarantee.
Costs in KL and Selangor
Published guidance is RM 650 to RM 3,800+ depending on crack length. KL Servis Rumah publishes PU high-pressure injection grouting from RM 300 per point and roof and concrete slab waterproofing on quote.
Compare approaches in PU grouting vs full membrane and use the waterproofing cost calculator.
Related Problems
- Condo balcony leaking below — whole deck, often with a JMB notice.
- Ceiling leak after rain — finding which envelope failed.
- Roof leak in the rainy season — roof covering, not the floor slab.
Frequently Asked Questions
Is PU grouting a permanent fix?
PU foam is flexible and typically lasts 8 to 15 years because it moves with the slab. Written cover is up to 5 years.
Can you grout without accessing the top of the slab?
Yes. Injection is done from below.
How long does it take?
Most concrete slab crack projects finish in 2 to 4 hours — same day, no tile hack, immediate curing.
Seal the Crack, Don’t Just Paint the Stain
If the drip only appears in a downpour and follows a line, treat the crack — not the paint. KL Servis Rumah provides waterproofing across Kuala Lumpur and Selangor.
Send a photo of the stain line during rain — WhatsApp our waterproofing team.