How do I fix damp patches on my ceiling in Hong Kong

I live on the top floor of my building and damp patches appear on the ceiling after heavy rain. How do I repair or reseal the roof-top?

If safe, get up on the roof and check for any cracked or damaged areas where water could be getting in or concave areas where water is building up. If you're fortunate enough to isolate areas that correspond with the damp patch indoors, remove the damaged surface with a scraper or wire brush and allow the area to dry out completely before patching it.

If the roof surface is concrete you can use quick dry cement ($25 per kilogram) which comes in powder form that you mix with water. It dries very quickly, even in damp conditions, so make sure the surface is clean and ready to be patched up before you begin. If the roof is topped with a bitumen Layer you can match up with black bitumen emulsion ($340 per 20 litres).

If you can't find the source of the problem, you could try coating the entire surface with Kangaroo Waterproofing Membrane, available in grey or white.

To avoid a sore back, apply with a roller attached to an extension pole. All materials available from Yuen Fat Ho tel 2546 0201.

What Causes Ceiling Damp in Hong Kong Apartments?

Living on the top floor is not the only reason you get ceiling damp in Hong Kong. In my nearly 30 years working in flats across the territory, I have seen ceiling damp caused by all sorts of things. Here are the most common culprits:

• Upstairs neighbour leaks — This is by far the most common cause if you are not on the top floor. A leaking shower tray, cracked bathroom tiles, or faulty waste pipe in the flat above will send water straight through the concrete slab and onto your ceiling. The stain often appears a metre or two away from the actual leak source because water travels along the underside of the slab before dripping down.
• Pipe condensation — During Hong Kong's humid summer months (April to September), cold water pipes running through the ceiling void can sweat heavily. The condensation drips onto the back of your ceiling plaster and creates damp patches that look identical to a leak. The giveaway is that the patches appear and disappear with the seasons.
• Air conditioning condensate — Split-unit AC systems produce litres of condensate water daily. If the drain pipe is blocked, cracked, or improperly routed, that water ends up inside your ceiling. I see this at least twice a month, particularly in older buildings where the AC drain runs through the ceiling space.
• Roof waterproofing failure — For top-floor residents, the bitumen or membrane waterproofing on the roof degrades over time due to UV exposure and Hong Kong's extreme temperature swings. Cracks develop, water pools in low spots, and eventually it comes through.

Before you spend money on repairs, you need to identify the actual source. A useful trick is to tape a sheet of aluminium foil over the damp patch and check it after 24 hours. If moisture appears on the outside of the foil, you have a condensation problem. If moisture appears between the foil and the ceiling, water is coming through from above.

How Do You Stop Recurring Ceiling Leaks?

If the leak is coming from the flat above, your first step should be to speak to your upstairs neighbour — politely but directly. In my experience, most people are reasonable about it once they understand the problem. If they are uncooperative or the flat is unoccupied, you can report the matter to your building's Building Management Office (BMO). Under Hong Kong's Building Management Ordinance, the BMO can facilitate mediation and, if necessary, arrange for a plumber to inspect the source.

For persistent disputes, the Joint Office for Investigation of Water Seepage Complaints (a joint initiative by the Food and Environmental Hygiene Department and the Buildings Department) can conduct a formal investigation. You can file a complaint by calling 1823 or submitting online. They will carry out moisture tests to identify the source and can issue legal notices requiring the responsible party to carry out repairs.

Once the source of water is fixed, here is how to properly repair the ceiling:

• Wait for it to dry completely — This is critical. In Hong Kong's humidity, allow at least two to three weeks with a dehumidifier running in the room. Painting over damp plaster just traps the moisture and the stain will return within weeks.
• Scrape away damaged plaster — Use a broad scraper to remove any flaking or bubbling paint and plaster back to solid material.
• Apply a stain blocker — Use a shellac-based primer like Zinsser B-I-N (around HK$180 for a small tin from paint suppliers) to seal the stain. Water-based primers will not block old water marks.
• Re-plaster and paint — Apply a skim coat of finishing plaster, sand smooth, then paint to match. A 5-litre tin of Nippon or ICI ceiling white costs around HK$150-$250.

For pipe condensation issues, the fix is to insulate the cold water pipes with foam pipe lagging. You can buy self-adhesive pipe insulation tubes for around HK$10-$20 per metre from hardware stores. If you need help repairing the wall surface after dealing with damp damage, or if the water has caused cracking in your walls, those are separate jobs worth tackling at the same time.

When Should You Call a Professional?

Ceiling damp is one of those problems where getting it wrong the first time makes everything worse. Call a professional if:

• The damp patch is spreading rapidly or the ceiling plaster is sagging — this means significant water is pooling above and could collapse
• You cannot identify the source after basic investigation
• The leak involves the building's common drainage or water supply pipes, which are the building management's responsibility
• You need to access the roof but do not have safe access or the roof is locked (many buildings in Hong Kong restrict roof access)

A professional can also use thermal imaging cameras to trace hidden leaks without opening up the ceiling, which saves a great deal of mess and guesswork. For more home repair advice, check our full range of interior repair articles.

Last updated: March 2026