Flat RC roofs leak at outlets, laps and parapets

Last updated 31 October 2026.

A flat roof in this climate is a tray with a hole in it. The hole is the outlet. Leaves, a pigeon nest, a cooling-tower pad and a new screed that creeps across the grate all close the hole. Water then stands, finds the lap in the sheet, and shows up on the ceiling as a brown map that people call a “slab leak”. The slab is wet because the outlet forgot its job. Open the outlet, watch the next storm, and see whether the map returns before you buy a new membrane.

If you are on the top floor of an HDB block, that roof is not your bathroom and not your private contract. HDB’s ceiling-leak page tells owners to maintain the interior and to contact the town council for leaks from common property. A block roof is common property. Call the council named on the S&CC bill. HardwareZone threads are full of top-floor owners who were sent between the council and HDB. The page to start is still the council, with photos taken during rain and again when the slab has had a dry day. A private torch-on on a council roof is how you pay for a membrane you may not be allowed to touch.

Landed houses and private RC roofs are different. You own the tray. The failures we actually see, without inventing a survey, are the ones the detailing predicts: ponding at a low spot because the fall was lost when a new screed was added; a blister in a bituminous sheet where vapour was trapped; a crack at the parapet upstand, which moves differently from the field; and a pipe or a railing post through the slab with no collar. A coating rolled across the field and stopped at the base of the pipe leaves the penetration, which is the leak.

Match the repair to the sheet you have. A liquid acrylic or a traffic coat is a maintenance layer when the membrane underneath is still stuck, the laps are closed, and you have found one split you can bridge. It is the wrong product when the sheet is brittle across the whole bay, the laps have opened, or water is running under the sheet and blistering it. Then the old membrane comes off, or a new system is designed over a prepared surface, and the upstand is carried up the parapet far enough that a puddle cannot overtop it. Torch-on bitumen is a roof system. It is a bad bathroom system, and the membrane-types guide explains why. Injection of polyurethane into a crack is a crack treatment. It does not give the roof a new outlet or a new upstand.

A ponding test on a roof is a controlled flood of one bay, outlets plugged, a mark on the upstand, and a watch on the ceiling below. Competitors describe 24-hour floods. HDB’s bathroom water-test line does not set the hours for a roof, and we will not invent a pass mark in millimetres. What we will not accept is a hose on one patch for five minutes. That wets the coating and leaves the lap ten metres away untested. Do the bay you can supervise. Do not flood a roof that already drains into a neighbour’s yard without telling them. Electrical plant and solar isolators on the roof stay dry. If you cannot isolate them, you do not flood that bay.

Concrete that is popping, with rust stains and exposed aggregate, is spalling. The small spalling patch on the ceiling page is S$150 to S$300 when that sentence matches. A roof slab that is spalling from above is not a ceiling patch and not a coat of acrylic over loose concrete. The spalling page is the structural repair. A polycarbonate or metal awning over a yard is the awning page, including its gutters. Do not ask a flat-roof quote to include a sheet awning, and do not ask it to include curtains or a blind on the skylight.

Homey Furnishing Pte. Ltd. (ACRA UEN 202018148K), 27 Mandai Estate, #06-10 Innovation Place, Singapore 729931, invoices permit-exempt leak checks, regrouting, silicone, and making-good after the water has stopped. V On Construction Pte. Ltd. (ACRA UEN 201611708H), on the HDB Directory of Renovation Contractors, invoices an HDB renovation permit when bathroom finishes are hacked or a new membrane is laid under new tiles. A concealed or new water-supply pipe is a PUB licensed plumber. Homey Furnishing is not that plumber and does not publish a waterproofing warranty term.

Curtains, blinds and roller shades are not part of this booking. Fabric window treatments are a different product, and these pages do not quote them.

Rain is the test you already have. Use it before a flood test, and do not stand on a wet roof alone.

1. Photograph the ceiling stain during rain and six hours later. A stain that fades may be condensation on an uninsulated slab. A stain that drips is a path.
2. If there is a floor above you, this is not your roof. Use the inter-floor guide.
3. On your own roof, look at the outlet first. Clear it. Watch the next rain.
4. Walk the parapet and every pipe. The field can look perfect while the upstand has a crack you can fit a card into.
5. Only then decide coat, new sheet, or a crack injection. Write down which one, and why the others were rejected.

Who owns the roof decides the phone call. The detailing decides the repair, after ownership is clear.

Acrylic and similar maintenance coats need a clean, dry sheet and a day the weather will leave alone. Applied over morning dew they blister by the next afternoon. Bituminous torch-on needs laps you can see and a skilled torch, because a cold lap is a planned leak. PVC or TPO sheet is welded. A glued pretence at a weld opens. Crystalline slurry on the ceiling below is a negative-side treatment: it may slow damp coming through concrete, and it leaves the roof outlet untouched. Use it, if at all, after the top is repaired, not instead.

Good roof work leaves the outlet proud and protected with a grate you can lift. The upstand is continuous around posts. A water test is done by bay. Sloppy roof work paints the field, stops 20mm short of the parapet, buries the outlet in coating, and calls you back in the next monsoon. Ask to see the outlet before the invoice is due.

The PAA question is “what is the best way to seal it”. The honest answer is the outlet first, then a system that matches the sheet you have. There is no best product on this page.

Make them walk you to the outlet and the worst upstand before you discuss a brand.

1. Is this roof mine, the town council’s, or the MCST’s?
2. What happens to the outlet in your detail?
3. Is this a coat over the existing sheet, or is the sheet coming off?
4. How is a pipe or a post treated?
5. What flood test will you do, and which ceiling will you watch?
6. Which published HomeySG sentence is the price? If none, say quote on inspection.

HDB top floor: town council, with rain photos. If they say the leak is from your own aircon or your own bathroom, then it is not the roof and you change pages. Do not start a private membrane while that argument is open.

A roof over a bedroom with a dripping light is an electrical problem as well as a wet one. Isolate. Spalling concrete that is falling is the spalling page and, if pieces are coming off a block facade, the town council again. Nobody works the roof edge without a way to stop a fall. We will not “just have a look” over a parapet.

How you're charged

A leak or waterproofing job that already has a sentence on HomeySG uses that sentence. Nothing in this table is a fuel surcharge, and the handyman hour is not added on top of a band unless the written scope says so. A membrane, a balcony, a planter, a roof, an epoxy-grout system, or a listening survey with no matching row is quote on inspection.

Who is allowed to do the work

HDB’s renovation pages split a coat of silicone from a hacked bathroom. Replacing toilet wall and floor finishes is not allowed for 3 years from the completion date of the block, because a membrane sits on the screed. After that, a replacement uses pre-packed waterproofing screed, a membrane upturned at least 150mm at walls, kerbs and pipes, a UPVC floor-trap grating with a long collar, and a water test. Fines for unauthorised work are already written on the permit guide as up to S$5,000. Homey Furnishing does not sign the permit invoice.

• Regrout, a silicone zone, hollow-tile injection and a stain repaint after the water has stopped stay on the published rows. They do not replace a membrane.
• Opening the bathroom floor or wall finishes is V On Construction when the flat is HDB and the work needs a permit.
• PUB’s leak leaflet says it is an offence under the Public Utilities Act to engage an unlicensed worker for plumbing work. A concealed or new supply pipe is a licensed plumber. An exposed trap is a different visit.
• External walls of an HDB block are common property. The town council on the S&CC bill is the first report, not a private injector.
• SCDF does not allow hacking of household-shelter walls, floor slabs or ceiling slabs to chase a pipe. The leak is found another way.
• A window-frame leak is the window sealant page. Exterior rivets and stays stay with a BCA approved window contractor.
• Curtains, blinds and roller shades are not quoted here.

Ways to keep the bill smaller

• Photograph the stain, the wet room, and the water meter before anyone hacks. A hack that destroys the evidence is a second problem.
• Run one test at a time. Shower, flush, aircon and rain on the same afternoon make a useless log.
• If the joint is only mouldy silicone, the published zone is S$80 to S$150. That is not a membrane.
• If the ceiling is already dry and the source is fixed, the hairline repaint is S$150 to S$400. Paint on a live leak is how the stain returns.
• Clear the roof outlet before the monsoon week you already know is coming. A grate you can lift is cheaper than a membrane you do not need.

WhatsApp the photos to +65 8561 0630.

Roof work is quote on inspection unless the photo is actually a small ceiling patch or a small spalling patch after the water has stopped.

Anything not in that table is quote on inspection.

WhatsApp the photos to +65 8561 0630. The reply uses a band already printed on HomeySG, or it says quote on inspection.

Official pages, not a legal opinion

Ownership of an HDB roof is a common-property question. HDB’s ceiling-leak page is the pointer. We do not quote a warranty in years for a roof system.

• HDB ceiling leaks. Common-property leaks, including the sort that come from outside the flat, go to the town council.
• HDB building works. Bathroom and balcony membrane rules are not a specification for a block roof.
• Parliament on external seepage. Town councils maintain external walls. Roofs of the block sit in the same common-property idea. Confirm the element with the council.
• BCA shelters. Do not open a shelter slab because a roof leak stained a corner of it.

• Which membrane belongs on a roof
• Awnings and gutters
• Spalling concrete
• If it is a balcony, not a roof
• Handyman price list