Polycarbonate skylights leak at the caps, bars, gaskets and kerb flashing, not through the sheet — here is how to find which one, and how to repair it so it stays repaired.
Polycarbonate sheet is not the weak point. The sheet is tough, it takes impact far better than glass, and it rarely fails in the middle of a panel. What fails is everything holding it in place: the end caps that close a multiwall sheet, the rubber gaskets in the glazing bars, the screws that pass through the sheet, the sealant at the perimeter, and the flashing where the whole assembly meets the roof covering. That is why a skylight can look perfect from below and still drip every time it rains hard.
It matters because the fix follows the failure. If the sheet is sound and the gasket has perished, resealing the perimeter with more silicone treats the wrong component. If the flashing at the kerb has lifted, nothing done to the skylight itself will stop the water, because the water is not coming through the skylight at all — it is coming past it. Working out which of the four paths is live is the whole job, and it is worth an hour with a torch before anyone quotes.
| Leak path | What you see | What it usually means |
|---|---|---|
| End caps and open flutes (multiwall sheet) | Dirt, algae or standing water visible inside the vertical channels of the sheet; cloudy streaks that will not clean off | The sheet ends were never properly closed, or the tape and cap have failed — water is sitting in the flutes |
| Glazing bar gaskets and screw line | Drips along one bar, or a neat line of drips following the fixing row | Perished rubber, or screws over-tightened into the sheet so the seal is crushed and the sheet has cracked around the hole |
| Perimeter sealant | Water at the edge of the opening, often after wind-driven rain rather than steady rain | UV-degraded sealant that has hardened, cracked and pulled away from one face of the joint |
| Kerb flashing and roof junction | Staining on the ceiling beside the opening rather than under it; water appearing during heavy rain only | The flashing that carries water around the skylight has lifted, corroded or was never dressed properly into the roof covering |
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💬 Get Your Instant QuoteA quick discipline that saves a great deal of guessing: stand inside during rain with a torch and watch where the first drop forms, not where the puddle is. Water tracks along a glazing bar or the underside of a frame before it falls, so the drip point and the entry point are often a metre or more apart. The same logic applies to any roof opening — window leak repair works through it for windows and frames.
This is the mechanism behind most skylight leaks, and it is not obvious. Polycarbonate expands and contracts noticeably more than the aluminium or steel frame it is fitted into. On a Malaysian roof the sheet sees direct sun through the middle of the day and then a cold downpour in the afternoon, and it does that cycle every day of the year. So the sheet is permanently moving relative to its frame.
The correct installation allows for that movement. Fixing holes are drilled oversized so the screw can sit in the middle of a slot and the sheet can slide; screws are fitted with a washer and tightened only until the washer seats, not until the sheet is clamped; the sheet is left with a clearance gap at the ends rather than jammed into the profile; and the weather seal is a gasket or a purpose-made profile that can flex, with sealant as a backup. The wrong installation clamps the sheet solid and relies on a rigid bead of sealant to keep the water out. That bead is then asked to absorb daily movement it was never designed for, and it fails in one of two ways: the sealant splits, or it holds and the sheet cracks around the screw instead, producing the star-shaped cracks you can see from below.
This is why so many skylight repairs come back. Adding a second bead of sealant to a rigidly clamped sheet does not remove the movement, so it fails the same way for the same reason, usually faster than the first one did.
Three sheet problems are worth knowing, because they change the answer from “reseal” to “replace”.
If the sheet is yellowed, crazed or streaked internally, the honest advice is to price replacing the panel rather than resealing around a panel that is already at the end of its life. Skylight cost covers what a new unit or a re-sheet involves.
A skylight is a cold surface in a humid country, and Malaysian homes produce a great deal of moisture. Water on the underside of a skylight is not automatically a leak, and treating condensation with sealant achieves nothing at all.
| Observation | Points to a leak | Points to condensation |
|---|---|---|
| Timing | Only during or shortly after rain | Mornings, cool nights, after a hot day, with no rain |
| Pattern | Drips from one or two definite points | An even film or fine droplets across the whole underside |
| Location | Follows a bar, a fixing line or one edge | Worst in the middle of the panel and around the coldest edges |
| Response to wind | Worse when rain is driven from one direction | Unrelated to wind |
| What helps | Nothing until the path is closed | Ventilation, an extractor, less unvented moisture in the room |
There is a hybrid case: condensation forming inside the flutes of a multiwall sheet because the end seals are wrong. That looks like a leak, behaves like condensation, and is cured by closing the sheet ends correctly rather than by sealing the perimeter. Condensation versus water seepage covers the wider diagnosis.
A skylight that sits flush in a roof plane is a permanent problem, because water arriving down the slope has to be diverted around it with nothing but a seal. A skylight that sits on a raised kerb has a fighting chance: the roof covering is dressed up the sides of the kerb, an apron flashing carries water from the uphill side around and out below, and the frame sits above the water line rather than in it. On a tiled roof that means a dressed metal flashing tucked under the tiles uphill and over them downhill; on a metal roof it means a properly formed upstand and a sheet lap that runs past the opening.
If your ceiling stains beside the skylight rather than under it, suspect the flashing first. That is the same detail, and the same failure, as the wall and chimney junctions covered in tile roof leak repair, and it is repaired the same way — by cutting out the failed detail and forming a new one, not by adding sealant on top of the old one.
Sealant choice matters more on polycarbonate than on almost any other substrate, because the wrong chemistry damages the sheet rather than merely failing to stick. The rule to work to is simple: use a sealant whose supplier explicitly states compatibility with polycarbonate, and treat anything else as unsuitable until proven otherwise.
| Sealant family | How to recognise it | On a polycarbonate skylight |
|---|---|---|
| Acetoxy-cure silicone | Strong vinegar smell while curing; the cheapest general-purpose tube | Generally not recommended — it releases acetic acid as it cures. Avoid unless the supplier states it is suitable |
| Neutral-cure silicone | Little or no vinegar smell; usually sold as neutral, low-modulus or glazing grade | The usual choice, but still confirm the datasheet names polycarbonate |
| Polyurethane sealant | Sold as a construction or joint sealant, paintable | Strong and flexible, but compatibility with polycarbonate varies by product — check before use |
| Butyl tape and gaskets | Non-curing tape, or a moulded rubber profile in the glazing bar | Often the better answer at a moving joint, because it flexes instead of curing rigid |
Two practical notes. Sealant needs a clean, dry, dust-free surface to bond to, and a skylight edge in Malaysia is neither of those things without preparation — most failed beads failed because they were applied over grime, not because the tube was wrong. And never treat a bead as the primary weather line at a moving joint; it is a backup to a gasket, a cap or a flashing. DIY waterproofing sealants covers the families in more detail.
The pattern is familiar to anyone who has had a skylight “repaired” three times. A bead is run over the leaking edge; it works for a season; it cracks; a second bead goes over the first. Each round has the same three problems. The movement that broke the first bead is still there. The new bead is bonded to old, hardened sealant rather than to the sheet or the frame, so it is only as good as what is under it. And the accumulated layers make the eventual proper repair messier and more expensive, because all of it has to come off before anything can bond properly.
It also disguises the diagnosis. If the real path was the kerb flashing, the sealant was never going to work, but three failed attempts have now cost more than the flashing repair would have. The general case for fixing the component instead of the symptom is set out in why waterproofing fails.
| Condition | Sensible answer | Why |
|---|---|---|
| Sheet clear and sound; gasket perished or sealant cracked | Reseal and re-gasket, correcting the fixings | The cheapest honest repair, provided the fixings are freed so the sheet can move |
| Cracks radiating from screw holes | Replace the affected sheet, oversize the holes, refit correctly | The crack will keep growing; it is a movement problem, not a seal problem |
| Yellowed, brittle or crazed sheet | Replace the sheet | The material has aged out; no seal repair will outlast it |
| Streaking or algae inside the flutes | Replace the sheet and close the ends properly | Flutes cannot be cleaned out once contaminated |
| Frame corroded, or the kerb flashing failed | Rework the junction, or replace the unit | The leak is around the skylight, not through it |
| Repeated failures across several bays | Price a full replacement | Repeated repairs on an aged assembly rent time rather than buy a fix |
Skylight work is priced as a roof repair, and the number is driven by access, by how much has to come apart and by whether sheet is being replaced (market estimate 2026, indicative Klang Valley):
| Work | Market range (2026, Klang Valley) | Notes |
|---|---|---|
| Tiled / metal roof repairs | RM500 – RM3,000 per job | resealing, re-gasketing, flashing rework or replacing a sheet |
On our own side, ClickBina leak detection and assessment is RM300 – RM800 per visit, waived if you proceed with the repair — that is our price, not a market range, and it is worth taking when the drip point and the entry point clearly are not the same place. A new skylight or a full re-sheet is a different budget again and is covered in skylight cost; the wider roof numbers are in roof repair cost.
Two constraints drive the programme more than the materials do. The first is that nobody should stand on a polycarbonate sheet. It is not a walking surface, it gives no warning before it fails, and a fall through a roof opening is the most serious risk on this kind of job. Proper work uses crawl boards spanning the structural members, or access from below with a tower, and a contractor who is happy to walk across the panels is telling you something about how they will treat the rest of the job.
The second is weather. Sealant will not bond to a damp surface, and a skylight opened up in front of an afternoon storm puts water straight into the room below. Work is planned in dry windows, in sections, with each section closed before the crew leaves. In the monsoon months that usually means mornings rather than full days. The monsoon roof preparation checklist covers what is worth doing before the wet season rather than during it.
Ask for a hose test on completion: water played over the skylight and the roof above it, working from the low side upwards so each detail is loaded in turn, with someone inside watching. Run it long enough to matter — a brief splash does not reproduce a storm. If the original leak only ever appeared in wind-driven rain, say so in advance and agree what happens if it returns, because a hose cannot reproduce wind.
Afterwards, the maintenance is mostly cleaning and looking. Wash the sheet with plenty of water and a mild soap using a soft cloth, and keep ammonia-based glass cleaners and solvent cleaners away from it — those are a common cause of crazing on polycarbonate. Once a year, before the wet season, check the end caps, the gaskets along the bars, the perimeter bead and the flashing, and clear leaves and debris from the uphill side of the kerb where they dam water. That annual look is the difference between a re-gasket and a replacement, and it fits alongside scheduled roof maintenance.
ClickBina repairs skylights, roof lights and polycarbonate awnings across Kuala Lumpur and Selangor as part of a roofing and waterproofing team, so the skylight, the flashing around it and the roof it sits in are all handled by the same crew instead of three. We diagnose the path before quoting rather than running a bead over whatever is wet, we say plainly when a sheet has aged out and resealing would be a waste of your money, and where the pattern is unclear we do a paid detection visit at RM300 – RM800 per visit and waive it if you proceed. Browse all of our waterproofing and leak repair services, or send photos of the skylight and the ceiling stain on WhatsApp for a fixed quote.
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