The short answer
A conservatory roof usually leaks because a seal or joint has failed, not because the glazing itself is porous. The usual causes are perished gaskets and sealant, a split or lifted flashing where the roof meets the house wall, cracked sheets or panes, blocked gutters, and an end closure that has come away. What looks like a leak can also be condensation, so check before paying for a repair.
Joints fail before panels do
Think of the roof as panels sitting in a frame, with water going wherever the panels stop. Polycarbonate sheet and glass are both waterproof in themselves. What keeps the roof dry is everything that joins them: rubber gaskets pressed against the glazing, cappings clipped over the bars, sealant at the ridge, and metal flashings against the brickwork.
Those parts are softer than the glazing and are exposed to sun, frost and movement. Rubber hardens and shrinks. Sealant cracks and lets go of the surface it was meant to grip. The frame moves a fraction every time it warms and cools, and over the years the gap at a joint opens a little more. That is why leaks so often start at the same places, and why the water you see on the floor may have travelled some way from its entry point.
The usual suspects, one by one
Perished gaskets and sealant
The most common cause on an older roof. A gasket is the rubber strip between the glazing and its bar. When it shrinks, a channel opens along the edge and rainwater runs in. Dark staining along a glazing bar is a clue.
The flashing at the house wall
At the junction between conservatory roof and main house, a strip of metal bridges the joint. This is the flashing. Lead is the usual material. If it lifts, cracks or loses its seal to the brickwork, water runs behind it and drips inside. It is a very common leak, and one that sealant alone seldom cures.
Cracked or slipped glazing
A hairline crack in a polycarbonate sheet or a chipped glass pane lets water through. So can a sheet that has slipped in its bar and no longer sits tight.
End closures and ridge details
Multiwall polycarbonate has hollow channels. Strips called end closures seal the open ends. If one comes away, water and dirt get inside the channels and drain out later, which can look like a leak coming from the middle of a sheet.
Drainage and the frame
Gutters cause trouble even on a sound roof. A blocked or sagging gutter overflows, and the water can be thrown back against the frame and driven up under the edge of the glazing. If you see water stains on the eaves or the wall plate, look at the gutter before anything else. Our page about the conservatory gutter covers how these systems fit and fail.
The structure matters too. A frame that has moved, rotted at the base or settled on one side can pull glazing out of its seats, and that opens gaps that no seal can close. If you can see gaps at the corners, bars that no longer line up, or timber that feels soft, the leak is a symptom of a deeper problem. Our guide to conservatory roof structure explains what holds a roof up and what to look for.
Is it a leak or is it condensation
People call out a roofer for a leak, and find the roof is dry. Condensation produces droplets and running water on the underside of the glazing, and it can run down onto a sill in a way that looks very like a leak. It is a moisture issue, not a weathertightness one.
Some tests help. Condensation tends to appear in the cold mornings and in the evening, on a clear day with no rain, and it forms across the whole roof surface rather than at one joint. A genuine leak appears during or just after rain, and it gathers at a seam, a bar, a corner or the wall junction. Staining that traces back to a single joint also points to a leak.
Condensation needs different fixes, chiefly ventilation and better insulation. Our page on a cold conservatory looks at why these rooms lose heat and attract moisture.
What to do about it
Start by finding out where the water enters. Choose a dry day, look along the bars and the wall junction from outside, and look for dark lines, gaps, lifted edges and cracked sheet. Photographs help whoever comes to look at it. Do not smear thick sealant over a joint you have not understood. It can hide the entry point and leave the water to find a second route.
A repair is the right answer when a defined cause turns up: a flashing to reseal, one sheet to change, a gutter to clear. If drips show up all over, the sheets are brittle or the seals have gone throughout, patching only moves the problem around. We lay out the way to decide on our page about whether to repair or replace.
If water is coming in now, say so when you contact us. Our first step is to stop the room taking in water, then we plan the proper repair or replacement. For the length of a full replacement, see our page about conservatory roof replacement time.
Keeping the next leak away
Prevention is mostly attention. Look at the roof at the start of autumn and again after the worst of the winter. Clear leaves from the gutters and from the bar channels. Watch for green growth along the edges, which holds water against the seals. Check the wall flashing for lifting. Do not use harsh tools or a pressure jet on the roof surface.
At the survey you see the exact joint at fault in pictures of your own conservatory, and nothing is charged for it. If the roof as a whole has had its day, replacement is quoted at one fixed written price, and the pictures help you compare that with the cost of resealing every joint again in a few winters. The overview of every roof type is on our conservatory roof replacement page.
