Leaking conservatory roof: where the water gets in, fault by fault
Five joints and one look-alike are worth checking first, and you can narrow it down from inside without setting foot on the roof.
A conservatory roof usually leaks at a joint rather than through the glass: the glazing bar caps and gaskets, the flashing where the roof meets the house, the box gutter, the ridge, or the taped ends of polycarbonate sheets. Following the wet line uphill, and checking for drips on dry mornings, tells you which, or whether it is condensation.
The short version
- Water travels before it drips. Follow the wet line back uphill along the glazing bar or the wall before you decide where the leak is.
- Drips that appear on cold mornings when it has not rained are usually condensation, and the cure is ventilation and heat, not sealing the roof.
- A leak along the house wall can come from the house, not the conservatory. Rain soaking into the wall above can run down inside the cavity, and new flashing will not stop that.
- Sealant smeared over caps and joints is a stopgap. It can trap water as easily as stop it, and it has to be cut off wherever new gaskets or flashing will go.
Before you start
Find where the water enters, not where it lands
The wet patch on the floor is rarely under the fault. On a conservatory roof, water that gets past the outer seal runs down the inside of the glazing bar, or along the wall plate, until it reaches a low point or a fixing, and drops off there. So the first job is to follow it back uphill.
Do it in rain, while the trail is still wet, along the underside of each glazing bar and the line where the roof meets the house. Photograph each wet spot, because whoever looks may come on a dry day.
Then note when it happens: in all rain, only in heavy or wind-driven rain, or on dry, cold mornings as well. That alone rules out several of the causes. If you are not sure the water is rain at all, for example because a bathroom or a boiler overflow sits above the conservatory, start with how to tell a rain leak from a plumbing leak.
Do not climb onto the roof. HSE lists rooflights and glass as fragile surfaces and says every roof should be treated as fragile until a competent person has confirmed otherwise. Treat polycarbonate sheets and glazing bars as if they will not take your weight, and do not lean a ladder against them.
The six usual causes
Where conservatory roofs leak, fault by fault
These are the six places a roofer looks first. The table puts them side by side, and the sections below go through each one.
| Fault | Where the water usually shows | What you can check yourself | The proper repair |
|---|---|---|---|
| Glazing bar caps and gaskets | A drip line along one bar, often reaching the eaves end | From an upstairs window: a lifted top cap, a gasket shrunk back from the panel. From the ground: a missing end cap at the eaves | Lift the cap, reseat the panel, fit new gaskets and end caps |
| Wall flashing | Along the house wall, worst in wind-driven rain | Peeling or cracked flashing tape, lead lifting from its chase, mortar missing from the joint above | Lead or a proper flashing chased into the wall, not tape |
| Box gutter | Along one line, mainly in heavy rain | Leaves or standing water seen from above; a downpipe that runs slowly | Clear it, then reseal or line the joints |
| Ridge | At the top of the room, near the centre | A loose finial or ridge cap; gaps where bars meet the ridge | Reseat slipped panels, refit the ridge cappings |
| Polycarbonate sheet or end tape | Green or dirty lines inside the sheet, drips at the eaves | Water or algae in the flutes, yellowing, cracks | New tape and closures, or new sheets if they are brittle |
| Condensation | Drips all over, on cold dry mornings | Wipe the underside dry and watch whether it comes back without rain | Ventilation and heating, not roof work |
Faults 1 to 3
Glazing bars, the wall flashing and the box gutter
Glazing bar end caps and gaskets
Most conservatory roofs are glass or polycarbonate panels held between aluminium or uPVC glazing bars, with a top cap clipped or screwed over each bar and rubber gaskets pressing on the panels either side. The gaskets shrink and harden with sun, and the panels themselves creep downhill as they expand and contract. Between them, gaps open up at the top of the bar and under the cap, and water gets into the bar.
From an upstairs window you can often see it: one cap sitting higher than its neighbours, a black gasket pulled back from the edge of the panel, or a panel that has slid down and left a visible gap under the ridge. From the ground, look at the eaves end of each bar: a missing end cap lets wind drive rain straight in, and it is the cheapest fault here to put right. The proper fix is to take the cap off, push the panel back into place, fit a stop so it cannot slide again, and replace the gaskets.
The wall flashing
Where a lean-to roof or the back of a conservatory meets the house, the joint is covered by a flashing. The good version is lead, tucked into a mortar joint in the brickwork and dressed down over the roof. Designing Buildings gives Code 4 lead as the minimum it recommends for flashings because it resists the fatigue splits caused by expansion and contraction, and names a conservatory roof meeting a house wall as a place for stepped flashing. Many conservatories have something cheaper: a self-adhesive flashing tape, or a bead of gel or silicone along the wall. Those do not last as long as lead, and peel or crack as they age.
Look from the ground or an upstairs window for tape lifting at the edges, lead that has come out of its joint, or mortar missing from the chase above it. If the flashing looks fine and the wall still runs wet inside, look higher up. Rain soaking into the house wall above can run down inside the cavity and come out over the conservatory, which flashing cannot stop. A cavity tray just above the roof line turns that water outwards; small weep vents in the brickwork above the flashing usually show one is there. If the leak is worst in wind-driven rain and the wall above faces the weather, read what water coming in only during driving rain tells you before you pay for new flashing.
The box gutter
A box gutter is the flat-bottomed channel built into the roof where a conservatory meets the house on two sides, or where two roof slopes meet. It is usually aluminium, made in lengths joined with seals, and it drains to an outlet at one or both ends. It leaks in two ways: the joints between lengths fail and it drips along its line, or the outlet blocks, the gutter fills, and water rises over its upstand and under the roof edge. The second only happens in heavy rain, so it often seems fixed when it is not.
Check whether the downpipe from the box gutter runs freely in rain, and whether you can see leaves or standing water from an upstairs window. Clearing a box gutter usually means reaching over the roof, so do not try it from the panels. Once it is clear, the joints can be resealed or the whole gutter lined. The same logic is behind gutters that overflow only in heavy rain.
Faults 4 to 6
The ridge, polycarbonate sheets, and condensation
The ridge
On a Victorian or Edwardian roof, every glazing bar meets the ridge at the top, under a ridge capping, often with a decorative finial at the end. A drip near the middle of the room, at the highest point, usually comes from here.
The causes are slipped panels, tired seals, or a ridge cap or finial that has worked loose, and from an upstairs window you can see whether the cap sits straight and the panels still reach it. The repair is to reseat the panels and refit the cappings, not to fill the ridge with sealant.
Polycarbonate sheet failure and end tape
Multiwall polycarbonate is hollow, with flutes running down the slope. The usual detail is a solid tape over the top ends, so water cannot get in, and a breathable tape and end closure at the bottom, so any moisture inside can drain and dust stays out. When those tapes perish, the flutes fill with water and then algae. You will see green or grey lines inside the sheets from below, and drips at the eaves. Old sheets also turn yellow and brittle, and crack around their fixings.
Sound sheets with failed tape can be cleaned and re-taped. Sheets that are discoloured, cracked or brittle across the roof are at the end of their life.
Condensation mistaken for a leak
A lot of “leaking” conservatories are not leaking at all. The government’s guidance on damp and mould describes condensation as moisture from inside the home cooling and condensing onto colder parts of the building, and a single-layer roof is often the coldest surface in the house. The signs are drips spread across the whole roof rather than one line, worst on cold, still mornings, and present when it has not rained.
The test costs nothing: wipe the underside dry on a dry evening and look again in the morning. If it is wet, it is almost certainly condensation. Drying clothes in there, an open door to a warm kitchen and shut roof vents make it worse; open vents and a little steady heat make it better. You do not need a roofer for this. The same reasoning is in how to tell condensation in the loft from a roof leak.
The sealant question
Why sealant over everything only buys time
Sealant is the first thing most people reach for, and it can stop a drip for a while. It is rarely a repair. A sealant has to bond to both sides of a joint and stretch with whatever movement the joint has.
Designing Buildings’ guide to sealants puts it plainly: the right one is matched to the materials, with adequate bonding properties, and is flexible enough to tolerate the movement expected. A conservatory roof moves every day as the sun comes and goes, the joints are usually damp and dirty when the sealant goes on, and the movement that let the water in is still there afterwards.
So the bead peels off a surface it never properly stuck to. Sealant run over a cap or gasket blocks the drainage paths the roof was designed with, so water that does get in is more likely to end up in the room. And a roof coated in sealant, or a painted-on repair compound, has to be scraped clean before anyone can fit new gaskets.
So what is the best sealant for a leaking conservatory roof? For a stopgap on a single crack or a lifted flashing, a sealant made for external glazing that says on the cartridge it is compatible with the material it is touching, polycarbonate in particular, since not every silicone is. Apply it to a clean, dry joint, and treat it as holding the line until the real fault is fixed. If the roof also needs covering after storm damage, what is safe to do yourself as a temporary repair sets out the limits.
Repair or replace
When a repair is right, and when replacement is worth considering
Repair is right when the leak is in one place and the rest of the roof is sound. A slipped panel, failed flashing, a blocked box gutter or a missing end cap is a repair job, and a new roof for one of them is money spent on the wrong thing.
Replacement starts to make sense in four situations: the polycarbonate is yellow and brittle across the roof; several bars leak at once and the gaskets have perished throughout; the frame itself is distorting, with doors binding or gaps opening at the house wall, in which case get the movement explained first, because a new roof will not fix a base that is moving; or the real complaint is that the room is too hot in summer and too cold in winter, and the leak is the last straw.
The rules differ by type. A new glazed or polycarbonate roof on the same frame normally keeps the conservatory exempt from building regulations, because Schedule 2 of the Building Regulations 2010 exempts a conservatory of up to 30 square metres at ground level as long as its glazing meets the safety glazing rules. A solid or tiled roof is different. It is building work that needs approval, insulation to the current standard and a check that the frames and base can carry it. Our guide to replacing a conservatory roof with a solid roof covers that decision in full.
What to do next
What to do about a leaking conservatory roof
- Next time it rains, follow the wet line uphill from inside, photograph it, and note what kind of rain it was.
- Do the dry evening wipe test. If the drips come back without rain, deal with ventilation first.
- From the ground and an upstairs window, look for missing end caps, lifted top caps, peeling flashing tape, and leaves in the gutters. Clear the eaves gutters from a ladder footed on the ground if you can do so safely, never from the roof.
- Check whether a downpipe from the house roof empties onto the conservatory. That much water landing on one panel will find any weakness.
- If the cause is at a cap, the flashing, the box gutter or the ridge, get it repaired properly rather than sealed over. Ask what will be replaced, not just sealed.
Conservatory leaks are part of our roof repairs work and our wider roofing services, including the gutter repairs a box gutter often needs. Thomas Hannen comes out to look at the roof himself, and you get a free written itemised quote that says what he found and what the repair involves.
Questions
Common questions
What is the best sealant for a leaking conservatory roof?
For a stopgap, an external glazing sealant that says on the cartridge it is compatible with the material it touches, especially polycarbonate, applied to a clean, dry joint. It is not a repair: sealant run over caps and gaskets can block the roof's drainage and has to be removed before the real fault is fixed.
Why does my conservatory roof only leak in heavy rain?
Usually because a gutter or box gutter is partly blocked or its outlet is too slow, so it overflows only when the flow is high, or because wind-driven rain is getting past a flashing or into the house wall above. Light rain drains away before it reaches the weak point.
Is my conservatory leaking or is it condensation?
Wipe the underside of the roof dry on a dry evening and look again in the morning. If it is wet without rain, it is almost certainly condensation.
Can I go on my conservatory roof to find the leak?
No. HSE lists glass and rooflights as fragile surfaces, so treat polycarbonate sheets and glazing bars as if they will not carry your weight. Look from the ground, from an upstairs window, or from inside.
Is it worth repairing a conservatory roof or should I replace it?
Repair it if the leak is in one place and the rest of the roof is sound. Consider replacing it if the polycarbonate is brittle throughout, several bars leak at once or the frame is moving. A solid roof almost always needs building regulations approval.
Read next
Related advice
The guides below pick up where this one leaves off, and the roofing page covers the work itself and the places we cover, if you would rather have the job looked at.
Who wrote this
Thomas Hannen
Thomas owns Bournemouth Roofing & Building and prices every job himself instead of sending somebody else. The company is Bournemouth Roofing & Building Ltd, number 15471719, registered in England and Wales. If you want to talk something through before you commit to anything, the site visit and the written itemised quote are both free.
Free quote
Conservatory roof leaking?
Thomas comes out, looks at the roof from the ground and inside, and puts what he found in a free written, itemised quote, including whether it needs a repair, a new roof covering or nothing more than better ventilation.