Conservatory Leaking At The Wall Junction

Key takeaways

• This is the one conservatory leak that damages the main house, so it deserves more urgency than a ridge drip.
• The flashing must be cut into a mortar joint and dressed down over the roof. Sealant smeared over the top is not the same thing.
• Box gutters between a conservatory and the house block with debris and then overflow inward.
• Damp appearing inside the house next to the conservatory, rather than in the conservatory itself, points straight at this junction.
• Rendered walls hide the problem for longer, because the water travels behind the render before it shows.

What should be there

A correctly formed junction has a flashing, usually lead or a proprietary alternative, with its top edge cut into a raked out horizontal mortar joint, wedged, and pointed over. It then dresses down over the conservatory roof or into the top of a gutter section, so that water running down the house wall is caught and directed outward onto the roof. On many conservatories, particularly where the roof sits below a window or where levels are awkward, there is also a box gutter: a lined channel running along the wall to collect that water and carry it to one end. Both details are straightforward to build correctly and both are frequently skipped or rushed, because the conservatory installer is a glazing specialist rather than a roofer.

The sealant shortcut

The most common defect at this junction is not damage but a shortcut: instead of cutting into the mortar joint, the flashing has been stuck to the face of the brickwork with silicone or a bituminous compound. It works for a couple of years. Then ultraviolet light, thermal movement and the sheer difficulty of bonding sealant to porous brick take their toll, the top edge lifts, and water runs behind the flashing rather than over it. From the ground it still looks like a flashing, which is why this defect survives casual inspection. If you can see a continuous bead of sealant along the top edge of the flashing rather than a mortar line, that is what you are looking at.

The box gutter problem

A box gutter along the house wall is a trap for everything that comes off the main roof above: moss, grit, leaves and the sludge those make when wet. Because it is tucked behind the flashing and below the conservatory ridge line, nobody ever looks at it, and it can be completely blocked without any external sign. Once it fills, water rises above the upstand at the back and goes into the wall, or it overflows at the front into the conservatory. Both symptoms come from the same cause. Any conservatory more than a few years old with a box gutter should have that gutter checked as the first step in any leak investigation, before anyone starts talking about the roof panels.

Why the damp shows up inside the house

This is the detail that makes this junction different from every other conservatory leak. Water getting behind the flashing enters the wall of the house itself, not the conservatory, and it then behaves like any other penetrating damp: it travels through masonry and emerges on the internal face, sometimes in the room next to the conservatory, sometimes in the room above it. Homeowners frequently report damp in a dining room or a bedroom and never connect it to the conservatory outside, because the conservatory is completely dry. If damp has appeared on an internal wall that backs onto or sits above a conservatory, this junction should be the first suspect.

Rendered and painted walls

Where the house wall is rendered, the flashing detail is harder to form well and much easier to fudge, because there is no obvious mortar joint to cut into and the render surface tempts people into simply sealing against it. Water then gets behind the render, spreads laterally, and emerges some distance away, which makes diagnosis slower. Hollow sounding render, blown patches or a horizontal crack near the flashing line are all signs that water is tracking behind it. The correct approach on a rendered wall is to cut a proper chase for the flashing and make good, rather than to rely on any surface bond.

What to look at, from the ground and from inside

From the garden, look along the line where the conservatory meets the house. You want to see a metal flashing whose top edge disappears into a mortar line, not one that stops at a bead of sealant. Look for gaps, lifted corners, and any point where the flashing has been cut around a downpipe or a window sill, because those transitions are weak spots. From an upstairs window you can often look down onto the box gutter if there is one, which is the easiest way to see whether it is full of debris. Inside the house, check the wall shared with the conservatory for staining, and check whether it worsens after rain rather than in cold weather.

Why this junction deserves more urgency than a ridge drip

Treat this with more urgency than a ridge leak. Water entering the house wall does slow structural damage to plaster, timber and any embedded joists, and unlike a conservatory drip it is not confined to a room you can shut off. Deal with it promptly if damp is visible inside the house, if plaster is blowing, or if there is any suggestion the water is reaching a floor structure. It can wait a short time for settled weather, since flashing work needs dry conditions to be done properly, but this is not one to leave for a season. A blocked box gutter, by contrast, can often be cleared quickly and may be the whole fix.

Getting it repaired properly

The right repair cuts a chase into a mortar joint, fits new flashing in appropriate lengths with laps, wedges it, and points it in. If there is a box gutter, it should be cleared, its lining checked, and its outlet confirmed to run freely. Ask specifically whether the flashing will be cut in or sealed on, because that single question distinguishes a lasting repair from a two year one, and it is the question most likely to reveal how the original was done. It is also worth asking who is doing the work: this is roofing and leadwork rather than glazing, and conservatory installers are not always the right trade for it. One further point is worth raising before work starts. Lead expands and contracts a great deal with temperature, so it has to be fitted in limited lengths with laps between them rather than as one continuous strip along the whole wall. A single long piece will buckle and split within a few seasons no matter how well it was fitted on the day, and that is a common reason a flashing that looked perfect at handover has failed by its third winter.

Frequently Asked Questions

Why is my house wall damp next to the conservatory?

Almost certainly the junction where the conservatory roof meets the wall. Water getting behind a failed or badly fitted flashing enters the house wall rather than the conservatory, then travels through the masonry and appears inside. That is why the conservatory itself can be completely dry while a room next to it is damp.

What is a box gutter and why does it matter?

It is a lined channel running along the house wall that collects water coming off the main roof above and carries it to one end. Because it sits hidden behind the flashing, it silts up with moss and grit unnoticed, and once blocked it overflows either into the wall or into the conservatory.

Can I just seal the gap with silicone?

It will hold for a season or two at best. Sealant does not bond reliably to porous brick or render, and the joint moves as temperatures change. The lasting fix cuts the flashing into a mortar joint so gravity and geometry keep water out rather than adhesive.

Who should do this work, a roofer or a conservatory company?

This is leadwork and roofing rather than glazing. Many conservatory installers are glazing specialists and the wall junction is the part of the job most often rushed. Whoever does it should be able to explain how the flashing will be cut into the wall and how the gutter, if there is one, will drain.

Why does it only leak sometimes?

Because most failures here are gaps rather than holes. They shed ordinary rain and fail when water arrives in volume or is driven at the wall by wind, or when a blocked gutter finally fills. This is why the same junction can be fine for months and then produce damp after one particular storm.

Is this covered by my conservatory warranty?

It depends on the age and terms, and on whether the wall junction was part of the original installation. It is worth checking the original paperwork before commissioning work, because a junction that was formed incorrectly at installation is a workmanship matter rather than wear and tear.