If you own a Victorian or Edwardian terrace in South London and you have a damp patch at the top of an upstairs wall, the odds are strongly that your parapet is the cause and your roof covering is fine.
This is the most common misdiagnosis in London roofing. The homeowner sees water inside, assumes the roof, and gets quotes for roof work. Meanwhile the water is entering thirty centimetres above the roof, at coping level, and running down inside the wall.
What is a parapet wall, and why do London terraces have them?
A parapet is an external wall that carries on upwards past the roof, so the roof sits behind it rather than running out to an open edge.
On a London terrace you will usually find them in two places: across the front elevation, where the parapet hides the roof from the street and gives the facade its flat top, and along the party walls, where the wall between you and your neighbour rises above both roofs.
They exist for two reasons. Aesthetically, Georgian and Victorian street architecture wanted a clean parapet line rather than a row of visible roof slopes. Practically, a party wall taken up above the roof line was and remains a fire separation between terraced properties.
Both reasons are sound. The consequence is that a large stretch of masonry is exposed to weather on three faces, capped with stone, and joined to the roof at its base. That is a lot of detail in a small area, and detail is where roofs leak.
Why do parapets leak so often?
Because a parapet gets wet from every direction and has more junctions per square metre than any other part of the building.
Think about what an ordinary wall has to deal with. Rain hits its outer face, runs down and evaporates, and it is protected above by the roof overhang and below by a damp proof course.
A parapet has none of that. It is exposed on the outer face, on the inner face towards the roof, and across the top. It has no roof over it. It usually predates any reliable damp proofing at high level. And it has to be joined, watertight, to the roof covering at its base.
The specific failure points, in the order we find them:
Open joints between coping stones. The mortar joints between copings are horizontal, exposed and constantly moving with temperature. They are the first thing to fail and the most common single entry point.
Cracked or spalled copings. Stone and concrete copings crack, and once frost gets into a crack it opens further each winter.
Copings that have shifted or lost their fall. A coping should shed water outwards. Bedding that has washed out over decades lets stones settle unevenly, and a coping that now falls inwards is directing water onto the roof side of the wall.
No damp proof course under the coping. On older work there is often nothing under the stone at all. Anything that gets past the joint has an uninterrupted path down inside the wall.
Failed pointing on the wall faces. Eroded mortar on a wall exposed on both faces absorbs water rather than shedding it.
The junction at the base. Where the roof covering turns up the parapet, it has to be dressed, sealed and terminated properly. On old work this is often a mortar fillet that has cracked, or a felt upstand that has pulled away.
Blocked or missing outlets. Because the roof sits behind a wall, water cannot run off the edge. It has to leave through an outlet or a chute. Block that and the parapet becomes the side of a tank.
That last one is worth dwelling on. A parapet roof has no free edge. Every drop of water that lands on it must leave through a hole. When the hole blocks, the water rises against the parapet and finds the first weakness in it.
How do you tell a parapet leak from a roof leak?
Three signals: where it shows, when it shows, and what the ceiling below is doing.
Where it shows. A parapet leak appears at the top of an upstairs wall, or in the corner where the ceiling meets an external wall, or as a stain running down the inside face of a chimney breast or party wall. A roof covering leak usually appears in the middle of a ceiling, away from the walls.
When it shows. Parapet leaks are frequently wind-driven. They appear after storms that blew rain against a particular elevation, and not after a still day of ordinary rain. If your damp patch has a favourite kind of weather, that points at a wall rather than a roof.
What it looks like between events. A parapet holds a lot of water in the masonry, so the staining often stays damp for days after the rain has stopped, and it can reappear without new rain as the wall dries inwards.
What is above it. Go outside and look up from across the road. If the damp inside is directly below a section of parapet where you can see displaced copings, an open joint or vegetation growing out of the wall, you have your answer.
There is one more test worth knowing, because it saves money. If a contractor has already replaced or repaired the roof covering and the leak persists unchanged, the leak was never in the covering. That sounds obvious written down, and it is remarkable how often the next quote is for more roof work.
What does the water actually do inside the wall?
It travels, and it does not travel in a straight line. This is why parapet leaks get misdiagnosed and why they come back.
Water entering an open coping joint enters the core of the wall. Victorian parapets are often solid masonry with a rubble or poorly filled core, so there is a route down through the middle of the wall rather than across a clean cavity.
From there the water follows whatever path is available: down a mortar bed, along a timber wall plate, across a joist end bearing into the wall, until it reaches something that stops it or something that shows it. That point can be a metre or more from where it entered, on a different elevation, at a lower floor.
Two consequences follow, and both matter commercially.
The first is that sealing the point where the damp shows achieves nothing at all. The water is not entering there.
The second is that joist ends and wall plates bedded in a wet parapet rot quietly for years. By the time a ceiling stains, timber in the wall has usually been damp for a long time. That is the argument for dealing with a parapet leak in the year you notice it rather than the year it becomes unavoidable.
How is a parapet leak actually fixed?
By treating the parapet as one continuous detail from the top of the coping to the roof, rather than as a list of separate repairs.
The work depends on the survey, but the sequence is usually some combination of the following.
Repoint or rebed the copings. Where the stones are sound and the joints have gone, they are raked out and repointed with an appropriate mortar. Where stones have shifted, they are lifted, rebedded to a proper outward fall and reset level.
Replace what cannot be saved. Copings cracked right through or badly spalled are replaced rather than filled.
Repoint the wall faces where the mortar has eroded, so the masonry sheds water rather than absorbing it.
Waterproof the top surface and the joints as a continuous layer. This is where a liquid-applied system earns its place, because a parapet is nothing but awkward geometry. A seamless system can be taken across the top of the copings, down into every joint, around the corners and down the roof face of the wall as one unbroken layer with nothing to cut and nothing to lap. Every alternative involves forming a joint somewhere, and the joint is what failed in the first place.
Carry the waterproofing into the roof. The upstand where the parapet meets the roof covering is dressed and reinforced so that the wall and the roof become one continuous waterproof surface rather than two surfaces meeting at a line.
Clear and, where needed, enlarge the outlets. A parapet roof depends entirely on its drainage. Outlets and chutes are cleared, checked for adequate size and added to the maintenance schedule.
Where the parapet sits alongside a pitched roof rather than a flat one, the same principles apply at the abutment, and the flashing or secret gutter behind the parapet becomes part of the same detail. Our page on pitched roof repairs covers that side of it.
What does this mean if you are buying or own a terrace?
Look at the parapet before you look at anything else, and look at it from the street.
If you own the house, a set of binoculars and ten minutes from the opposite pavement will tell you more about your next five years of roofing costs than anything you can see from inside. What you are looking for: coping stones that are cracked, missing, sitting unevenly or leaning; open joints you can see daylight or shadow in; plant growth out of the wall or the joints, which means there is already enough moisture and organic matter up there to support it; and staining or streaking down the face of the wall below a particular joint.
If you are buying, ask what the parapet detail is and whether there is any damp proofing at coping level. On a Victorian terrace the honest answer is often that nobody knows, which is itself useful information for the price.
And if you already have a damp patch at the top of an upstairs wall, get the parapet surveyed before you accept a quote for roof covering work. K & M Roofing Solutions surveys parapets, copings and abutments as part of every roof survey across South and West London, and the survey is free.
If the roof behind the parapet is flat, our flat roof repair and waterproofing service explains how the upstands and outlets are detailed as part of the same continuous layer. For the waterproofing system itself and the substrates it is approved for, see liquid applied roof waterproofing.