What is a parapet wall, and why do they leak so often?
A wall that continues above the roof line is wet on both faces and at the top, with nothing above it. Why that makes parapets and their copings the most exposed masonry on a building, and what actually fails.
- Difficulty
- intermediate
- Time
- 9 min
- Read
- 4 min
- Safety
- warning
Short answer
A parapet is a wall carried up past roof level. Unlike the rest of a wall it gets rain on both sides and on top, it cannot dry in one direction, and it is thermally exposed on both faces so it moves more. The coping on top with its damp proof course and drip, and the tray at its base, are what keep water out of the building. Failures here show up as damp at the top of the wall inside.
Parapets are common on terraces, on flat-roofed extensions, on Georgian and Victorian frontages and on almost anything with a hidden gutter. They are also the part of a building that takes the worst of the weather with the least protection, which is why the same handful of failures come up again and again.
Safety
Step by step
- Understand what makes a parapet different.An ordinary wall is sheltered above by the eaves and dries inwards and outwards. A parapet is exposed on two faces and on its top edge, and it stands proud in the wind. It wets more, dries differently and moves more.
- Look at the coping.The stone, concrete, clay or metal capping along the top. Its job is to shed water clear of both faces, so it should overhang each side and have a drip groove on the underside of each overhang.
- Look for the damp proof course under the coping.There should be a horizontal damp proof course immediately below the coping, stopping water that gets past the joints from running down inside the wall. It is a standard detail and a standard omission.
- Look at the joints between coping units.Open, cracked or mastic-repaired joints are the usual entry point. Movement along a long run of coping opens them, which is why long parapets should have movement joints.
- Find the tray at the base of the parapet.Where the parapet meets the roof there should be a cavity tray and flashing arrangement taking water out and preventing it tracking down into the wall below the roof. This is the same principle as an abutment tray anywhere else.
- Check the outlets and the gutter behind it.Many parapets conceal a gutter. A blocked outlet turns that gutter into a tank, and the water backs up over the flashing and into the building. Outlets on hidden gutters are the single most valuable thing to keep clear on this kind of roof.
- Read the internal damp.Damp appearing across the top of an upstairs wall, at ceiling level, on the elevation with the parapet, is the classic pattern. It follows heavy rain, and it often gets misdiagnosed as condensation because it appears high up.
Tips
- A parapet with no drip grooves under the coping overhangs will run water down both faces, and the resulting staining is usually visible from the pavement.
- Sulphate attack shows as horizontal cracking and expansion in the mortar of a persistently saturated parapet in older brickwork. It is a sign the wall is staying wet, not just that the pointing is old.
- Repointing a parapet in a hard cement mortar when the bricks are soft traps moisture in the brick, and the brick faces spall off in frost instead. Match the mortar to the masonry.
Common mistakes
- Sealing coping joints with mastic year after year — It is a two-year fix on a detail that needs the damp proof course beneath it to be working. Mastic on top of a missing DPC just moves where the water goes in.
- Painting a damp parapet with a waterproof coating — A parapet needs to dry, and it can only dry through its faces. Sealing one face traps water in the wall and drives the damage inwards or into frost damage.
- Ignoring a hidden gutter until it overflows — By the time water appears inside, it has usually been standing above the flashing for some time and the roof deck beneath has been wet for months.
If it doesn't work
Damp along the top of an upstairs wall after heavy rain
Cause: Water entering through the parapet — coping joints, a missing DPC under the coping, or a failed tray at the base — Fix: Inspect the coping and the gutter outlets from a safe position first. The order of investigation is outlets, coping joints, DPC, tray — cheapest and most likely first.
Bulging or horizontal cracking in a parapet
Cause: Sustained saturation, sulphate attack, or corroding embedded metal — Fix: Keep clear of the base, and get it inspected. A parapet is at height, its failure mode is falling masonry, and it is not a place to wait and see.
Staining running down both faces of the parapet
Cause: Coping with insufficient overhang or no drip groove — Fix: Confirm from the ground with binoculars. Replacing or re-detailing coping is specialist access work, but knowing that is the cause stops money being spent on the wrong thing.
Questions people ask
Is a parapet structural?
It is part of the wall and it is loaded by wind, so yes in the sense that matters. A leaning, bulging or cracked parapet is a structural and a safety issue because of what is beneath it.
Can I re-render a parapet to stop it leaking?
Only if the render is designed for it, with the coping, DPC and drainage details working. Render over a parapet with no working coping detail traps water behind it and accelerates the damage.
Who owns a parapet on a terrace?
Usually it is part of the wall it sits on, which on a party wall means shared. In England and Wales work to it may be notifiable under the party wall legislation, and on a leasehold flat it is normally the freeholder's responsibility as part of the structure.