What is a parapet, and why does it leak where it meets the roof?
A parapet is a low wall around the edge of a flat or low-slope roof, and it leaks mainly where it joins the roof. The join is a line where two different materials meet, and it needs a flashing that bridges them.
The roof membrane runs up the inside face of the wall and must be sealed to it. Above the membrane, the wall must be capped so that rain does not soak into the top of the wall core. Each of those two jobs fails differently, and a leak can come from either one.
How does clay tile coping protect a parapet, and how does it fail?
Clay coping sheds water off the top of the wall and down both faces, and it works as long as its pieces are tight and the joints between them are sealed. It fails when the bed under it cracks or when the joints open and let rain into the core.
The Kensington house in the city report had a parapet lined with clay roof tiles, which is a finish that depends on careful jointing. A tile that is set well sheds water for decades. A tile set on a bed that has gone soft lets water in along its underside, and that water finds the easiest route down.
When coping is lifted, the technician looks at the bed and the sealant at the joints. Old mortar that has turned to sand is cut out, and the new bed is laid to fall outwards so the water leaves the wall rather than sitting on it.
Why is base flashing so often the cause?
Base flashing is the strip that carries the membrane up the wall and it is the first thing to fail when it is nailed through the face without a proper lap. Once it lifts, water gets behind the membrane and runs down inside the wall.
Flashing that was fixed with the nails in the wrong place, or that was lapped the wrong way round, lets water in at each joint. The new flashing is lapped so water always runs over the top of the joint and never into it.
Sun-baked membrane and sealant also shrink and crack, and the flashing that holds them down is lifted by that movement. A repair that renews the flashing and the membrane edge together keeps the joint working for the life of the roof.
What does the inside of the wall tell you?
The room below a leaking parapet shows the pattern: a stain close to the wall, a bloom of white salt where water has dried, or bubbling paint. The place of the stain on the wall is the strongest clue to where the water entered.
White salt on the inside face of a parapet means water has passed through the wall and dried there, sometimes many times. It is a sign of a repeat leak and a reason to open the wall rather than paint over the stain.
A stain that appears only after heavy rain points to flashing or coping. A stain that appears after any rain points to a lower entry, such as a crack at the bed of the wall, and that needs a different repair.
A parapet that is clad in plaster or stucco shows its faults differently. Hairline cracks at the top of the wall let rain in beneath the cap, and the stain then appears on the inside of the wall rather than on the ceiling. The technician checks the cap and the top few courses before deciding whether the crack is a surface fault or a route for water into the core. A crack that opens and closes with the temperature is treated as a movement joint that needs a flexible seal, not as a fault that a mortar patch will close for good.
