Why does a parapet leak into the wall rather than the roof?
Because the water runs over the top of the wall and down the inside face, where the cap has a gap. The membrane may be perfect and the wall still takes the water.
Rain falls on the cap and sheds to either side. If the cap is cracked or a joint is open, water runs into the top of the wall instead, and the wall takes it down into the cavity. From there it can reach the top-floor ceiling or run down an inner wall, and it can stay in the masonry for a long time before anyone notices.
That is why a roofer checks the parapet top before the roof membrane. The membrane may be sound and the damage still be at the cap.
- Check the cap before the membrane
- Look for stains on the inside face of the top-floor wall
- Note whether the stain follows a joint in the cap
How is counterflashing fixed to the wall?
It is set into a reglet cut in the wall or fixed to the face as the detail requires, and it laps the membrane so water sheds down over it.
Counterflashing is the top edge of the roof flashing, and it has to lie over the base flashing so that water runs off it. When it has been fixed with the wrong sealant, or when the reglet has cracked or been filled, the top edge opens and water enters behind the base flashing.
A roofer checks each length, resets it where it has lifted, and seals the joints with a product the membrane manufacturer names. Where the counterflashing is missing altogether, the detail is rebuilt to the standard the roof needs.
- Check that the counterflashing laps the base flashing
- Reset any length that has lifted from the wall
- Use the sealant that the membrane detail names
When is a parapet repair not enough?
When the wall has been wet for a long time, when the masonry is cracked through, or when the coping is failing along its whole length.
A parapet that has been wet for a long time may have damaged masonry or lintels at the top. A roofer can see the cracks that are on the surface, but not what lies inside the wall, and where the readings suggest a structural problem the roofer says so and recommends a structural check.
Where the coping has failed along most of its length, a full replacement of the cap is quoted rather than repeated patching, because patching a failing run tends to move the leak to the next joint.
- Cracks that run through the masonry, not just the surface
- Readings that stay high after the wall has dried
- Coping that has failed along most of its length
How should a parapet be inspected for the first time?
From the roof and from the top-floor room below, on a dry day and again after rain. A first inspection gives the baseline that every later check is measured against.
A parapet inspection works best as a pair of looks. From the roof, the roofer walks the top and marks every crack, gap and loose piece. From the top-floor room, the roofer or a homeowner looks at the inside face of the wall under the parapet for stains, bubbled paint and any ceiling line that is darker at the wall. Together the two views show whether the water is entering at the top and where it runs.
Do the look on a dry day first, then again after the next rain. Water shows its route on the second look in a way it does not on the first, and a stain that appears only after rain points to a specific joint. Write down the date of each look, so a repair can be checked against the baseline later.
A homeowner can do the room-side look without going on the roof. The roof side is the roofer's job, and it should be done from a secure position with the right fall protection. Keep the record of both looks in the same file as the roof's other records.
- A dry-day look from the roof and from the room below
- A second look after the next rain, noting where water appears
- A dated written note of each look kept with the roof records
