Flat Roof Parapet Walls: Problems and Repairs

A parapet wall is the low wall that runs around the perimeter of many flat roofs. While it gives the roof a clean architectural finish, it also creates multiple opportunities for water ingress if not properly maintained and detailed.

Why Parapet Walls Cause Leaks

Water can penetrate a parapet wall from the top (through failed coping stones or mortar), from the face (through the brickwork or render), or at the junction with the roof membrane (through failed upstand flashing). All three entry points need to be addressed.

Coping Stones

Coping stones should be weathered (sloped to shed water) and have a proper overhang to throw water clear of the wall face. They should be bedded and jointed in a suitable mortar. Failed or missing mortar joints are a very common cause of water ingress.

Upstand Flashings

At the junction between the flat roof surface and the parapet wall, the roofing membrane must be turned up the wall face by a minimum of 150mm and secured with a metal flashing. This flashing must be properly dressed into the masonry and sealed.

Common Repairs

• Re-pointing coping stone joints
• Replacing cracked or displaced coping stones
• Re-dressing lead or aluminium flashings
• Re-applying upstand membrane in conjunction with roof repair or re-roof
• Applying waterproof render or coating to parapet wall faces