Flat Roof Upstands: Why They Matter and How They Should Be Detailed

Upstands are one of the most critical details in flat roofing. They're where the horizontal roof surface meets a vertical element — a wall, a parapet, a rooflight kerb, or a chimney. Get upstands wrong and you'll have leaks. Get them right and the roof will perform for decades.

What is an Upstand?

An upstand is the vertical portion of the roofing membrane where it turns up to meet a wall or other vertical element. The membrane must extend up this vertical face by a minimum of 150mm above the finished roof surface, and be terminated and weatherproofed at the top.

Why 150mm?

The 150mm minimum is specified in British Standards and building regulations to ensure that windblown rain and water splashing off the roof surface cannot drive up behind the upstand. In exposed locations, more may be advisable.

Termination at the Top

At the top of the upstand, the membrane must be properly terminated. For wall abutments, this means a metal flashing — lead, aluminium, or code 3 lead — dressed into a mortar joint in the brickwork and pointed in with a flexible sealant.

Common Upstand Failures

• Upstand height below 150mm — water drives behind the membrane in heavy rain
• Failed or missing flashing at the top of the upstand
• Membrane not adequately bonded to the vertical face
• Timber firring at the upstand has rotted, pulling the membrane away from the wall
• Sealant failed at the flashing/brickwork junction