Signs Your Flat Roof Is Failing (Common on 1960s-70s Housing Stock)

Updated 14 September 2026 · 4 min read

A huge amount of Britain’s 1960s and 1970s housing - extensions, garages, dormer bungalow flat sections, and whole properties on some estates - was built with flat roofs, and a lot of that felt or asphalt covering is now well past its original design life.

Why flat roofs fail differently from pitched roofs

A pitched roof sheds water quickly by design; a flat roof (in practice, almost always a very shallow slope, not perfectly level) relies on the covering itself staying watertight, since water sits on the surface far longer before draining. Once the felt or asphalt covering cracks, blisters or perishes, water has much more opportunity to find a way in than it would on a steep pitch.

Signs to look for

• Blistering or bubbling in the felt surface, usually from trapped moisture beneath the covering
• Ponding water that doesn’t drain within 24-48 hours after rain, which accelerates wear at that spot specifically
• Cracking or splitting, especially around the edges and any upstands (where the flat roof meets a wall)
• Internal ceiling staining directly below the flat roof section - often the first sign anyone notices, by which point the covering has usually already failed
• Visible sagging in the roof deck itself, which can indicate the timber structure underneath has started to absorb water

Typical lifespan by covering type

Traditional felt (bitumen) typically lasts 15-20 years. GRP fibreglass and modern single-ply membranes can last 25-30 years or more with proper installation. If you don’t know when your flat roof was last covered and it’s an older property, it’s worth finding out rather than waiting for a leak to tell you.

Repair vs full re-cover

A small, isolated blister or crack can sometimes be patch-repaired if the rest of the covering is sound. Widespread cracking, multiple problem areas, or a covering that’s clearly at the end of its expected life is usually better addressed with a full re-cover rather than chasing leaks one at a time.