The figures you read everywhere are roughly right. What those figures do not tell you is why one thirty-year-old roof is still in fine condition and another is already leaking after twelve years.
I come across both, sometimes in the same street. The difference almost never lies in the brand of the covering.
What really decides the lifespan
The fall. The most important factor of all. Water that stays standing puts permanent strain on the covering, collects dirt and speeds up ageing. See
fall on a flat roof.
The details. Upstands, roof edges, roof penetrations. If they are made generously and neatly, the roof surface lasts a long time. If they are made too tight, ageing starts there and not in the main area.
Whether there has ever been water in the insulation. A roof that has leaked for a while is never the same again afterwards. Wet insulation stays wet and keeps the whole build-up cold and damp. See
wet insulation.
“A flat roof is not killed by the sun. It is killed by water that has nowhere to go.”
Fouad, owner of Astrodak Service
Signs that the end is near
A flat roof gives clear warnings before it starts to leak. If you look at it once a year, you will see them coming.
Fine cracks. Hairline cracks in the top layer, especially at the bend of an upstand. The material has lost its stretch and that does not come back.
Bare patches. On bitumen with a mineral top layer: patches where the slate chippings have gone and the bitumen is exposed. That is where UV starts its work.
A spongy feel. The roof gives when you walk on it and leaves an impression. That means water in the insulation, and that is a different conversation from a repair.
Have it checked before it becomes a job
Most of the damage I repair was still a small fix a year earlier. Have your roof checked with no obligation and you will know exactly where you stand.
Overlay or replace
On a bitumen roof an extra layer can sometimes buy a few more years, but only if the build-up underneath is dry and the number of layers stays limited. If there are already three layers of bitumen on it, you are mainly adding weight to a problem that sits underneath.
And overlaying means you cannot see what state the insulation is in. On a roof of twenty years or older, that is exactly the question you want answered, because that is where your comfort and your energy bill come in.
What you can do to get the full lifespan
Two things, and they cost almost nothing. Keep the outlets clear, because a blocked outlet means standing water, and standing water is the biggest enemy of any covering. And have the roof looked at once a year, so a loose roof edge profile or a small crack at an upstand is spotted before it becomes a leak.
Why a roof of the same age can be different
Two flat roofs that are twenty-five years old can be completely different. On one, the water runs neatly to the outlet and the covering has aged evenly everywhere. On the other, there is always a puddle in one spot, and there the top layer reaches the end years before the rest.
That means a flat roof is rarely worn out everywhere at once. On a roof with one bad spot and an otherwise healthy covering, a targeted repair plus improving the fall is often the smart route, not a full replacement.
Frequently asked questions about the lifespan of a flat roof
