How long does a flat roof last?

Why flat roofs wear out

A flat roof is never truly flat; it should fall gently towards a gutter or outlet. It sits exposed to full sun, frost and rain with no tiles to shade it, and it expands and contracts every day. Over the years these cycles stress the covering, especially at joints and corners.

Ageing is gradual, so a roof rarely goes from fine to failed overnight. Small signs build up first, which is why a yearly look is worthwhile.

The deck underneath

The covering is only as good as the boards it is bonded to. Chipboard decks swell and lose strength once they get wet; plywood and oriented strand board cope better but still rot if damp is trapped for long. A soft, bouncy deck flexes the covering every time it is loaded, and that movement opens up splits.

Condensation from a warm room below can also soak the deck from the inside on a poorly insulated roof, even when the top surface is perfect.

Falls and ponding

Water left sitting on a roof for days does three things: it adds weight, it keeps the covering constantly wet, and it finds any weakness in a joint. Shallow puddles that clear within a day or two after rain are common; large pools that remain are a sign the falls are inadequate or the deck has sagged.

Correcting falls is normally part of replacing a roof rather than a repair, because the boards or firrings beneath have to change.

Upstands, edges and outlets

The open field of a flat roof is rarely where it fails first. The weak points are the details: the turn-up against a wall, the trim at the edge, the joint around a rooflight and the outlet where water leaves the roof. If these were rushed when the roof was built, they shorten its life regardless of the material.

Outlets and gutters blocked by leaves or moss hold water back onto the roof, which is a simple cause of early wear.

Foot traffic and things on the roof

Most domestic flat roofs are not designed to be walked on regularly. Repeated footfall, ladders resting on the edge, satellite dishes, air-conditioning units and plant pots all concentrate load and can puncture or crease the covering. If a roof needs regular access, it should be designed for it, with a suitable walkway or terrace finish.

Different covering types in general terms

Single-ply rubber membranes, glass-reinforced plastic and built-up felt each age in different ways. Rubber is laid in large sheets with few joints; fibreglass cures into a hard seamless layer; felt is built up in bonded layers with a protective top. Each can perform well when installed properly on a sound, well-drained deck.

The guide to EPDM, GRP and felt compares how they behave. The choice of covering matters, but the deck, falls and detailing usually decide the lifespan more than the material itself.

Signs a flat roof is near the end

Look out for:

• Leaks returning in different places after repairs.
• Soft or spongy areas when the roof is walked on.
• Large pools that stay for days after rain.
• Widespread cracking, crazing or blistering of the surface.
• Covering pulling away from edges or walls.
• Damp or mould on the ceiling below that keeps coming back.

Repair or replace

One defect on an otherwise healthy roof is a repair; our guide to a flat roof leaking explains the usual entry points. Several of the signs above together usually mean the roof has reached the point where replacement is better value than patching.

If replacement is likely, flat roof replacement cost explains what goes into the price. Our flat roofing page covers the work we do on extensions, garages and other flat roofs. We look at the roof, explain the options and give a clear written quotation before any work starts.

Frequently asked questions