How long does a flat roof last?

Anywhere from ten to fifty years depending entirely on the material and the detailing — and the reputation flat roofs have for failing comes almost entirely from one older material and from poor installation rather than from flatness itself.

The materials, roughly in ascending order of life:

Built-up felt, layers of bitumen felt. The source of the bad reputation: older systems blistered, cracked and split, and expectations of around 10 to 20 years were common. Modern torch-on systems are considerably better than the material's reputation suggests.

Single-ply membranes — synthetic sheets, hot-air welded at the seams. Light, quick to install, and typically quoted at 20 to 30 years or more.

Liquid-applied systems, resins painted on to form a seamless membrane. Excellent around complex detailing and penetrations where sheets are difficult, with life depending heavily on the product and preparation.

EPDM rubber, a single sheet covering an entire small roof with no joints. Very durable, frequently quoted at 30 to 50 years, and its strength is precisely that seams are where roofs fail.

Lead, zinc and copper, traditional metals with very long lives and high cost.

Why flat roofs actually fail, which is rarely the membrane itself:

Standing water, because a "flat" roof must not be flat — a fall of around 1 in 80 is typically required, and inadequate fall is the commonest design fault.

Detailing at edges, upstands, outlets and penetrations, which is where nearly all leaks occur.

Blocked outlets, which is the most preventable cause and the least maintained.

Thermal movement, cracking rigid materials.

Trapped moisture in the build-up, causing blistering.

The warm-roof question. Modern practice places insulation above the deck, keeping the structure warm and avoiding condensation within it. Older cold-roof construction, with insulation below and a ventilated void, is more prone to interstitial condensation and rot.

Guarantees are typically split between product and workmanship — and the workmanship one matters more.