Flat roof guarantees explained: what a "20-year guarantee" actually means

Flat roofing adverts love a big number. Ten-year guarantee. Twenty-year guarantee. "Fully guaranteed." If you're comparing quotes in Mansfield, Nottingham or anywhere in between, those numbers are doing a lot of selling — so it's worth knowing exactly what they cover, because two "20-year guarantees" can be completely different promises.

Two guarantees, not one

Every properly quoted flat roof comes with two separate layers of cover, and they protect you against different failures.

• The manufacturer warranty covers the material itself. On an EPDM rubber roof, the membrane maker warrants the sheet — in our case Elevate (formerly Firestone) EPDM carries a 20-year manufacturer warranty.
• The workmanship guarantee covers how it was fitted — the trims, the upstands, the outlets, the detailing around pipes and rooflights. Ours is 10 years, in writing, on every job.

Here's the part the adverts skip: flat roofs almost never fail because the membrane wore out. They fail at the details — a poorly dressed upstand, a rushed corner, an outlet that ponds. In other words, they fail on workmanship. A 20-year headline that's really the manufacturer's number, attached to workmanship nobody stands behind, protects you against the failure that almost never happens and leaves you exposed to the one that does.

The question nobody asks: who backs it?

A guarantee is only as good as whoever has to honour it in year seven. Before the length of the guarantee, ask about the strength of it: is it in writing, with a certificate in your hand on completion? Does it name who backs it? And has the company actually been around long enough for its own guarantee to have ever been tested?

• Red flag: a guarantee that's only verbal, or "insurance-backed" with no insurer or scheme named.
• Red flag: a company incorporated more recently than the guarantee it's offering has ever run.
• Red flag: a big number on the flat roof, and silence about who fits it and what training they've had.

What we put in writing

We're an approved Elevate EPDM installer — trained and audited by the manufacturer, which is what keeps their 20-year membrane warranty valid on the roofs we fit. On top of that, every job carries our 10-year workmanship guarantee, in writing, handed over with the paperwork on the day we finish. You can check our Trading Standards Buy With Confidence approval yourself.

That's what a guarantee should look like: named backers, written certificates, and a company with 19 years of roofs you can go and look at.