The short answer: a failing flat roof announces itself in seven main ways: standing water that outstays the rain, blisters, splits and crazing, open laps, cracked upstands and flashings, stains indoors, and greenery sprouting from the drainage. Here is what each sign looks like, what it means, and how quickly it needs someone on a ladder.

1. Ponding that outlasts the rain

Puddles after a downpour are normal. Puddles still there two days later are not; the common industry rule of thumb treats water standing 48 hours after rainfall as ponding worth investigating. It usually means a blocked outlet or a deck that has sagged, and while ponding alone rarely leaks, it ages every lap and detail it sits on. Urgency: moderate, but rising with every wet winter.

2. Blisters in the membrane

Raised bubbles, soft or crisp, mean air or moisture is trapped between layers that have lost their bond. Intact blisters can behave for years, which tempts people to forget them; the trouble starts when foot traffic, frost or age bursts one and hands water an open door. Urgency: monitor small ones, act before winter on large or clustered ones.

3. Splits, cracks and crazing

A surface that looks like dried mud or old paintwork tells you the material has oxidised and lost its flexibility; a visible split tells you it has already given way somewhere. Crazing is the early warning, splits are the event. Urgency: crazing earns a place on a watch list; any open split justifies a repair visit now.

4. Laps lifting or fishmouthing

The joints between sheets are the membrane's seams, and when an edge lifts or wrinkles open into a "fishmouth", capillary action does the rest. Failed laps are quiet leaders of leaks because they let water in slowly and invisibly. Urgency: high; a lap you can lift by hand is an entry point today, not a risk for later.

5. Cracked upstands and flashings

Where the roof turns up a wall or parapet, movement between materials cracks the detailing first. Look for hairline gaps at the top edge of flashings, mortar falling from chase joints, and detailing that has pulled away from the brickwork. Most leaks blamed on "the roof" start here. Urgency: high, because these faults funnel water straight into the structure.

6. Stains indoors, away from the obvious

Water travels along decks, joists and vapour layers before it drops, so a stained ceiling is often metres from the actual fault. The stain is proof of entry, not a map. This is where guessing gets expensive and a methodical survey of the whole roof pays for itself. Urgency: the highest on this list; the structure above is already wet.

7. Vegetation in the outlets

Moss on the surface is cosmetic; grass and buddleia growing from a gutter or outlet are structural gossip. Plants only grow where debris and moisture have sat undisturbed for seasons, which means drainage has been compromised for at least that long. Urgency: the clean itself is trivial, which makes ignoring it inexcusable.

How often should you look for these signs?

Twice a year is the rhythm most roofs deserve: once in autumn, before the leaves and the weather arrive together, and once in spring, to see what winter did. Add a quick look after any storm severe enough to make the news. Each check takes minutes from safe vantage points: an upstairs window over the extension, the garden looking back at the garage, a ceiling scan indoors.

The habit matters more than the expertise. You are not diagnosing; you are noticing change, and change is the whole story with flat roofs. A blister that grew, a puddle that stayed longer than last month, a new tidemark on a bedroom ceiling: each is a data point that turns "the roof is probably fine" into an informed decision, made while the options are still cheap.

What should you do with a positive match?

Count the signs, then act on the count:

  • One early-stage sign: a diary note and a check after the next storm.
  • Two or more signs, or any water indoors: professional eyes on the roof, because combinations are how small faults compound.
  • An old roof nobody has examined: skip the checklist and get the whole picture surveyed once, properly, with photographs you keep.