How Do You Know a Flat Roof Needs Replacing?

Check One: Count the Patches

The most reliable single signal, and the easiest. Look down at the roof from an upstairs window and count the visible repairs - patches of different colour, squares of newer material, blobs of black sealant.

One patch with a known cause is a fault that was fixed.

Three or four patches of different ages, in different places, is a covering that has been producing faults rather than having them. That pattern predicts what a proper inspection finds more reliably than anything else you can see.

And two patches on top of each other in the same place means the material around the first one could not hold it.

Check Two: Look Two Days After Rain

Not during rain - two days after, when everything that is going to drain has drained.

Pools still sitting there mean the roof is not shedding water. A flat roof is not flat: it should be laid to a fall so water runs to the outlet.

Ponding does not itself make a hole, but it keeps the covering permanently wet, magnifies any small defect, accelerates ageing, and in freezing weather puts ice loads where you do not want them.

More practically: a covering that spends its life under water is a poor candidate for patching, because a patch has to bond and stay bonded in the worst conditions on the roof.

A tidemark or a ring of silt showing where water sits, even when it is dry, tells you the same thing.

Check Three: Crazing Versus a Split

This distinguishes a fault from an ending, and you can usually see it from a window or with a zoomed photograph.

A split, tear or lifted edge in one place, with the rest of the surface looking even, is a fault. Repairable.

Crazing - a network of fine cracks spread across the whole surface, like old paint - is the material at the end of its flexible life. There is no useful patch for that, because it will crack next to wherever you patch it.

On felt, add the mineral finish. If the grey or green granules have weathered off and the bitumen underneath is exposed and shiny, the covering is well through its life.

Check Four: Is the Ceiling Ever Dry?

From inside. Does the damp patch below the flat roof dry out between spells of rain, or does it stay damp?

A patch that appears when it rains and fades afterwards is water getting in and then stopping. That is a leak with a specific route.

A patch that stays damp regardless is more serious. It usually means water is sitting inside the roof build-up rather than passing through it - saturated insulation in a warm-deck roof, or a soaked deck. Neither dries out in place, because there is a vapour barrier below and a waterproof covering above.

That is the finding that most often turns a repair into a replacement, and it is one you can spot from your own ceiling.

Check Five: What Is It Made Of and How Old Is It?

On a Widnes estate house, if the garage or extension roof is original and has never been recovered, it is somewhere between fifty and seventy years old and it is probably built-up felt on a chipboard deck.

Neither of those is a scandal - they were standard - but both matter. Felt of that age has lost its flexibility, and ordinary chipboard swells and delaminates once wet and does not recover.

If it has been recovered in the last twenty years, the covering may well be EPDM or GRP and the deck may have been replaced at the same time, which changes the picture entirely.

Ask whoever quotes what the deck is made of and what it felt like underfoot. That is the question that decides the size of the job, and it has a specific answer.

What These Five Tell You

Patches few, no ponding, no crazing, ceiling dries out: repair.

Patches several, ponding, crazing across the surface, ceiling permanently damp: the conversation is about replacement, and the useful next step is somebody establishing the state of the deck and whether the insulation is wet.

Anything in between is genuinely a judgement, and it is worth having somebody look rather than deciding from a window - but you will now know what questions to ask, which is most of the battle.

And do not walk on it to check. A flat roof that has taken water is exactly the sort that gives way, and a garage roof is high enough to break a leg.