Flat roofs are not flat
That is the whole thing, really. A flat roof is supposed to be laid to a slight fall so water runs off it, and the vast majority of the problems I get called to come back to that fall being wrong, having settled, or never having existed. If water stands on a covering for days after rain it will find a way through eventually, whatever the covering is made of.
You can check yours without going up. After a shower, look for a dark patch that is still there the following afternoon, or for a tide line of silt and moss in a rough oval shape - that is where the water sits. A roof with proper falls dries out evenly and looks the same colour all over.
The other usual suspect
Where the flat roof meets a wall. The covering has to turn up the brickwork and be capped or tucked into a joint, and that upstand is where movement concentrates. A hairline split at the back of an extension roof will let in more water over a winter than a hole in the middle of the field, because everything drains towards it.
Repair or re-cover
The deciding question is not the state of the surface, it is the state of the deck underneath. If the chipboard or ply is dry and firm, almost anything on top of it is fixable and it is worth fixing. If it has gone soft - and you can often feel this underfoot as a slight sponginess, or see it as a sag in the ceiling below - then a new covering laid over it is money spent on a rotten base.
When I re-cover one I take it back to the deck, replace whatever timber has gone, and put the falls right with firrings before anything waterproof goes down. That is the part that takes the time and it is the part that makes the difference. Anyone quoting a flat roof without having lifted a corner to see what is underneath is guessing.
Signs it has gone past patching
- The deck moves or feels soft when someone walks on it.
- Blisters across the field rather than in one spot - that is water already trapped inside the build-up.
- The ceiling below has stained in more than one place, or the stains have moved.
- You are on the third repair. Two is bad luck. Three means the problem is not where the patches went.
Sun does more damage here than rain
East Kent is one of the sunnier and drier parts of the country, and flat roofs feel that more than pitched ones do. A dark covering sitting in full sun on a still July afternoon gets far hotter than the air temperature, then drops back overnight. That cycle, repeated over years, is what makes old felt go brittle and crack along the lines where it is already stressed - the upstands and the seams.
It is a good argument for a light-coloured mineral finish or a reflective chipping layer on anything large and south-facing, and a good argument against ignoring a small split in August because it is not raining. The splits appear in the dry months. They just do not announce themselves until October.