A damp patch is a symptom, and it has several possible causes that cost very different amounts to put right. Working out which one you've got is most of the job.
If water is actively coming in right now — especially near a light — start with what to do first instead.
First: is it actually coming from outside?
Before assuming the roof, rule out condensation. The two look similar on a ceiling and are completely different problems.
| Condensation | Water getting in | |
|---|---|---|
| Shape | Diffuse, spread out, often in corners | Defined patch, often with a tide-mark edge |
| Colour | Grey, sometimes black mould spotting | Tea-coloured or ringed, from dust and timber |
| Timing | Worse in cold weather, better with ventilation | Responds to rainfall |
| Where | Cold outside walls, window reveals, unventilated rooms | Anywhere, including the middle of a ceiling |
| In the loft | Black mould growing on the timbers themselves | Wet timber, usually traceable back to one point |
The quick test is rainfall. If it grows during a downpour and settles afterwards, it's ingress. If it's worst on a cold, dry morning, look at ventilation first.
The loft is the giveaway people miss. If you can get up there safely, look at the timbers rather than the ceiling below: a condensation problem shows up as black mould growing on the timber itself. That's a ventilation fault, not a hole, and nothing done to the outside of the roof will change it.
The usual causes, roughly in order of how often we find them
Blocked gutters
The most common and the cheapest. A gutter that can't carry water overflows down the wall, and brickwork soaks it up until it appears inside — often at the top of an upstairs wall or where the ceiling meets it. There's no hole in your roof at all.
Worth checking after autumn, and worth checking before assuming anything more expensive. We'd recommend having your guttering cleared at least once a year as a matter of course — it's the cheapest maintenance there is on a house, and it heads off a problem that otherwise arrives looking like a roof leak.
Rain getting in behind tiles or slates
On a pitched roof, wind-driven rain can get past the covering. That's what the breathable membrane underneath is for — it catches what gets through and takes it back out. Where the membrane is old, torn, or was never there, water carries on into the roof space.
Failed flashings and mortar at junctions
Roofs leak at their edges and junctions, not usually in the middle of a slope. Chimney stacks, valleys, verges and anywhere the roof meets a wall are where the lead work and pointing does its job — and where age shows first.
This is also the source of the fallen mortar people find on their drive: a weathered chimney stack, or a gable verge where the mortar bedding has cracked. It's a sign of the same wear that eventually lets water in, which is why it's worth a look before it becomes a stain.
A failed flat-roof covering
If the patch is underneath a flat roof, the covering itself is a real possibility. Flat roofs almost never fail in the open middle — they fail at the joins where one sheet laps over the next, and at penetrations: skylights, vents, outlets, anything cut through.
Condensation in the roof space
A cold deck flat roof needs the void above the insulation to breathe. If it can't, moisture gets trapped against the underside of the deck and condenses there. It looks exactly like a leak from below and no amount of sealing the outside will fix it.
Why the stain isn't under the hole
Water runs. It travels along the underside of the covering, down a rafter, across a joist, sometimes along a cable, and drops at the first low point it finds. The stain marks where it landed, not where it got in — and the two can be metres apart, on a different part of the roof entirely.
This is why "just put some sealant round that vent" so often fails. The vent was visible; the actual entry point wasn't.
What's useful to have before we come out
- A photo of the patch, and one of the room so we can place it in the building.
- A photo of the roof from outside, standing back far enough to get most of it in frame, plus close-ups of anything obviously wrong.
- When it happens — every time it rains, only in driving rain from one direction, or once after something exceptional. That detail narrows it down more than anything else you can tell us.
- If you can get into the loft safely, where the water actually appears up there.
Send them by WhatsApp or text with a short description. We can often advise straight from the pictures.
And if the answer is that it's fine
Then that's what we'll say. A single stain after exceptional weather, with a roof that's otherwise sound, doesn't need a re-roof — and telling you so costs us nothing.