The short answer
Look at the timing and the pattern. A roof leak shows up during or soon after rain, in one place, and often grows from a single point. Condensation shows up in cold, still weather, spreads across a wider area, and collects on cold surfaces such as the underside of felt, nail tips and window glass. If the damp appears whatever the weather, or is worst on cold mornings, think condensation and check ventilation.
A practical way to tell moisture from the roof above from moisture made by your own household, using what you can see from the loft.
Timing and weather give the first clue
Start with when the damp appears. A leak follows rain. The ceiling stain or drip usually shows up while it is raining or within a few hours of a heavy shower, and it eases when the roof dries. Wind-driven rain can make a leak appear only when the weather comes from one direction, which is a strong hint that water is entering at a vulnerable spot on that side of the house.
Condensation follows the temperature and the way you live. It tends to build up on cold mornings, in still weather, after cooking, showering or drying washing indoors, and it can be present on a dry day. Water droplets form when warm, moist household air meets a cold surface, such as the underside of the roof covering. Keep a simple diary for a week, noting the weather, whether the heating was on and what the damp looks like.
If the pattern matches rain every time, the roof is the suspect, and our page on a roof leaking after heavy rain is the better place to continue.
What to look for in the loft
Pick a dry afternoon, take a torch, and go up only if the hatch is easy to reach and you can stay on the joists. Study the underside of the roof covering. Leaks leave a trail. You may see a dark, wet stain around a single tile or a nail, a streak running down a rafter, or wet insulation directly below a spot where daylight shows. Water often travels along timber before it drops, so the wet patch may sit a little way from the entry point.
Condensation looks different. Felt or boarding can be damp over a broad area, with beads of water on nail points and fine black mould speckled across the timber. Insulation may be damp on its upper face, and the damp is often worse on the north-facing slope or around bathroom extract points that vent into the loft instead of outside.
Take pictures of every stain, bead of water and patch of mould, and note the date on each. A roofer can read those images well, and they can save a visit when the answer turns out to be a ventilation fix rather than a repair to the covering. Include one wide shot of the whole roof space, so the stain can be placed against the rafters and the eaves.
Why condensation forms in a roof space
Every household makes moisture. Breathing, cooking, bathing and drying clothes all add water vapour to the air, and the warmer the room, the more vapour the air can hold. When that air rises through gaps around hatches, light fittings and pipes into a cold loft, it condenses on the first cold surface it meets. Tight modern houses and blocked eaves make the effect worse because the moist air has nowhere to go.
British Standard BS 5250 covers the control of moisture in buildings. Marley's guidance for a cold roof laid with non-breathable (HR) underlay gives eaves ventilation of 25mm along the length of the eaves for pitches of 15 degrees or less, and 10mm for steeper pitches. Insulation pushed hard into the eaves can block those gaps, and the cure may be as simple as fitting eaves vents or tray inserts that keep an air path open above it.
Extractor fans that vent into the loft instead of to the outside, and loft hatches that do not seal, are other frequent causes. Sorting these out may stop the damp without any roof repair at all.
Can it be both, and what happens next
Yes, and that is why a proper look matters. A small leak can sit alongside condensation, and one can disguise the other. A wet patch under a cracked tile may be joined by damp across the felt, so mending the tile fixes only half of the story. A roofer who surveys the loft and the roof together can sort the two apart.
If the cause is a leak, the repair is the covering, flashing or gutter that is failing. If it is condensation, the answer is ventilation, sealing the paths that move warm air into the loft, and sensible habits in the house, such as using extractor fans and drying washing in a vented room. Do not seal the loft up tight, since cutting off ventilation can make condensation worse.
Whichever it is, dry out wet insulation and treat mould, then see our page on a damp ceiling after a roof leak. If you cannot tell, send us your loft pictures along with a note of the weather on the days the damp appeared, and our survey will start with the spots those clues point to. Our emergency and general roof repairs page shows how we approach urgent leaks.
More on this in Emergency & General Roof Repairs in Crawley
Two more damp questions
Does black mould in the loft mean the roof leaks?
Not by itself. Mould grows where damp air meets cold timber, so it is a common sign of condensation. A leak tends to leave a darker tide mark or a rotted patch around one spot. If the mould is spread widely and the roof covering looks sound from the ground, ventilation is the likelier cause.
Will more insulation stop condensation?
Not on its own. Insulation keeps heat in, which makes the roof space colder, and it can block the eaves if it is laid carelessly. Moisture still needs a way out, either through proper ventilation or a sealed ceiling that stops warm air rising. Add insulation with the air path in mind, not instead of it.