Why a dry roof still drips
Every home makes moisture. Cooking, showers, wet washing and simple breathing all add water vapour to the air, and warm air carries more of it than cold air. That air drifts upwards, finds gaps around light fittings and loft hatches, and meets the underside of the roof deck.
On a cold night the deck is cold, and the vapour condenses there as water. The boards go damp, the insulation soaks and timber begins to rot, while the surface above shows no fault. Stains on the ceiling and black mould in the corners are often blamed on a leak, when there may be no hole at all.
The clue is timing. A leak shows after rain. Condensation tends to appear in cold weather, whether it has rained or not, and it is worse in rooms with a lot of steam. If a roof stayed dry for years and then started to show damp once the ceiling was insulated or the windows were sealed, the build-up has probably changed and moisture is now settling in a new place.
Staining that sits in a ring or runs along the line of the joists is another hint, since the cold timber is where the vapour lands first.
Cold deck roofs and ventilation
A cold deck roof keeps its insulation between or below the joists, with a void left above it, under the deck. The deck stays cold, so the void must be ventilated to carry moisture away. Approved Document C, paragraph 6.11, relies on that ventilation to meet the requirement for cold deck roofs.
Ventilation only works if air can move. Typical faults:
- insulation squashed up against the deck, so no air can pass,
- vents painted over, hidden behind a fascia or never fitted,
- gaps in the ceiling below that let warm room air into the void,
- a void split by cross-members, so air cannot travel from side to side.
BS 5250:2021 is the current code of practice for moisture in buildings, and designers use it for exactly this question. It is guidance, not a checklist, and the answer for a given roof depends on its shape, size and use. A house with a flat extension often needs the void looked at on site, since the arrangement differs from the main roof next door.
Warm roofs and the vapour control layer
A warm roof turns the idea round. Here the insulation is laid over the deck, which keeps the deck and the structure warm, so no ventilated void is needed. The waterproof covering sits on top of the insulation.
A vapour control layer, a membrane that holds back moist air, goes beneath the insulation on the warm side. If it is torn, badly lapped or missing at the edges and around pipes, moist air reaches the cold layers above and condenses just as it would in a cold roof. The layer is only as good as its joins.
Which type is yours? A look from inside the roof void, or at a lifted soffit, often shows whether there is airspace above the insulation. Our comparison of warm roof vs cold roof sets out how each is built.
When a flat roof is re-covered, Approved Document L 2021 treats it as renovation, so thermal upgrades come into play as well. Our page on flat roof Building Regulations goes through what that means for the build-up.
What to do about it
Start with the rooms. Extractor fans in the kitchen and bathroom, lids on pans and shut doors while showering all cut the supply of moisture. Wiping condensation off windows before it runs onto the sill helps too.
If damp is already in the roof, a survey has to find where the moisture arrives. The roofer may lift a ceiling section or an area of the covering to look at the deck, check whether the vents are open and see if the insulation is wet. Wet insulation does not dry out by itself, and boards that have stayed damp for a while may be soft.
The fix depends on what is found. It can be as small as clearing a vent or sealing gaps below, or as large as taking the roof apart and rebuilding it with a vapour layer. Either way, laying a new waterproof covering over a roof that is damp inside only hides the fault.
Photos of the stains, and of the room under them, are the place to start. The written total arrives after the survey. The wider service is on the flat roofing page.
