The short answer
A loft turns damp when damp air drifting up from the rooms below meets cold roof timber and condenses into water. The cures are to stop that air reaching the loft, keep the eaves open so air can cross the space, and make sure the underlay and insulation suit the roof. A leak and condensation can look alike, so the cause needs checking before anything is repaired.
How moisture gets into a cold loft
Everyday life puts water into the air indoors. Cooking, bathing, drying washing and plain breathing all add it. Warm air carries more water vapour than cool air, and it drifts upward through every gap in the ceiling: round the hatch, the light fittings, the pipes and the heads of partition walls. Once it reaches a loft that is cold, it meets timber, felt and nail points colder than the air itself, and it leaves its water there.
That is why the signs gather in a recognisable pattern. Frost on nail tips, damp rafters, dark mould under the sarking and drips onto the insulation tend to be worst in a cold snap, when the temperature gap is widest. They often dry off in summer, which tempts owners to ignore them until the timber is soft.
A leak is different. It leaves a stain in one spot, often below a tile, flashing or valley, and it follows rain, so it is worst after a downpour and not in a cold snap. Our page on roof leak or condensation helps you tell them apart, and a survey settles it. Approved Document C wants a roof to resist rain and to keep condensation within the structure from harming its performance.
Ventilation, BS 5250 and the eaves gap
A cold loft stays healthy when air passes through it, carrying off whatever moisture got in. The guide to managing moisture in buildings is BS 5250, and the 2021 edition is the current one. Approved Document C ties its condensation requirement to that standard (it cites the 2002 edition) and says ventilating cold deck roofs will meet it, while roofs that are very small, like a porch, may not always need it.
Marley, the tile maker, sets out what a roof lined with HR underlay (the kind that does not breathe) should have at the eaves: a 25 mm opening along their full length where the pitch is 15 degrees or less, and 10 mm on anything steeper. A breathable, or LR, underlay can cope with the small leakage built into ordinary eaves, provided the ceiling below is tightly sealed and the product holds independent third-party certification.
These are guides to what a competent installer checks, not a specification for your house. The right answer depends on the underlay, the pitch and the ceiling, so we read your own roof. Our guide to pitched roof ventilation goes into more depth.
Blocked eaves and the wrong underlay
Two faults often turn a manageable loft into a wet one. First, insulation shoved hard into the eaves. Extra quilt gets rolled out over the loft floor to save heat, and its edge ends up jammed against the roof slope, covering the very openings that let fresh air in. Air stops flowing, and the damp stays put. The remedy is to pull the quilt back and fit an eaves tray or similar part to hold the gap open.
Second, an underlay unsuited to the roof. Older roofs were lined with a non-breathable felt and depend on air crossing the loft. A breathable membrane lets some vapour pass, but it still has to be laid as its maker specifies. Our guide to breathable membrane versus roofing felt sets the two side by side.
Other contributors are a hatch that does not close tightly, extractor ducts ending inside the roof space, and gaps round pipes. Approved Document C wants a proper draught seal fitted to the loft hatch, a small part that is cheap to get right.
What we check and what you can do first
Start indoors. Seal round the hatch and pipe holes, keep the door shut on steamy rooms, make sure kitchen and bathroom fans discharge outside, and avoid drying washing in rooms with no air movement. If you can reach the eaves from the hatch without leaving the joists, check that the insulation has not been packed against the roof.
When we survey, we photograph the underside of the roof, the underlay, the eaves and the insulation. We note whether the staining sits along the line of a tile or flashing, which suggests rain, or spreads across the timber in a broad film, which suggests air. If it is condensation, we explain the cause and set out the choices: clearing the eaves, adding vents, sealing the ceiling, or renewing the underlay when the roof is being re-covered anyway. Replacing underlay means lifting tiles, so it makes sense alongside other work, such as renewing battens. Where the roof is otherwise sound, vents and a sealed ceiling may be all it takes.
Price follows the roof's size, how it is reached and whether tiles come off. One written total reaches you after the survey, and it names what is included. For the bigger picture, start from the pitched roofing hub, which links the other tile and underlay topics.
