Where this starts
A cold roof space is meant to be cold and dry. It gets damp in two quite different ways, and they present so similarly at the hatch that the wrong one is often chased for a season.
CONDENSATION arrives as water vapour from the house below — from cooking, washing, drying clothes, breathing — which rises through gaps in the ceiling and meets the coldest surface available. That surface is the underside of the roof covering. The signature is EVEN: droplets or frost across a wide area of the underside, on the nails, sometimes dripping onto the insulation below, and worst after cold nights and in winter.
A LEAK arrives from outside and obeys gravity. The signature is LOCAL: one rafter with a dark run down it, one patch of insulation compressed and stained, a line of drips that always lands in the same place, and it follows rain rather than temperature.
THE TIMING SEPARATES THEM CHEAPLY. Go up after a cold, still, dry night in winter: water then is condensation, because no rain has fallen. Go up during or just after heavy driving rain: water then, in a localised place, is a leak. A roof can of course have both, which is why finding one does not close the question.
THE THIRD POSSIBILITY, and the one most often missed, is that the loft is ventilated adequately and something is DELIVERING moisture into it directly — a bathroom extractor ducted into the roof space instead of outside, an unsealed loft hatch under a bathroom, or downlighters punched through the ceiling of a kitchen. Those put a concentrated stream of warm wet air into a cold space, and no amount of eaves ventilation copes with a fan aimed into it.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Moist air from the house reaching a cold roof space
Most commonDroplets, frost or dampness spread EVENLY over the underside of the covering and on nail points, worst after cold nights, with no relationship to rainfall.
How to check: Go up after a cold dry night. Water present when no rain has fallen is condensation, and the pattern will be general rather than local.
Ventilation openings blocked or never provided
Most commonThe same general condensation pattern, with insulation pushed hard into the eaves, no visible daylight at the eaves, or a re-roof or insulation upgrade in the recent past.
How to check: Look along the eaves for a clear air path above the insulation. Insulation stuffed to the rafter feet closes the inlet completely.
An extractor or vent discharging into the roof space
CommonLocalised heavy wetting around a duct, a duct that ends loose in the loft, or a concentrated damp area directly above a bathroom or kitchen.
How to check: Follow every duct to where it actually terminates. A duct ending in the loft is delivering the problem, not venting it.
A defect in the covering letting rain in
CommonLocalised staining following gravity — a run down one rafter, one wet patch of insulation, drips in a repeatable spot — appearing with rain rather than with cold.
How to check: Go up during or straight after heavy rain and look for a track. Water that follows rainfall and follows one path is entering.
Air leaking up through the ceiling at penetrations
CommonCondensation concentrated around downlighters, pipe and cable penetrations, or an untreated loft hatch, rather than evenly over the whole roof.
How to check: Whether the damp clusters around holes in the ceiling. Warm wet air takes the easiest route up, and those are it.
An impermeable underlay in a roof that assumes ventilation
Less commonA traditional bitumen felt with little or no eaves and ridge ventilation, or a breather membrane installed without the air gap its certification assumes.
How to check: What the underlay is and whether the roof has openings. A closed sheet with no ventilation path has nowhere to send the moisture.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
The underside of the covering, evenly, is condensation. A rafter, a joist or one patch of insulation is a leak. That single observation splits the whole problem.
- 2
Two visits in different weather settle the condensation-against-leak question definitively, and neither requires anything but a torch.
- 3
Insulation pushed into the eaves is the commonest cause of a loft that was dry for twenty years and became damp after an upgrade.
- 4
An extractor discharging into the loft delivers warm wet air continuously, and nothing done to the ventilation will keep up with it.
- 5
Surface damp dries; timber that is soft to a probe, discoloured through, or showing growth has been wet long enough to matter structurally.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where roof timbers are soft to a probe, discoloured through their thickness, or showing fungal growth, which is decay rather than surface condensation.
- Where the loft has been damp over more than one winter, since prolonged wetting of structural timber is a different question from a wet morning.
- Where an extractor or vent discharges into the roof space, because re-ducting it correctly through the roof or the wall is a roofing job with a weathering detail.
- Where the roof has recently been re-covered or the insulation upgraded and the damp followed, which points at the ventilation design of the work rather than at a defect.
- Where there is any sign of wetting around a chimney, a valley or an abutment, since those are the details that leak and the repair is not a surface one.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.