Reading the signs: leak or condensation
The two causes can look alike from the hatch, but they behave differently, and a short check will usually tell you which one you have. Go up on a dry morning after a cold night, then again after a wet day, and compare notes between the two visits, since a leak and a bout of condensation can leave similar stains.
| What you see | Leak | Condensation |
|---|---|---|
| Where | One patch under a fault | Spread across the roof underside |
| When | During or after rain | Cold, still or frosty mornings |
| Look | Stained or dripping timber | Beads of water on nails and felt, black mould |
| Dries out | When the weather dries | Slowly, in warm weather |
A leak follows rainfall and wind direction. Condensation follows the temperature and how much moisture the house produces, with cooking, showers and clothes drying indoors all adding to it.
If you can see both, they may be happening together. A small leak can add water to a loft that is already condensing, which is why a roofer checks the outside and the inside before naming a cause.
How a loft is meant to breathe
A cold roof has its insulation at ceiling level, which keeps the loft above it cold. That loft only stays dry if air moves through it and carries the moisture away. BS 5250:2021 covers moisture control in buildings, and Approved Document C expects the loft of a cold deck roof to be ventilated.
The usual route is a gap along the eaves, where the roof ends at the gutter, so air can enter, with a way out at or near the ridge. Marley's figure for HR underlay on a cold roof is 25mm along the eaves for pitches of 15° or less, and 10mm for steeper ones. The page on the eaves ventilation gap shows how it is formed.
HR underlay is the older, non-breathable underlay, and it depends on that ventilation because vapour cannot get through it. A breathable membrane works differently, and breathable membrane against felt compares them. Which one is already on your roof decides how much airflow it needs, and if the roof was re-covered in the past, the age and type of the underlay are worth asking about.
What blocks the air
A damp loft may have been cut off from its air supply by a change, sometimes a well-meant one. Changes that do it include:
- Loft insulation pushed into the eaves, covering the gap at the lower edge of the roof.
- Boxes and stored goods packed hard against the roof slope.
- A bathroom or kitchen fan that vents into the loft instead of outside.
- A loft hatch that does not seal, letting warm house air rise into the cold space.
- A re-roof that replaced old felt with a different underlay and left the ventilation as it was.
Fixing it is mostly a matter of restoring airflow, not adding more insulation. Eaves vent trays hold the insulation back and keep the gap open, the hatch can be sealed with a draught strip, and extractor fans should discharge outdoors through the wall or roof.
None of this is a roof repair as such, but a roofer sees all of it while looking at the roof from inside, and the page on roof underlay and battens covers the layers that go with it. A damp loft is often a house problem that shows up in the roof.
When damp means the covering needs work
If the loft is damp in one place after rain, the cause is outside. The patch is traced back along the rafter to find where water is entering, which could be a slipped tile, a failed valley, a cracked flashing at a chimney or a blocked gutter. Each is a small repair when caught early.
Wet timber needs attention too. Roof timbers that stay damp for long enough can rot, and a ceiling below can stain or come down. Take a photograph of the damp, and note whether it was worse after rain or after a cold night, because the roofer will ask which. Bring that photo to the survey so it can be compared with the loft as it looks on the day.
The survey looks at the roof covering from outside and the roof space from inside, then gives one written price for whatever is found. If the answer is ventilation and not a repair, you are told that plainly, because a new covering will not cure condensation.
The overview of tiled roof work is on the pitched roofing page.
