Is the wet patch in my loft a leak or condensation?

Both look like a wet loft and they need opposite remedies: a leak is fixed on the roof, and condensation is fixed inside the house by stopping warm moist air getting up there and by letting the roof space breathe. Getting this wrong means paying a roofer to fix a roof that is watertight, which is a common and expensive outcome.

Safety

What you'll need

• Head torch
• Crawl boards
• Particle mask
• Camera
• Hygrometer (optional)

Step by step

1. Go up on a cold, still morning first.That is when condensation is at its most visible. Beads of water or frost on the underside of the membrane, glistening nail points coming through the felt, and damp along the ridge are the signature.
2. Then go up during or just after heavy rain.A leak shows itself in the wet. Look for active drips, running water, and a wet trail on a rafter or a purlin that is dry either side of it.
3. Look at the distribution.Condensation is general — the whole underside of the roof or a whole cold area of it. A leak is a point and a path: a wet spot on the membrane, a stain running down one rafter, a wet patch on the insulation directly below it.
4. Follow any stain upwards.Water runs down and sideways along timbers before it drops, so the entry point is usually higher and often offset from the wet insulation. Trace the dry-edged stain trail back to its top.
5. Check the usual leak points.Around the chimney and its flashings, at abutments where a lower roof meets a wall, at valleys, around soil vent pipes and roof vents, and at the eaves where the membrane is exposed and brittle.
6. Check the usual condensation causes.A bathroom or kitchen extract duct discharging into the loft rather than through the roof or wall, an unsealed loft hatch, unsealed pipe and cable penetrations, a stud partition open at the top, and blocked eaves ventilation.
7. Look at the membrane type.Old bitumen sarking felt does not let vapour through, so a loft with felt depends entirely on ventilation at the eaves and ridge. A modern breather membrane lets vapour out and is much more forgiving, though it still needs the ventilation to have been designed with it.
8. Check the insulation itself.Wet insulation from a leak is soaked in one area with a clear boundary. Wet insulation from condensation is damp over a wide area, often heaviest at the coldest edges of the loft.
9. Act on what you found.A leak means a roofer, from proper access. Condensation means sealing the ceiling penetrations and the hatch, redirecting any duct that discharges into the loft, and restoring the eaves ventilation.
10. Dry the loft out and recheck.Remove sodden insulation, let the timbers dry, and go back up after the next spell of the relevant weather. Prolonged wetting of roof timbers leads to rot, so this is not something to leave for a season.

Tips

• Photograph everything on each visit with a note of the date and the weather. The comparison between a wet-weather visit and a cold-morning visit is what settles the diagnosis.
• Daylight visible through the roof on a bright day, with the loft light off, is the single fastest way to find missing tiles and holes.
• If the duct from a bathroom fan is simply hanging in the loft, that is both the condensation cause and a five-minute job to identify. It is far more common than people expect.

Common mistakes

• Calling a roofer before checking on a cold morning — Loft condensation is extremely common and is caused inside the house. A roofer will find and charge for something, and the loft will still be wet next winter.
• Sealing the eaves and ridge vents to keep the loft warmer — A cold roof space has to be ventilated. Sealing it traps the moisture that inevitably leaks up from the house, and the timbers get wetter.
• Leaving wet insulation in place to dry — Sodden mineral wool holds water against the ceiling for a very long time and its insulating value is gone while it is wet. Take it out, dry the area, and replace it.

Questions people ask