What does condensation in the loft tell you about the roof?

Warm air carries water vapour. Cooking, washing, drying clothes and simply breathing add moisture to the air in a home, and that warm air drifts upwards through gaps around hatches, light fittings and pipe holes. Up in the loft it reaches the chilly underside of the roof, cools quickly, and leaves its water behind as condensation on the felt, the battens and the rafters.

A loft with insulation laid over the ceiling is colder than the rooms below, which makes the effect stronger. The roof was not necessarily built wrongly. It is the pairing of a warm, damp house with a cold roof that causes the trouble. Droplets on the underlay on a cold morning, which dry out later in the day, are the classic sign of it.

The two can look alike but behave differently. A leak appears after rain, in a patch below one particular fault, and the stain has a distinct edge. Condensation is more widespread and turns up in cold weather, often across many rafters and nail points at once, and it can happen without any rain falling.

Other clues help. Water drips from the points of nails and clips with condensation, whereas a leak tends to run along a timber from a single entry point. Black spots of mould on the timbers suggest long-term damp air. Photograph what you see and note the weather, since this helps a roofer judge it. Our guide on checking a roof from the loft shows how to look safely without stepping off the joists.

A cold roof is meant to breathe, with air coming in low at the eaves and going out high near the ridge. Marley, the tile maker, advises a continuous gap along the eaves for roofs fitted with the older type of felt, with a wider opening for shallow pitches and a narrower one for steeper pitches. BS 5250:2021, the code of practice for managing moisture in buildings, is listed by BSI as current, under review.

The usual fault is blocked airflow. Insulation pushed hard into the eaves, stored boxes or a lining over the gaps all stop air getting in. Painted-over vents, or a fascia fitted with no gap behind it, do the same. Clear the way and the moisture often clears too. Our page on checking your roof ventilation goes into the details.

Cutting the moisture before it rises helps as much as ventilating the roof. Seal the gaps around the hatch and the pipe holes, make sure kitchen and bathroom fans vent to the outside air and never into the roof space, and keep the bathroom door shut while in use. Piling on more insulation is no cure for damp, and it can leave a cold loft colder.

Spray foam needs care. The March 2023 consumer guide from RICS lists dampness, such as uncontrolled condensation, among the risks of incorrect installation, and notes that removal could potentially damage the structure. The Energy Saving Trust does not recommend spraying foam over a roof that leaks. If your loft already has foam and you see damp, have it assessed before anyone adds more.

Timing tells you a lot. Damp that builds through the cold months and fades in warmer weather points to condensation. Damp that appears only after a spell of rain, in one place, points to a leak. If both are present, deal with the leak first, since water entering from outside makes every other measure harder.

A winter morning check is the most useful. Look at the underside of the felt and at the nail points when the loft is coldest, and compare it with the same spot on a dry, mild afternoon. Notes and dated photographs make the pattern plain, and they save a roofer a good deal of time. Share them with us when you book and the survey starts from what you have already seen.

We examine the felt, the timbers and the vents from inside, photograph the damp, and decide whether the cause is air from the house, a blocked gap at the eaves or a genuine leak. You receive pictures of your own loft and a clear answer on what the roof needs. If work is called for, a written quote follows the visit at one total price.

Often the cure costs less than people fear: clearing the eaves, adding vents or sealing the ceiling below. Where the felt itself has perished, a repair or a re-cover may be the right answer, and we will say which. See our roof inspections and surveys page for how a visit runs from the first look onward.