Where the water comes from
Cooking, showering, wet washing and even breathing add moisture to the air indoors. Warm air carries that moisture upward, through ceiling gaps, around light fittings and past a hatch that fits badly. In the loft it finds the underlay and the timber beneath the slates, which sit close to the outside temperature, and the vapour condenses into water.
On a slate roof this happens out of sight. The slates stay dry on top, so nothing looks wrong from the street. The moisture gathers on the underlay and the rafters, drips onto the insulation and stains the ceiling below.
From inside the house it can pass for a leak. A visit may find every slate in place and a loft that is damp from within. Cold, calm, humid weather brings it on. A leak follows rainfall, while condensation follows a drop in temperature and eases when the weather warms.
Knowing the difference saves money, because new slates will not cure a ventilation problem, however carefully they are laid and fixed on the roof.
Signs to look for in the loft
From the hatch, a torch shows the telling signs without you stepping on joists or boards you cannot see. Look for these:
- Beads of water or a damp sheen on the underlay and the rafters, with no daylight showing through.
- Dark speckled patches of mould on the timber, especially near the eaves and the ridge.
- Insulation that is flattened, wet to the touch or stained.
- Rusty nail heads or fixings in the roof timbers.
- Ceiling stains that appear in winter without a rainy spell beforehand.
Compare those with the signs of a true leak, which are tide marks and drips in one place directly under a defect, and which get worse after rain. A leak also tends to leave the timber around it dark and wet, not simply speckled. Condensation is spread widely and is worst where the air is stillest.
If you find daylight, or the damp is concentrated below a chimney or valley, call someone to look, because that points to a different fault. For the wider picture of damp in a loft, see the page on why a loft is damp.
Ventilation and underlay
Roofs are built to let moisture escape. Approved Document C meets its moisture requirement by ventilating the roof space, and BS 5250:2021 is the British Standard on moisture in buildings. Air has to move through gaps at the eaves and the ridge for that to work, and the gaps only help while they stay open.
Marley, which publishes guidance for tiled roofs, asks for an eaves opening of 25 mm for pitches of 15° or less and 10 mm for steeper ones, where a cold roof uses a non-breathable underlay. Your slate roof's data sheet and specification set the actual requirement, not that figure.
Two things can go wrong in an older roof. Loft insulation gets pushed into the eaves and chokes the airflow, and the original underlay is a bitumen felt that passes no vapour. Felt that was fine decades ago can end up in a roof with more insulation and more moisture than it was ever meant to handle.
Checking means looking at the gap itself, covered in the eaves ventilation gap guide, and at what lies under the slates, covered in underlay for a slate roof.
Cutting the moisture and clearing the air path
The cure begins with the moisture rather than the slates. Extractor fans ducted outdoors take steam away where it is made, and a fan that discharges into the loft only moves the problem upstairs. Closing the bathroom door during a shower and drying washing outside or in a vented dryer all cut what reaches the ceiling.
Next comes the air path. Pull insulation back from the eaves so a clear gap remains, and fit eaves vents or trays where the structure allows. Seal gaps round light fittings and the loft hatch, so less warm air reaches the roof in the first place.
If the underlay has perished or is the wrong type for the roof, the real fix sits under the slates. When a roof is stripped for another reason, such as failed fixings, that is the moment to renew the underlay and the ventilation together.
Be wary of anything that closes the roof in more tightly. Spray foam on the underside hides the timbers, and RICS warns that once they are covered they cannot be properly inspected. A survey says which of these steps your roof needs. The work is described on the slate roofing page, and the quote that follows carries one written total.
