How is a slate roof ventilated?

Every household makes moisture. Cooking, showers, laundry and simple breathing load the air with water vapour, and the warm air climbs toward the loft. Where it meets cold timber and a cold underlay, it turns back into liquid. Over a winter that can mean wet rafters, mould, and eventually rot.

The guidance on managing moisture in buildings is BS 5250:2021, which BSI currently lists as under review. Its practical lesson for a roofer is simple: give moist air a way out, and stop warm air leaking up in the first place. Ventilation is the second half of that, and it only works properly when the first half has been done. The first is a sealed ceiling, with the hatch, the light fittings and the pipe holes fitted so they do not pass warm air upward. A roof that is ventilated but sits above a ceiling full of holes is fighting a losing battle, because the damp air arrives faster than any vent can clear it. Bathrooms and kitchens with extractor fans that vent into the loft make the same mistake worse.

On a cold roof, which has its insulation laid across the ceiling joists, the standard route is a continuous opening at the eaves. Cool air enters there and flows up under the slates and out at the high end. With a high-resistance underlay, Marley sets an opening of 25 mm at the eaves where the pitch is 15 degrees or lower, and 10 mm where it is steeper than that.

The openings are usually a ventilated strip or tray in the soffit or fascia, and they only help if they stay clear. Birds' nests, paint and, above all, insulation pushed hard into the eaves block them. A roofer checks them from inside the loft and from a ladder outside. They are small, easy to overlook and usually quick to put right, which makes them one of the most useful things a survey can check. Painted-over vents and tucked-in quilt are the faults seen most often by a roofer working in a loft. The slate roofing page lists the other checks that go with them.

A low-resistance, or breathable, underlay lets vapour pass through it. With a well-sealed ceiling, Marley allows the ventilation at the eaves to come from the ordinary 3 mm gap that eaves construction leaves, as long as the underlay's third-party conditions are met. That is a lighter requirement than the strip used with a high-resistance sheet.

It does not mean the roof can be left without thought. The manufacturer's instructions govern, and they depend on the pitch, the ceiling below and the insulation. The underlay and the ventilation approach should be written into the quote together, so the two match. Our answer on underlay for a slate roof sets out the types and how they differ.

Many older slate roofs were laid with no underlay at all. Air leaked in at every joint, and the timbers stayed dry because the roof was never airtight. When a house is later insulated and the ceiling sealed, the moisture starts to stay put. Owners then meet damp in a roof that was fine for generations.

This pattern turns up when a loft is insulated or converted, which is why the roof deserves a look before anything inside is changed. Nobody planned the old air paths, so nobody thinks to protect them. A re-slate gives the chance to add the underlay and eaves ventilation properly, and our guide to re-slating a roof describes the sequence. A repair that leaves the old build-up alone will not change how the roof behaves, whatever has been done to the loft below.

More insulation can reduce ventilation if it is fitted carelessly. Quilt pushed into the eaves closes the opening that feeds the roof space, and sprayed foam across the rafters changes the whole moisture balance. The Energy Saving Trust advises fixing a leaking roof before any foam goes on, because trapped water cannot escape and nobody can see it.

If you are planning insulation, ask what it does to the air path and keep the eaves clear. Our guide to spray foam and condensation explains what can go wrong in a foamed roof. A survey with photographs of the timbers will show whether the roof is breathing as it should. Whatever it turns up, you receive a single written price, covering only the work that cures the fault found. Where the answer is a cleared eaves tray and nothing more, that is all you are asked to pay for.