What Is the Membrane Under Slates For?

Slates overlap in rows called courses, and the overlap sheds rain. In a gale, though, rain does not fall straight. It blows sideways and upwards, and powder snow can find its way under the laps. Without a membrane, that water drops onto the loft timbers and the insulation.

Is the underlay what keeps my roof watertight?

No, the slates do that job. The membrane catches whatever slips past them and carries it along the fall of the roof until it drops into the gutter at the bottom edge. A membrane lies under the battens and over the rafters, so it works like a tray stretched across the whole slope. It is a safety layer rather than the main covering, and like any sheet material it ages with heat, damp and years of flexing in the wind. Old bitumen felt dries out, tears and sags between the rafters, and a sagging sheet can hold water against a batten, the timber strip the slates hang from. We look for these faults whenever slates are lifted.

That is why a re-slate is the right moment to renew it. The access is already paid for, and a new sheet gives fresh slates a sound surface to lie over.

There are two families. HR stands for high resistance. An HR underlay is non-breathable, which means damp air from the house cannot travel through it. LR stands for low resistance, and an LR underlay is a breather membrane that lets vapour from the house escape upwards.

Which type belongs under my slates?

The answer comes from how the roof is put together and from the maker's instructions for the sheet, which we follow to the letter. A cold roof has its insulation at ceiling level and an unheated loft above, and in that set-up the airflow needed at the eaves changes with the type of underlay. A warm roof, with insulation along the rafters, is planned differently again, so we ask which one you have before naming a product. Getting the pairing wrong is a familiar route to a damp, mouldy loft, and the damage is slow enough that it can go unseen for several winters.

Our answer on insulating a slate roof shows how the membrane works alongside the insulation, and why the two should be planned together rather than one after the other.

BS 5250 is the British Standard on moisture in buildings, and it is where loft ventilation planning begins. Where HR underlay sits over a cold roof, Marley's guidance asks for 25mm of opening along the eaves if the pitch is 15 degrees or less, and 10mm if it is steeper than 15 degrees. BS 5534 puts natural slate at 20 degrees or more, so on a slate roof the 10mm figure is the one that applies.

What changes with an LR membrane?

Marley treats a breather membrane differently. If the ceiling below is well sealed, the gaps in ordinary eaves construction, taken as 3mm, can supply the air, as long as the product carries third-party certification. Read the data sheet for the very sheet being fitted before relying on that, since certification differs between makers. The rule saves a lot of trouble: ventilation is chosen for the membrane, never the other way round. On a re-slate we confirm the opening at the eaves before the first batten goes down, and we show you the product sheet beside the photos. Our page on slate roof vents describes the vent choices, and the details at the lowest edge are explained in the slate eaves course.

The survey asks three questions. Is there a membrane at all? Some very old slate roofs went on without one, with the slates bedded straight onto battens. What condition is it in where we can see it, in the loft near the eaves and around the chimney stack? And is the loft already damp, with dark patches on the timber or water marks on the ceilings below?

Does an old slate roof need a new membrane?

Not for every repair, and a few replaced slates do not call for one. Where the sheet is whole and dry, we leave it alone and tell you so. A full re-slate is different, because it is the chance to fit a new sheet of the right type and set the eaves airflow to suit it. Our slate roofing work starts with a survey, and you are shown photos of your own underlay and battens before any price is agreed. Send a picture of the loft at the eaves, and we will say which parts we would examine first on site.