Understanding breathable membranes under slate roofs

What sits beneath the slates

The slates are the part everyone sees, but the layer underneath does much of the quiet work. Traditionally that layer was either nothing at all — just sarking boards in Scotland and parts of the north — or, from the mid-twentieth century onwards, a bituminous felt such as type 1F. That felt kept out wind-driven rain and snow, but it also trapped moisture. Any vapour rising from the rooms below, or any damp that found its way in past a slipped slate, had nowhere to go.

Breathable membranes were developed to solve that problem. They are vapour-permeable underlays, sometimes called low-resistance (LR) underlays, and they have largely replaced felt on new builds and full re-roofs across the UK. If you own an older property, understanding how they differ from what is already up there will save you money and avoid a few unpleasant surprises.

How a breathable membrane behaves

The principle is straightforward. A breathable underlay is highly resistant to liquid water but only slightly resistant to water vapour, so moisture can pass through it and disperse into the ventilated space above the insulation. Most products are described by their vapour resistance, expressed as an Sd value, with typical low-resistance membranes sitting well below 0.5 m. The lower the figure, the more freely vapour moves.

What they are not is a substitute for slates or a waterproofing layer in their own right. They are a secondary barrier and a wind barrier, catching the rain that gets past a slipped or cracked slate and preventing it from reaching the battens and insulation. Some modern membranes are also capillary-active, meaning small amounts of water that collect on the underside are drawn back up and allowed to evaporate rather than dripping onto the ceiling below.

When breathable underlay is the right choice

• New build and full re-roof: the obvious case. The membrane goes over the rafters, under the battens and counter-battens, and the roof is detailed from the start with the correct laps and ventilation.
• Warm roof constructions: where insulation sits above the rafters, the membrane is usually specified as part of a system, and the manufacturer's guidance should be followed closely.
• Cold roofs with a well-ventilated loft: a breathable underlay works happily here, provided the loft itself is adequately ventilated and no insulation has been pushed into the eaves to block the airflow.
• Over-boarding and re-covering: where the existing covering is stripped and the roof structure is sound, breathing membranes offer better moisture management than felt without changing the roof's appearance.

Old roofs need a different approach

An older property is a different proposition. If the roof still has bituminous felt, or sarking boards with a felt covering, the cavity between the slates and the felt may be poorly ventilated. That matters because the membrane beneath it is a high-resistance layer — vapour cannot pass through it, so it must be carried away by air movement instead.

Guidance commonly calls for a continuous ventilation opening at eaves level for this type of roof, with additional high-level ventilation where the pitch is low, the span is wide, or the roof space is complex. A figure often quoted is a continuous equivalent opening of around 25 mm at the eaves, but the right amount depends on pitch, span and the underlay in use, so it is worth taking advice rather than working from memory. On sarking-board roofs, ventilation is trickier still, because the boards themselves restrict airflow; retrofitting vents at the eaves and at ridge or high level is often the practical answer.

Watch for the classic warning signs of a poorly ventilated old roof: condensation forming on the underside of the slates in cold weather, damp staining on rafters and battens, rusted nails, and a musty smell in the loft. Once battens start to rot, the fix becomes a re-roof rather than a repair.

Details that decide how well it works

Most problems with breathable membranes come from installation rather than the product. A few points repay attention:

• Laps: horizontal and vertical laps should follow the manufacturer's figures exactly, and be generous on low pitches or exposed sites.
• Tension: the membrane should be taut but not drum-tight, with a slight sag between rafters so any water that gets past the slates runs down rather than pooling.
• Fixings: use the specified nails or staples, and avoid over-driving them through the membrane.
• Penetrations and junctions: valleys, ridges, chimneys and pipe vents all need proper detailing, and taping or sealing where required.
• Exposure limits: most membranes are only designed to be left exposed for a limited period, often a few months, so don't let the roof sit uncovered through a winter.

Keeping an eye on things afterwards

Once the roof is on, a little routine maintenance goes a long way. Clear gutters and downpipes each autumn, and check the eaves for moss and debris that can block ventilation paths. Look at the loft after a spell of cold weather — if the underside of the membrane is beaded with condensation, the ventilation is not working as it should. Check for slipped slates after storms and get them refixed promptly; a slipped slate lets water reach the membrane, and while the membrane will cope briefly, it is not designed to be the only thing keeping the rain out.

Get the roof looked at by a competent roofer every few years, and always ask what underlay is fitted and how it is ventilated. Two roofs can look identical from the street and perform very differently, and the difference is usually in the details you cannot see.