Roof Underlay and Membranes: Breathable vs Non-Breathable

The layer you never see does one of the most important jobs on your roof. Underlay sits beneath the tiles or slates as a second line of defence against wind-driven rain, and it decides whether moisture from inside your home escapes or gets trapped in the loft. The big choice is a breathable roofing membrane versus a traditional non-breathable underlay, and getting it wrong causes condensation, damp timbers and premature rot. This guide explains the difference in plain terms, what the UK standards require, and how to choose the right one when you re-roof.

The two types of underlay

Roofing underlay splits into two families, and they are classified by how much water vapour they let through.

A non-breathable underlay is the traditional option, historically bitumen felt and now usually a reinforced plastic sheet. It is effectively a vapour barrier: it keeps rain out but also stops moisture from inside the house passing through it. In the standards this is a Type HR (high water vapour resistance) underlay to BS 5250.

A breathable roofing membrane is the modern option, a vapour-permeable sheet, typically a three-layer non-woven polypropylene composite with a microporous film in the middle. It keeps rain out while letting water vapour from inside the roof escape upward and out. This is a Type LR (low resistance) underlay. Well-known examples include the Klober Permo range, DuPont’s Tyvek Supro, and membranes from Cromar and Permavent.

The real difference: condensation and moisture

This is the point that matters. Warm, moist air rises from your home into the roof space. If it hits a cold, non-breathable underlay it condenses into water on the underside, and over time that damp attacks the rafters and battens. That is why lofts under old felt so often feel damp.

A breathable membrane lowers that risk by letting the vapour pass through it and disperse, rather than trapping it against cold timber. For most modern re-roofs on UK homes, that is the main reason breathable membranes have become the default choice.

Ventilation: the mistake people make

Here is the single most misunderstood point, and where DIY reasoning goes wrong: a breathable membrane is not a licence to seal the roof up completely.

With a non-breathable underlay, because vapour cannot pass through it, you must provide traditional ventilation both low down at the eaves and high up at the ridge, so air flows through the roof space and carries moisture away. This is set out in BS 5250.

With a breathable membrane, the vapour can escape through the sheet, which can reduce the need for ventilation. But both the British Standards and the NHBC require, as a minimum, high-level ventilation (at or near the ridge) even with a breathable underlay. In other words, the membrane helps, but you still need some designed ventilation. Assuming a breathable membrane alone makes a roof foolproof is exactly how condensation problems creep back in. A good roofer designs the underlay and the ventilation together, not one or the other. For more on this, see our guide to soffit and roof ventilation.

Wind, BS 5534 and installation

Underlay also has to cope with wind. When the wind blows over a roof it creates suction that can lift both the tiles and the underlay. BS 5534, the British Standard for slating and tiling, sets out how underlay is tested and installed to resist this uplift, including how much a membrane is allowed to balloon between rafters. This is why a competent installation follows the manufacturer’s fixing instructions rather than just draping the sheet loosely. Some air-open breathable types and certain designs may call for counter-battens or extra support to meet BS 5534, so it is a job for a roofer who knows the current rules, not a shortcut. You can read the standard’s scope via the British Standards Institution.

Cost and lifespan

Breathable membranes generally cost more per roll than basic felt, but the difference across a whole roof is modest next to the price of the tiles, battens, scaffolding and labour, so underlay is rarely the place to economise. A quality breathable membrane from a reputable brand typically carries a manufacturer guarantee of around 15 years and, installed correctly, lasts the life of the roof covering. For how underlay fits into the wider job, see our new roof cost guide and our explainer on how roofers price a re-roof per square metre.

Which should you choose?

For the large majority of UK pitched-roof replacements, a breathable membrane is the sensible default: it manages condensation better and suits modern, warmer, better-insulated homes. Non-breathable underlay still has its place, particularly where a specific ventilation strategy is being used, on some heritage work, or where a designer specifies it, but it always demands proper eaves-to-ridge ventilation to work safely.

The practical takeaway: do not buy underlay on price alone, make sure whatever goes on is paired with correctly designed ventilation, and use a roofer who installs to BS 5534 and BS 5250. If you are weighing up a full re-roof, our guides to roof tile and material types and how to find a good roofer will help you brief the job properly.

Frequently asked questions

What is a breathable roofing membrane? A breathable roofing membrane is a vapour-permeable underlay, usually a three-layer polypropylene composite, that sits beneath the tiles or slates. It keeps rain out while letting water vapour from inside the roof pass through and escape, which reduces the risk of condensation in the loft compared with traditional felt.

What is the difference between breathable and non-breathable underlay? Non-breathable underlay resists water vapour, so it traps moisture and requires full eaves-and-ridge ventilation to work safely. Breathable membrane lets vapour pass through it, lowering condensation risk, though the standards still require at least high-level ventilation. Breathable is the usual choice for modern UK re-roofs.

Does a breathable membrane mean I don’t need roof ventilation? No. This is a common and costly misunderstanding. Both the British Standards and the NHBC require at least high-level (ridge) ventilation even with a breathable membrane. The membrane reduces the ventilation needed but does not replace it entirely, so it should always be designed alongside proper ventilation.

Is breathable membrane better than felt? For most modern homes, yes. It manages condensation far better than old bitumen felt and suits well-insulated roofs. Traditional non-breathable underlay still has valid uses, but only with a correctly designed ventilation strategy, so breathable is the sensible default for a typical re-roof.

What roofing membrane brands are used in the UK? Widely used breathable membrane brands include Klober (the Permo range), DuPont Tyvek (Supro), Cromar and Permavent. Choose a BBA-certified product installed to BS 5534, and check the manufacturer’s guarantee, which is often around 15 years.

Does the type of underlay affect the cost of a new roof? Only modestly. Breathable membrane costs more than basic felt, but across a whole roof the difference is small next to tiles, battens, scaffolding and labour. Underlay is not the place to cut corners, since a failed underlay can rot the timbers it is meant to protect.