28 Jul 2026
Breathable Roofing Membrane vs Traditional Felt: A UK Buyer’s Guide
A breathable roofing membrane lets water vapour escape from the roof space while keeping rain out, reducing condensation and often removing the need for extra roof ventilation. Traditional bitumen felt is non-breathable, heavier and cheaper, but can trap moisture. Most new UK roofs now use breathable membranes to comply with BS 5250/5534.
Breathable membrane vs felt: quick verdict
| Breathable membrane | Traditional bitumen felt | |
|---|---|---|
| Vapour permeable | Yes | No |
| Weight | Light — typically 100–120 g/m² | Heavy |
| Roll handling | One person can manage a roll | Awkward, usually two-person |
| Ventilation needed | Often reduced or eliminated | Eaves and ridge ventilation required |
| Condensation risk | Low | Higher |
| Lifespan | 30–50 years once covered | 20–30 years |
| Sagging | Minimal | Prone to sagging between rafters |
| Cost per m² | Moderate | Lower material cost |
| Certification | BBA approved products available | Traditional grades 1F / 1B |
For almost any new roof or re-roof in the UK today, a breathable membrane is the right answer. Traditional felt survives mainly in repairs to existing roofs and in situations where matching an existing build-up matters.
What is a breathable roofing membrane?
A breathable roofing membrane — also called a breather membrane or vapour-permeable underlay — is a lightweight synthetic sheet, usually spunbond polypropylene, laid over the rafters beneath the tile battens.
Its defining property is that it works in two directions. Liquid water cannot pass through it, so wind-driven rain and snow that gets past the tiles is stopped and drained away. Water vapour can pass through it, so moisture rising from the building into the roof space escapes outwards instead of condensing on the underside of the underlay.
Quality membranes are three-layer constructions: the outer layer forms a waterproof surface, the middle layer is the breathable waterproof membrane itself, and the inner layer resists abrasion and handling damage. They can be draped unsupported over rafters or laid fully supported directly on top of insulation, which makes them versatile across cold roof, warm roof and room-in-roof build-ups.
Beyond weather protection, they minimise the effects of wind uplift on slates and tiles, and provide a secondary barrier against wind-driven rain, snow and dust — the same core functions as traditional felt, but without the moisture penalty.
How traditional (non-breathable) felt differs
Traditional roofing felt is a bitumen-saturated sheet, usually reinforced, supplied in the classic 1F and 1B grades. It is effectively impermeable to vapour.
That impermeability is the whole issue. Every house pushes warm, moist air upwards into the roof space. With a non-breathable underlay, that vapour has nowhere to go. It reaches the cold underside of the felt, condenses, and either drips onto the insulation or runs down and collects at the eaves. Over years this soaks insulation, encourages mould, and keeps rafters and battens permanently damp.
The traditional answer is to ventilate the roof void heavily — continuous ventilation at eaves level and often at ridge too — so that moisture-laden air is flushed out before it can condense. That works when it is done properly. It fails when eaves vents are blocked by insulation pushed into the corners, when a loft is later converted without addressing ventilation, or when the felt sags and blocks the airflow path at the eaves.
Bitumen felt is also heavy and stiff. It is harder to lay, sags between rafters over time, and becomes brittle with age until it tears at the batten fixings.
Cost, weight, lifespan and ventilation compared
| Factor | Breathable membrane | Traditional felt |
|---|---|---|
| Material cost | Moderate | Lower |
| Installed cost | Often lower — faster to fit, one-person handling | Higher labour, heavier handling |
| Whole-life cost | Lower — longer life, fewer vent components | Higher — shorter life, ventilation components needed |
| Weight | 100–120 g/m² | Several times heavier |
| Coverage per roll | 50m² or 75m² typical | Smaller rolls, more joints |
| Lifespan | 30–50 years covered | 20–30 years |
| Ventilation | Often reduced or unnecessary | Eaves and ridge ventilation required |
The headline material price favours felt, but that is misleading. Once you account for faster installation, fewer roll joints on a 75m² roll, longer service life and the ventilation components you no longer need to buy and fit, the breathable membrane usually costs less over the life of the roof.
Do breathable membranes need roof ventilation?
Often not — but this deserves a careful answer rather than a flat no.
A vapour-permeable underlay allows moisture to diffuse out through the roof covering, which in many standard cold-roof build-ups means additional high-level ventilation is not required. That is one of the main practical advantages of the product and a large part of why it has become standard.
However, the membrane’s permeability is only one part of the moisture picture. The design still has to account for how much moisture the building generates, how well the ceiling below is sealed, and what the manufacturer’s certification actually states. Where there is a well-sealed vapour control layer at ceiling level, a breathable membrane above, and no unusual moisture load, ventilation can generally be omitted. Where the ceiling is leaky — recessed downlights, loft hatches, service penetrations — or the building has a high moisture load, ventilation should be retained.
The practical rule: seal the warm side properly with a taped VCL, use a certified breathable membrane on the cold side, and follow the membrane manufacturer’s guidance for your specific build-up. Where there is doubt, ventilating costs little and removes the risk.
Grammage explained (100 / 115 / 120 g/m²) and BBA certification
Grammage is the weight of the membrane per square metre, and it is the main specification number you will compare between products.
100 g/m² is the standard domestic specification — suitable for most house roofs, extensions and dormers, and the most cost-effective choice for straightforward work. 115 g/m² is a step up in robustness, better for larger roofs and more exposed sites, and noticeably easier to control in wind while you are laying it. 120 g/m² is a heavyweight, high-performance underlay for cold and warm roofs, airtight, water-resistant and the most durable of the three.
Heavier does not mean more breathable — permeability is a property of the membrane construction, not its weight. Grammage tells you about durability, tear resistance and handling.
BBA approval is the certification to look for. It means the product has been independently assessed by the British Board of Agrément for use in UK construction, and it is what building control and warranty providers recognise. A membrane without recognised certification may still perform, but you have no independent evidence of it.
One more figure to note: UV stability. Most membranes are rated for a limited period of exposure before the tiles go on — commonly up to four months. If a roof is going to sit dried-in over a winter, check that figure.
Which should you choose?
By project type:
- Full house re-roof — breathable membrane, 115 or 120 g/m², BBA approved. Buy the 1.5m × 50m (75m²) roll to cover faster with fewer joints.
- Extension, dormer or porch — breathable membrane, 100 g/m², 1m × 50m (50m²) roll suits most areas.
- Small roof, garage or outbuilding — breathable membrane, 1m × 20m (20m²) roll to avoid buying more than you need.
- Repair patch on an existing bitumen-felt roof — match the existing underlay type in the repair area rather than mixing systems on the same slope.
- Shed or summerhouse — a lightweight breathable membrane costs a little more than shed felt and lasts considerably longer.
- Flat roof — neither; use a torch-on felt system, which is a waterproof covering rather than an underlay.
Whatever you choose, order felt support trays at the same time. They hold the underlay at the eaves so it drains into the gutter instead of sagging behind the fascia — a small cost that protects the most vulnerable part of the roof.
Frequently asked questions
Is breathable membrane better than felt?
For most new roofs and re-roofs, yes. It is lighter, faster to fit, lasts longer, and lets moisture escape from the roof space rather than trapping it. Traditional felt has a lower material cost but usually costs more over the life of the roof.
Do you need ventilation with a breathable membrane?
Often not, because the membrane itself allows moisture to escape. It depends on the build-up, the moisture load and the manufacturer’s certification. Where the ceiling below is well sealed with a taped VCL, additional high-level ventilation can often be omitted.
What g/m² breathable membrane do I need?
100 g/m² suits most domestic roofs. 115–120 g/m² gives a heavier, more robust membrane for larger or more exposed roofs and handles better in wind during installation.
Can you use breathable membrane on a shed?
Yes, and it is a better long-term choice than traditional shed felt — particularly if the shed is insulated or used as a workshop, where moisture control matters.
Is breathable roofing felt waterproof?
Yes to liquid water. It stops rain and snow while allowing water vapour to pass through, which is exactly the combination a roof underlay needs.
Products mentioned in this guide
- 50m² Breathable Roofing Felt 1m × 50m 100 GSM BBA
- 75m² Eurovent Breathable Membrane 1.5m × 50m 115 GSM BBA
- 20m² Rubershield Eco Extra 1m × 20m 120 GSM BBA
- Felt Support Tray Eaves Protector
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