What Is a Breathable Roofing Membrane — and Do You Need One?
By Leonard Walton, owner & lead roofer3 min read
When you strip a roof back, one of the most important choices is invisible once it's finished: the membrane that goes under the tiles. A breathable roofing membrane is a second line of defence against water that also lets moisture vapour from inside the house escape out through the roof — so damp air doesn't get trapped against the timbers and turn into condensation and rot. On the right roof, it's the difference between a build-up that lasts and one that quietly sweats itself to bits.
What the membrane is for
Under your tiles sits a layer of underlay or membrane. It does two jobs: it catches any wind-driven rain or snow that gets past the tiles, and — if it's breathable — it lets water vapour from the rooms below pass out through the roof. Older felt underlays block that vapour, which is fine on a cold, well-ventilated loft but a problem where the roof is warmer.
Why it matters most on warm and habitable roofs
Here's the key idea. If the space below the rafters is lived-in and short on insulation or ventilation — a converted loft, a room in the roof, a bedroom in the eaves — warm, moist air rises and meets the cold underside of the roof. With nowhere to go, that moisture condenses on the timbers and, over time, causes damp and rot you can't see until it's serious. A breathable membrane lets that moisture escape instead of trapping it. It's exactly why, on a recent re-roof in Hull where the space below the rafters was habitable and under-insulated, we fitted a breathable quilt membrane rather than standard felt — the roof needed to breathe.
Membrane and ventilation work together
A membrane isn't the whole answer on its own — it works alongside proper ventilation at the eaves and ridge, guided by BS 5250, the standard for managing condensation in buildings. The right combination depends on how your roof is built and used, which is why we assess it rather than fitting the same thing on every job.
When it's fitted
The membrane goes on during a re-roof, once the old covering is off and before the new battens and tiles. It can't be added later without lifting the roof — so the re-roof is the moment to get it right, and worth making sure your roofer has actually thought about which membrane your roof needs.
Frequently asked questions
What does a breathable roofing membrane do?
It sits under the tiles as a second line of defence against water, while letting moisture vapour from inside the house pass out through the roof — so damp air escapes instead of getting trapped against the timbers.
Do I need a breathable membrane or standard felt?
It depends on the roof. A breathable membrane is especially important where the roof space is warm or lived-in and short on ventilation, because that's where trapped condensation does the most harm. We advise per roof, not one-size-fits-all.
Can the wrong membrane cause condensation?
Yes. On a warm or habitable roof, a non-breathable underlay with poor ventilation can trap moisture against the timbers and cause damp and rot. Matching the membrane and ventilation to how the roof is used is what prevents that.
Is the membrane fitted during a re-roof?
Yes — the membrane goes on when the old covering is stripped and before the new battens and tiles. It's not something that can be added later without lifting the roof, which is why getting it right at re-roof stage matters.
Planning a re-roof and want it built for how your house is actually used? We fit the right membrane and ventilation for your roof — re-roofing across Hull, Beverley, Hessle and the East Riding. Free written quotes. Call 07447 352091.