The short answer
Often yes, though usually less than with a non-breathable felt. A vapour-permeable membrane lets moist air escape through the fabric, so a cold roof with one can need smaller vents than with an old-style felt. It still needs a clear route for air in some roofs, and a sealed ceiling, a long span or a low or lean-to pitch can change the answer. Follow the membrane maker's instructions and BS 5250.
Two kinds of breathable membrane
The word breathable covers more than one product, and the difference affects ventilation. A vapour-permeable membrane is a sheet that lets water vapour pass through while stopping liquid water. It is sometimes written as LR, for low resistance. An air-open membrane goes further: it is also permeable to air, so air can move through the fabric itself.
The reason is simple. Steam from cooking, washing and breathing drifts up from the rooms below and meets the chilly roof, where it turns back into droplets. That is the usual source of damp patches and mould in a loft, and the same problem follows you into a converted loft, as loft conversion roofs explains. A breathable membrane gives moisture a second route out. It does not make the problem disappear, and an old bitumen felt, which is non-breathable (HR), gives it no route at all.
Underlay, the sheet stretched beneath the battens, is there to catch rain that gets past the covering. Whichever type is used alters how the void beneath behaves, and that shapes every ventilation decision after it.
What ventilation a cold roof needs
A cold roof has insulation at ceiling level, and the space above is left cold and ventilated. Marley quotes BS 5250 for the ventilation needed, and its figures show how much the membrane changes things.
Start with the older non-breathable (HR) underlay and insulation laid at ceiling level. Marley's guidance is a continuous 25 mm gap at the eaves for pitches up to 15 degrees, falling to 10 mm for anything steeper. The guidance then adds a 5 mm strip high up if the pitch tops 35 degrees, the span goes beyond 10 m, or the roof leans to one side.
Fit a breathable (LR) underlay instead and the requirement branches. Two things steer it: how leaky or tight the ceiling is, and how much air the tiles themselves let through. In one Marley example, a sealed ceiling beneath an air-permeable covering can get by with 3 mm at the eaves, provided the product has UKAS third-party certification, or 5 mm higher up at the ridge. These are the manufacturer's readings of the standard, not ours, so read the certificate for the product on your roof.
When a breathable membrane still needs vents
Several situations call for ventilation even with a breathable membrane.
- A normal ceiling, not a sealed one. Recessed lights, loft hatches and gaps for pipes let warm, moist air into the roof void. A leaky ceiling raises the requirement.
- A covering that is not air-permeable. Close-fitting concrete or slate coverings behave differently from loose clay tiles, which let air through.
- A low pitch. Marley's numbers change at 15 degrees, and a flat-ish roof dries out slowly. Our page on how to measure roof pitch shows how to find your own.
- A long span or lean-to. Air has further to travel, so high-level vents are added.
- Moisture-heavy rooms below. Bathrooms and kitchens put a lot of vapour into the air, and extractor fans should vent outside, not into the loft.
Blocked eaves are a frequent fault. Insulation pushed hard into the eaves can close the vents, which is why a ventilation tray is used to hold it back. The same goes for stored boxes stacked against the eaves, and for a roof that has been re-felted over a loft previously insulated by someone else. If you cannot see daylight or feel air movement at the eaves on a windy day, ask for the vents to be checked. Damp-stained timber near the eaves, or rusty nail tips showing through the underlay, tell a roofer the void has been staying wet.
Warm roofs, conversions and spray foam
A warm roof puts the insulation at the slope, so there is no cold roof void to ventilate in the usual way. The rules change, and the membrane and the insulation have to be chosen as a system. Manufacturers publish the build-ups they have tested, and building control will look for the product evidence.
This is where mansards and loft conversions come in. Our page on mansard roofs explains how the insulated room sits inside the roof shape. The flat top of a mansard is a separate case, since it needs its own detailing for drainage and vapour.
Foam sprayed onto the roof underside changes things again. RICS published a consumer guide in March 2023 that cautions about uncontrolled condensation and harm to parts of the property, and suggests owners ask a mortgage provider whether foam fits its lending policy. A BBC investigation reported in November 2024 that some lenders had declined such properties.
Getting the answer for your roof
The honest position is that no single rule fits every roof. The right answer comes from three things: the membrane you have or choose, the ceiling below it, and the covering above it. If any of those changes, the ventilation can change with it.
Our survey reads the underlay, the eaves and the loft as one system. Pictures record what is there, we test whether the vents are open, and we look at the timbers for the staining that condensation leaves. A new roof over an old void often needs the ventilation reworked at the same time. Once a re-roof passes the Building Regulations threshold, insulation may need upgrading too, and where the job is notifiable we handle that part as well.
A re-covering is priced firmly on paper after the survey. The bigger picture is on the pitched roofing in Bath page.
