Breathable membrane
Should a new roof have a breathable membrane?
The short answer
Usually yes on a pitched re-roof, but the right answer turns on the way the roof is built and how it is aired. A breathable membrane lets water vapour from the house escape through the underlay while still shedding rain that gets under the tiles. A non-breathable underlay can also be used, provided the roof is ventilated in line with the maker's rules. What matters is that the underlay and the ventilation are chosen as a pair.
What a breathable roof membrane does, where a different underlay still works, and the ventilation checks that go with either choice.
What the membrane is there to do
Under every tile and slate sits a layer called the underlay, fixed to the rafters and held in place by the battens. Its job is to carry away any rain or snow that blows past the covering, and to act as a back-up barrier behind the tiles. A breathable membrane does this and has one further talent: it lets moist air from the rooms below pass through it, rather than trapping it in the loft.
That matters because a family produces a surprising amount of vapour from cooking, washing and breathing. In a cold, poorly ventilated loft that moisture settles on cold surfaces as condensation, and over years it feeds mould and rot in the timbers. A membrane that lets vapour out is one way of managing the problem, though never the only one, and it works best when the loft below is also aired and the ceiling is sealed against steam rising from the rooms.
Our guide to roof insulation when re-roofing explains how the layers beneath fit with this.
Breathable and non-breathable underlay compared
Traditional bitumen felt is not vapour-permeable. Marley calls this type a high resistance underlay, and the BS 5250 moisture standard, now listed by BSI as current but under review, is the code of practice behind its ventilation advice. Modern breathable membranes are low resistance: vapour passes through them far more easily.
The difference shows up in how much ventilation the roof needs. Marley's advice for an unheated loft under a high resistance underlay is a 25 mm opening along the eaves on pitches of 15 degrees or below, and 10 mm on steeper ones. Pair a low resistance underlay with a well-sealed ceiling and the same source treats the small gaps in normal eaves construction, around 3 mm, as sufficient, if third-party test conditions are met for the product.
So the swap to a breathable type can simplify ventilation, but it does not remove the need to think about it.
Where the choice is not straightforward
Some roofs do not suit one answer, whatever a brochure says. A shallow pitch may need an underlay rated for its angle. A roof with insulation fitted tight against the underside of the boards, a warm roof, has a different moisture path from a cold loft and its own rules. Sloping ceilings in a converted loft leave little air space and call for a specific build-up.
Older houses are a particular case. A roof that was never lined behaves very differently from one with a modern membrane, and adding the membrane changes how air and vapour move through the structure. On a listed or period building, the choice also has to suit the conservation brief.
Instead of reaching for a stock answer, we look closely at your loft on the survey, note the pitch and the insulation, and specify the underlay to suit. It goes on the written quote, so the choice is visible to you before the work starts.
Ventilation, eaves and the loft
An underlay is only half of the system. The other half is air movement: eaves vents or a ventilated fascia to let air in at the bottom, and where the design needs them, ridge or high-level vents to let it out. Loft insulation packed tight into the eaves can choke that airflow, which is a well-known cause of condensation in lofts that were otherwise sound.
Bathrooms and kitchens need their own extract fans ducted outside, never just into the roof space. Pushing steamy air up into the loft overwhelms even a good membrane.
Part of what we check when we strip a roof is whether the old eaves detail let air through at all, and whether new insulation will cut it off. If either is a problem, we fix it as part of the re-roof rather than after the first winter. The roof replacement page describes the rest of the job.
What to ask a roofer about the underlay
Nobody expects you to know the trade, yet a handful of questions will tell you plenty. Ask which underlay will be used, and whether it is breathable. Ask who made it, and what roof pitch the maker rates it for. Ask how the eaves and the ridge will be ventilated, and what happens where insulation meets the eaves. A roofer who can answer clearly has thought about the whole build-up.
Ask also whether the membrane is covered by the quote. A re-roof quote that names the tile and says nothing about the layers below leaves a gap in your understanding of the job. Ours names the underlay and the ventilation and gives you one total price, which makes it easier to compare like with like.
Our answer on re-roofing over existing tiles explains why the underlay is also a reason not to cover over an old roof.
Related questions
Underlay questions
Is a breathable membrane a legal requirement?
Not in itself. The Building Regulations care about the result, which is a roof that keeps moisture under control, and guidance such as BS 5250 shows ways to achieve it. A breathable membrane is one accepted route, and the maker's instructions for the exact product set the ventilation to go with it.
Can I swap felt for a breathable membrane on a cold loft?
Often, but the ventilation needs reviewing at the same time. The gaps at the eaves that suited a felt may differ from those the new product's maker specifies, and the loft insulation must not block them. The survey checks both before an underlay is chosen.