Breathable membrane
What is a breathable roof membrane, and why does a new roof need one?
The membrane is the layer under the battens that deals with moisture. Below: what it is, the two underlay types, why the choice matters and what to check.
The short answer
A breathable roof membrane is a vapour-permeable underlay laid over the rafters and under the battens and covering, so that moisture from inside the house can pass out through it. A new roof carries an underlay beneath the covering as a second layer of protection, and which type goes on decides the eaves ventilation the loft needs. Felt is the older name for the same layer.
5 min read · Part of Roof replacement
Where the membrane sits and what it does
Looking from the loft upwards, the order is rafters, underlay, battens, then tiles or slates. The underlay is the layer that catches whatever gets past the covering, and it also has to deal with moisture rising from the rooms below, which is where the word breathable comes in. Because the membrane is hidden once the roof is finished, the time to ask about it is at the quote.
A vapour-permeable membrane lets that moisture pass. A high-resistance underlay, written HR on data sheets, does not, so it is the opposite of a breathable one. The two are laid in the same place but behave differently, and the roof is ventilated differently for each.
A new covering normally goes onto a new underlay, because reusing an old one would keep the holes left by the earlier battens and nails. The size of the new battens and the way they are fixed sit under BS 5534, the code for slating and tiling, which covers batten selection, fixing methods and wind uplift calculations. BSI lists the 2014 edition with its 2018 amendment as current.
Two types, two ventilation answers
Marley sets out the eaves ventilation for a cold roof, which is a roof insulated at ceiling level with the loft space above it left cold.
| Underlay | Eaves ventilation | Condition |
|---|---|---|
| High resistance (HR), lets no vapour through | 25 mm along the eaves at 15° or less; 10 mm above 15° | None beyond the figures |
| Breather type (LR) | About 3 mm from normal eaves construction | Ceiling well sealed; roof covering air permeable; certification met in full |
For the breather-type route, Marley attaches a certification condition, which names UKAS Third Party certification and must be fully met. The 3 mm in that row is not an extra gap to cut; it is what normal eaves construction already provides. The point is that the underlay is not a free choice made in isolation. A breathable type, with a sealed ceiling and air-permeable tiles or slates over it, needs far less ventilation at the eaves, while an HR type needs the full gap. Mixing the assumptions, such as fitting an HR product and leaving the eaves unvented, is the error a quote should rule out.
Why the choice matters for the loft
Moisture in a loft comes from the house itself: cooking, washing and breathing add water vapour that drifts upwards. The underlay and the eaves openings are two halves of how that moisture leaves the roof space. BS 5250, BSI's moisture code, carries the status Current, Under Review.
The same logic applies when the roof is insulated or when foam is involved:
- More insulation changes the temperature of the roof space, so the ventilation figure is checked when insulation is added.
- The Energy Saving Trust describes closed-cell spray foam as not vapour permeable.
- RICS says foam over the timbers stops them being fully inspected.
None of that makes a breathable membrane optional in every case. It means the membrane, the insulation and the ventilation are chosen as one design and priced as one job. A roof with the covering off is the easiest moment to get all three right, because the rafters are open. Loft conversions change the picture again, since the roof space is then lived in, so the specification is settled with building control first. Our page on insulation when re-roofing covers the energy side.
What to check on the quote and data sheet
A quote may give the underlay no more than a line, so ask the questions yourself.
- Which underlay is named, and is it described as vapour permeable or high resistance?
- Can you see the maker's data sheet for that product?
- What eaves ventilation does the data sheet call for, and is it built into the plan?
- If the product is a breather type, is the ceiling below well sealed, does the covering let air through, and is certification covered?
- Are new battens included, and are they fixed over the new underlay?
- Who fits the underlay, and is it checked before the battens go on?
Our page on whether a new roof needs new battens deals with the layer above. Once the survey is done you receive a written price, given as one total, and the roof replacement page explains the whole job. If anything on the data sheet is unclear, ask for it to be explained before you sign, since the underlay is out of sight once the roof is finished. Ask for the product name to appear on the invoice as well, so it can be traced if anything is queried later.
More questions
Is a breathable membrane the same as high-resistance felt?
No. HR stands for high resistance: that product type blocks vapour from passing, so a cold roof beneath it needs the full eaves ventilation, 25 mm at 15° or less and 10 mm above that. A breathable type is vapour permeable. The maker's data sheet tells you which kind a product is, and a quote should name it.
Do I still need eaves ventilation with a breathable membrane?
Marley's figure for a breather-type underlay is about 3 mm, supplied by ordinary eaves construction, on three conditions: a well-sealed ceiling beneath, a roof covering that is air permeable, and certification that is fully met. Ask the supplier to confirm all three in writing. If any condition is missing, the figure changes, so check the data sheet before the roof is closed up.
More on roof replacement
All guides and posts
Tell us what's wrong with the roof.
You'll get one written total price.