A breathable roofing membrane is a lightweight underlay that sits beneath the tiles or slates and lets water vapour pass out of the roof space while keeping wind-driven rain and snow out. It replaces traditional bitumen felt as the secondary weather layer and, because it is vapour permeable, it reduces the risk of condensation building up on the underside of the roof. It does not, on its own, remove the need to design roof ventilation properly.
What a breathable roofing membrane actually does
The membrane is the second line of defence. The tiles or slates shed almost all the water, and the underlay catches anything that gets past them, wind-driven rain, dust and the odd bit of snow, then drains it down to the eaves. A modern breathable underlay is usually a triple-layer spun-bonded polypropylene sheet with a micro-porous film in the middle. That film is the clever part: it is watertight from the outside but open enough to let moisture vapour from inside the building escape outwards.
That matters because warm, moist air is always rising up through a house. If it hits a cold, sealed surface it condenses into water, and water sitting against roof timbers leads to rot, mould and rusting fixings over time. A breathable membrane gives that vapour a route out instead of trapping it. You will find these underlays across most pitched roofs we supply, and they sit alongside the tiles, battens and fixings on our pitched roofing materials range.
Breathable vs non-breathable membrane: roofing felt vs membrane
The old standard was bitumen felt, often called 1F felt. It is heavy, it is effectively vapour-impermeable, and it tends to sag between the rafters into a trough that can pool water. Because it does not let vapour through, any moisture rising from below condenses on its cold underside and stays there. In an insulated roof that is a recipe for interstitial condensation.
A breathable membrane is lighter, stronger and vapour open. It handles moisture in the opposite way, letting it pass through and away. Here is the short version.
| Feature | Breathable membrane | Traditional bitumen felt (1F) |
|---|---|---|
| Vapour permeable | Yes, lets moisture out | No, effectively sealed |
| Weight | Light, easy to handle | Heavy |
| Sagging | Laid with light tension, minimal sag | Prone to sagging and pooling |
| Condensation risk | Lower, vapour escapes | Higher, vapour is trapped |
| Typical use today | Standard on most new pitched roofs | Occasional, some traditional specifications |
Some roofers do still choose a heavier non-breathable or traditional felt on certain jobs, and there is nothing wrong with that where the design calls for it. The point is to match the underlay to the whole roof build-up, not to assume one type suits every situation.
Does a breathable membrane remove the need for roof ventilation?
This is the question that catches people out. The word “breathable” makes it sound as though the sheet handles all the moisture on its own. In practice, most breathable underlays still need the roof to be ventilated, and that ventilation has to be designed in line with BS 5250, the British Standard covering the management of moisture in buildings.
The reason is that membranes fall into two camps. Some are vapour permeable but still fairly airtight, and some are both vapour permeable and air open. Only an air-open, vapour-permeable membrane that carries specific BBA certification can potentially be used without additional roof-space ventilation, and even then the NHBC will only accept it on that basis where the certificate says so. A good example is that even a widely used underlay like Cromar’s Vent3 range, which is BBA certified, still carries manufacturer guidance recommending ventilation at the ridge in cold roof applications.
So the safe rule is simple: follow the membrane manufacturer’s instructions and its BBA certificate, and design the eaves and ridge ventilation to suit the roof. Do not treat the membrane as a substitute for that. If you are unsure which underlay pairs with which ventilation detail, our counter staff have been specifying this for years and are happy to talk it through.
Which side of a breathable membrane faces up, laps and lifespan
Orientation matters. The membrane has a printed or darker face that is laid facing upwards, towards the tiles, because that side gives better UV resistance. On a Cromar Vent3 Classic, for instance, that is the light grey printed side up. Fit it upside down and you compromise how it performs.
It is unrolled starting at the eaves and worked up the roof, so each higher course laps over the one below and water always runs onto the sheet rather than under it. Head laps and side laps should follow the manufacturer’s fixing instructions and NHBC guidance, with the lap size linked to the roof pitch and wind zone rather than a single fixed figure. Leave a drainage and ventilation gap of at least 25mm between the underlay and the tiles, and turn the membrane up at least 100mm behind any abutment flashing.
Lifespan is generous when it is fitted and covered correctly. A quality breathable underlay protected by the roof covering can last for several decades. The main thing that shortens it is UV exposure, so most membranes are only rated for a limited spell uncovered, commonly up to around three months, before the tiles or slates should go on. Get the covering on in good time and the underlay quietly does its job for the life of the roof.
Underlay and roofing materials across Sussex, Kent and Surrey
We have stocked breathable membranes, felts and the full pitched roofing kit since Brian Gow started the business back in 1996, and we carry Cromar underlays alongside our other brands. You can collect from any of our four depots or have materials delivered across the South East. If you are local, our Burgess Hill roofing supplies branch keeps membranes and underlay in stock, and you can find your nearest branch in a couple of clicks.
Not sure which underlay and ventilation setup your roof needs? Tell us the build-up and we will point you to the right membrane. Get a quote or call the yard and we will sort it.
Frequently asked questions
What is a breathable roofing membrane?
It is a lightweight underlay fitted beneath the tiles or slates as a secondary weather barrier. It keeps out wind-driven rain and snow while letting water vapour from inside the building pass out through the sheet. That vapour permeability lowers the risk of condensation forming on the roof timbers, which is why it has largely replaced traditional bitumen felt on pitched roofs.
What is the difference between a breathable and non-breathable membrane?
A breathable membrane lets water vapour escape from the roof space, while a non-breathable underlay such as bitumen felt seals it in. The breathable type is lighter, sags far less and reduces condensation risk. Non-breathable felt is heavier and can trap moisture against the timbers. Both keep rain out, but they manage internal moisture in opposite ways.
Do you still need roof ventilation with a breathable membrane?
Usually, yes. Most breathable membranes still need the roof ventilated in line with BS 5250, at the eaves and often the ridge. Only an air-open, vapour-permeable membrane with specific BBA certification can potentially be used without extra ventilation, and only where the certificate and NHBC allow it. Always follow the manufacturer’s instructions rather than assuming the membrane does it all.
Which side of a breathable membrane faces up?
The printed or darker side faces upwards, towards the tiles or slates. That face carries the better UV resistance and is the side the manufacturer designs to sit outermost. On a Cromar Vent3 Classic that is the light grey printed side up. Lay it the wrong way round and you reduce how well it performs, so check the roll before you start.
How long does a roofing membrane last?
A quality breathable membrane protected by the roof covering can last several decades, often the working life of the roof itself. The biggest threat is UV exposure while it is uncovered, so most are only rated for a limited period exposed, commonly up to about three months, before tiling. Correct fixing and avoiding damage during the build both help it reach full life.
Can you use a breathable membrane on any roof?
Breathable membranes suit most pitched tiled and slated roofs, in both warm and cold roof designs. They are not a primary waterproofing layer, so they always sit under a proper roof covering, and they are not intended for flat roofs, which need their own systems. The right choice depends on the roof pitch, the build-up and the ventilation, so check the membrane specification for your job.