Breathable roofing membrane vs felt: which do you need?
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A breathable roofing membrane is a vapour-permeable underlay that goes under the tiles or slates. It sheds any rain that gets past the tiles but lets water vapour escape from the loft, so timber and insulation stay dry. Traditional bitumen roofing felt does the first job but not the second: it stops vapour, so the roof space has to be ventilated or condensation builds up. For a new or re-roofed pitched roof, breathable membrane is the modern choice. A 1.5m x 50m roll covers about 67m² once you allow 150mm laps. Our Breatheshield membrane costs from about £1.07 per m².
What is a breathable roofing membrane?
A breathable membrane (also called a breather membrane, breathable felt or a vapour-permeable underlay) is a thin, tough sheet laid across the rafters under the battens and tiles. It has two jobs:
- Keep water out. Wind-driven rain and snow get past tiles on every roof. The membrane catches it and runs it down to the gutter.
- Let vapour out. Warm, damp air from the house rises into the loft. The membrane lets that vapour pass through to the outside instead of condensing on the underside and dripping onto the insulation and timbers.
Our Breatheshield breathable roofing membrane comes in 50m rolls at 120gsm or 150gsm, in 1.5m and 1m widths.
What is the difference between breathable membrane and roofing felt?
Most people still call any roof underlay "felt", but the two behave differently:
| Breathable membrane | Traditional bitumen felt | |
|---|---|---|
| Keeps rain out | Yes | Yes |
| Lets vapour out | Yes, vapour-permeable | No, it is a vapour barrier |
| Loft ventilation | Still advised at the eaves unless the membrane's certificate says otherwise | Essential, at the eaves and ideally the ridge |
| Weight to carry up a ladder | Light, 50m roll | Heavy, shorter rolls |
| Behaviour in heat and cold | Stays flexible | Goes soft in summer, brittle in winter, tears with age |
| Typical use now | New roofs and re-roofs under tiles and slates | Repairs to older felted roofs |
Felt is not wrong, it has just been overtaken. If you are stripping a roof back to the rafters anyway, there is no good reason to go back with bitumen felt.
Do I still need roof ventilation with a breathable membrane?
Usually, yes, at least at the eaves. A breathable membrane lets a lot of vapour through, but a cold loft still benefits from air moving across it, and some membranes are only certified for an unventilated roof under specific conditions. The safe rule: if the membrane's certificate does not say the roof can be unventilated, ventilate it as you would for felt. An over-fascia vent (10mm) or 25mm along the eaves is the simple way to do it, and a vented dry ridge lets air out at the top.
Also keep a vapour control layer on the warm side of the insulation where the roof design calls for one. The less vapour gets into the roof in the first place, the less the membrane has to deal with.
120gsm or 150gsm: which weight do I need?
The gsm figure is the weight of the sheet, which is a fair guide to its strength and tear resistance.
- 120gsm is the standard weight and does the job on most domestic pitched roofs.
- 150gsm is heavier and tougher. It stands up better to wind, foot traffic and handling on bigger or more exposed roofs, and it is more forgiving if the tiles are not going on straight away.
On a sheltered house roof, 120gsm is fine. On a coastal or hilltop site, a big roof, or one where the membrane will be open to the weather for a while, the extra £5 a roll for 150gsm is cheap insurance.
How much breathable membrane do I need?
Work it out from the area of each roof slope (rafter length from eaves to ridge, times the length of the slope), then allow for the laps. Each course overlaps the one below, so you lose that much of every roll width.
| Roll | Price | Gross area | Usable at 150mm laps | Cost per m² (gross) |
|---|---|---|---|---|
| 120gsm, 1.5m x 50m | £79.99 | 75m² | about 67m² | £1.07 |
| 150gsm, 1.5m x 50m | £84.99 | 75m² | about 67m² | £1.13 |
| 150gsm, 1m x 50m | £69.99 | 50m² | about 42m² | £1.40 |
Worked example: a typical semi with two slopes, each 8m long with a 5m rafter, is 80m² of roof. Divide by 67m² per roll and you get 1.2, so order two 1.5m rolls. That also leaves enough for end laps, valleys and a bit of waste. Use the 1m roll for dormers, lean-tos, porches and cut-up sections where a wide roll just gets in the way.
How do you lay breathable roofing membrane?
- Start at the eaves and work up. Each course laps over the one below so water always runs down and out, never behind.
- Lap the courses. At least 100mm on pitches of 15 degrees and over, and 150mm on shallower roofs. Plenty of roofers lap 150mm as standard.
- Join roll ends over a rafter with a lap of at least 100mm, held down by the batten.
- Leave a slight drape between rafters, about 10mm and no more than 15mm. That gives a channel for any water to run down under the battens. Do not pull it drum tight, and do not let it hang in a belly either.
- Support it at the eaves. Dress the bottom course down into the gutter over an eaves support tray (£2.20 per 1.5m) so it cannot sag behind the fascia and hold a puddle.
- Fix it temporarily, then batten. Tack it with galvanised clout nails or 140-series staples from a hammer tacker (8mm for a single layer, 10mm for heavier sheet or doubled-up laps). The tile battens do the real holding.
Can I use roofing membrane on walls, floors or a shed?
- Timber-frame walls: use a membrane sold and certified for walls. Roofing underlays and wall breather membranes are tested for different jobs, so check the product before swapping one for the other.
- Floors and solid walls: that is a damp job, not a breathable one. You want a damp proof membrane (DPM), which blocks moisture completely. See our damp proofing range.
- Shed roofs: a shed with no tiles needs a top covering such as mineral shed felt. A breathable underlay is not made to be the finished surface and will break down in sunlight.
Common mistakes to avoid
- Laying it upside down or top course first. Read the print side and always lap down the slope.
- Pulling it tight. No drape means water sits on the battens and rots them.
- Blocking the eaves. Insulation stuffed into the eaves cuts off the airflow. Keep the gap clear.
- Leaving it uncovered. Membranes are not made to sit in the sun for months. Check the maker's exposure limit and get the tiles on.
- No support at the eaves. A sagging bottom course ponds water behind the fascia. Fit a support tray.
The short version
Breathable membrane and felt both keep rain out, but only membrane lets loft vapour escape, which is why it has replaced felt under tiles and slates. Use 120gsm for most house roofs and 150gsm for big or exposed ones. A 1.5m x 50m roll covers about 67m² with laps, from £79.99. Lay it from the eaves up, lap 100 to 150mm, leave a small drape and keep the eaves ventilated. Browse the full roofing membrane range and everything else for the job in roofing.
By Steve, Builders Hardware Direct. Steve runs the BHD trade counter in Gorslas and supplies roofing underlay, fixings and vents to roofers and builders across South Wales. Not sure which membrane or how many rolls? Call the counter on 01269 267030 or order online for fast UK delivery.
