Description
Under every tiled or slated roof there is a membrane doing quiet, unglamorous work: shedding any water that gets past the covering, while still letting moisture from inside the building escape rather than condensing in the roof space.
Get that layer wrong and you find out years later, in the form of damp timbers.
This is a 125gsm breathable roof and wall underlay, supplied as a 50m roll or as a complete taped kit.
- 125gsm breathable membrane for pitched roofs and insulated walls
- Water resistant from outside, vapour permeable from inside — no separate ventilation strategy needed for the membrane itself
- Two roll widths: 1m x 50m (50m²) and 1.5m x 50m (75m²)
- Complete kit option adds the lap tapes needed to seal it properly
Specification
| Weight | 125gsm |
|---|---|
| Roll sizes | 1m x 50m (50m²) and 1.5m x 50m (75m²) |
| Colour | Matt black |
| Performance | Water resistant and vapour permeable |
| Applications | Pitched cold roofs, pitched warm roofs, insulated walls |
| Complete kit contains | 1 membrane roll, 2 rolls single-sided lap tape, 1 roll double-sided lap tape |
Frequently asked questions
Roll or complete kit? The membrane only performs as a continuous layer, which means the laps have to be sealed. If you already have lap tape, buy the roll.
If you don't, the kit works out simpler than sourcing tapes separately — it includes both the single-sided and double-sided tapes the job needs.
Which width? 1.5m covers ground faster and leaves fewer horizontal laps, so it suits larger, simpler roof planes.
1m is easier to handle single-handed and wastes less on cut-up roofs with lots of hips, valleys and dormers.
How much do I need? Work from the roof slope area, not the footprint, and allow for horizontal laps between courses plus generous overlaps at ridges, hips and valleys. Round up to whole rolls.
How is it fixed? Laid horizontally starting at the eaves and working up, so each course laps over the one below, then held by the tile battens fixed through it into the rafters.
Keep it taut but with a slight natural drape between rafters so any water that gets through runs down the membrane rather than pooling on it.
Can I use it on a flat roof? No. This is a pitched roof and wall underlay. A flat roof needs a fully waterproof covering — see our EPDM, felt and GRP systems instead.
Is this the same as a vapour control layer? No, and they do opposite jobs. This is designed to let vapour out through the roof.
A vapour control layer is designed to stop vapour getting in, and sits on the warm side of the insulation.
What if my site needs more wind resistance than a loose-laid membrane?
For low-pitch or high-wind-exposure sites, our Self-Adhesive Peel & Stick Roofing Underlay bonds solid to the deck instead of relying on battens and laps alone.
Building a warm flat roof instead? You want our polythene vapour control layer and PIR insulation board. Covering a flat roof? See our 1.2mm EPDM membrane or torch-on felt system.
This underlay also sits beneath our bitumen architectural roof shingles and concrete interlocking roof tiles on a pitched deck.
Sarking a timber-frame roof rather than using OSB or ply? Our wood fibre sarking & insulation board is designed to work directly beneath this membrane in a fully breathable build-up.
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