A membrane is a thin, flexible layer that acts as a separator: it holds back water, air or water vapour without carrying any load itself. The word is also used for tensioned fabric structures, where the membrane does carry load — but only in tension.
Membranes used for sealing
| Type | Material | Use |
|---|---|---|
| Roofing membrane | EPDM, TPO, PVC, bitumen | Waterproof layer on a flat roof |
| Vapour control membrane | PE film, foil-faced material | On the warm side of the insulation |
| Breather membrane | Spun-bonded PP/PE | Under roof tiles or behind cladding |
| Damp-proof course (DPC) | Bitumen or plastic sheet | At the base of a wall against rising damp |
| Dimpled membrane | HDPE with studs | Against basement walls and under floors |
The order matters: the vapour control membrane belongs on the inside (warm side), the breather membrane on the outside. Reverse them and you trap moisture in the construction, which leads to wood rot and soaked insulation.
Membrane structures
In a membrane roof, the fabric is the structure. It is tensioned over masts and cables and works purely in tension; it cannot take compression or bending. Common materials are PVC-coated polyester fabric, PTFE-coated glass fibre fabric and ETFE film (often as an inflated cushion). Typical uses: stadiums, canopies, awnings and atria.
Points to watch when installing
- Always lap the seams in the direction the water runs
- Penetrations (pipes, rooflights, vents) are the weak spot: use preformed collars
- Membranes bond poorly to a dirty or damp substrate — work dry and dust-free
- Loose-laid roofing membranes must be ballasted or mechanically fixed against wind uplift
Related terms
- Vapour barrier
- Vapour permeable membrane
- Breathable construction
- EPDM
- Dimpled membrane
- Roof covering
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