What each layer does under the tiles
Underlay is a backup barrier fixed to the rafters, with the tile battens nailed on top of it. Tiles shed most of the rain, but wind pushes some of it up under the laps, and the underlay catches that water and guides it down into the gutter.
Traditional felt underlay is bitumen-based and sold as high-resistance, or HR. Vapour from a bath, a kettle or a drying rack cannot pass through it. That vapour drifts up into the loft and has to leave some other way, so an HR roof depends on a clear air path through the loft, entering low and leaving high.
A vapour-permeable membrane is a woven or spun plastic sheet that still holds back liquid water but lets vapour through. Some of that vapour can then escape upward through the sheet itself, instead of relying only on air moving across the loft.
Breathable is trade shorthand. It describes vapour movement, not air movement, and it never means a roof can be left unventilated by default. HR underlay is not breathable, so a quote that calls it breathable deserves a question.
What BS 5250 asks for
BS 5250:2021 is the current edition of the British code on moisture in buildings. It covers condensation in the roof space, and it applies whichever underlay you choose.
For a cold roof, meaning insulation laid at ceiling level with the loft above left cold, and HR underlay, Marley's published guidance gives an eaves opening of 25mm for pitches of 15° or less and 10mm for steeper roofs. Our eaves ventilation gap answer takes those two figures apart.
A vapour-permeable membrane arrives with its own instructions from the maker. Those state what ventilation the membrane needs on a cold roof, how it must be lapped and how it must be supported between rafters so it does not sag and hold water.
We follow the data sheet for the exact membrane named in the quote, not a rule of thumb. The sheets differ between makers, and the sheet is also the paper that the next roofer, or your surveyor at a sale, will ask to see when the roof is checked.
The two side by side
| Question | HR felt | Vapour-permeable |
|---|---|---|
| Lets vapour through | No | Yes |
| Needs eaves ventilation | Yes | As the maker states |
| Suits a roof with no loft | Seldom | Often, to the data sheet |
Felt stays sensible on a traditional cold roof where the eaves are open, the loft is clear and the ceiling below is sealed. A breathable membrane earns its place where the loft has been converted, the insulation sits between the rafters, or old eaves have been boxed in and cannot easily be reopened.
Fitting a membrane over a roof that is still blocked at the eaves does not clear damp by itself, and it can hide the symptom for a season or two. The cause has to be found first, and our damp loft guide lists the checks to make.
The build around the membrane matters more than what the sheet costs, because no membrane makes up for eaves that are blocked or a loft that cannot ventilate. A cheap sheet fitted into a roof that cannot ventilate costs more in the end than a dearer one fitted properly, because the damp it causes shows up in the timbers, the ceiling and the insulation.
Making the call on your own house
On a re-roof the underlay comes off with the old battens, so the choice arises at survey stage. It is settled by what already exists in the roof space, not by whichever product sounds newer.
Four points decide it:
- Whether the eaves are open, and how far the insulation reaches along the ceiling.
- Whether the roof space is lived in, or a plain cold loft.
- What the tile or slate maker requires for that product.
- What the membrane maker's data sheet says about laps and support.
Ask for the underlay to be named on the quote with its maker, and keep the data sheet with the house papers, because a buyer's surveyor may ask to see it.
The wider job is covered in roof underlay and battens, and every other tiled job sits under pitched roofing. Photos of the loft, the eaves and one slope are enough for us to say which route fits your roof. If the roof is being converted or insulated at the rafters, say so, because that changes the answer.
