Under almost every modern tile and slate roof there is a sheet that most owners have never seen. It is called the underlay or the sarking membrane, and it lies across the rafters beneath the battens, catching whatever the covering lets through. Wind drives rain and fine snow under tiles, and condensation forms on the underside of any cold surface, so the roof relies on this second line of defence. For many years that sheet was bitumen felt. Since the 1990s, breathable membranes have taken a large share of the market. They look similar from the loft, but they behave differently, and the choice shapes how the roof deals with moisture. This page explains both and when each makes sense.
What the underlay is for
Tiles and slates are not waterproof in the way a membrane is. They shed water by overlapping, and in a gale, rain and fine spray are blown up under the laps. Frost can open the joints, and a cracked tile lets water through. The underlay catches that water and drains it down to the eaves, where it exits into the gutter, which is why it is laid with an overlap and a slight drape between rafters. It also stops wind blowing through the roof, reduces dust and snow entering the loft and protects the timbers while the roof is being built.
The underlay is not a covering in its own right. If the tiles are missing or the roof is open for long, water will find its way through laps and nail holes. It is a safety net, not a roof. Our guide to roof battens covers the next layer up, which holds the tiles and forms the ventilated gap above the underlay.
Bitumen felt
Traditional underlay is a sheet of bitumen-coated fabric, usually reinforced with fibres or a polyester scrim. It is cheap, widely available and has a long record. It is waterproof and vapour-resistant: it stops liquid water and also stops water vapour passing through. That sounds like a benefit and in one way it is, since water cannot get in from above. But it means that vapour rising from the house, from cooking, bathing and breathing, cannot pass out through the felt, and so it must escape another way. In a roof with a felt underlay, the loft needs good ventilation at the eaves and ridge, so that moist air is carried away before it condenses on the cold underside.
Felt has weaknesses. It is relatively heavy and stiff, so it is harder to lay neatly. It degrades with UV light and age, becoming brittle and cracking. Old felt in a roof is often torn, perished or missing in patches, which is one reason we recommend renewing the underlay when a roof is re-covered. Felt can also tear in the wind before the tiles go on, leaving gaps. Bitumen felt for roofs is made to a British Standard, and the grade matters: a reinforced felt is the minimum we would use on a new roof.
Breathable membranes
A breathable membrane, also called a vapour-permeable underlay, is made from layers of polyethylene or polypropylene, often laminated, with microscopic pores or a permeable film. Liquid water cannot pass through it, but water vapour can. Warm moist air from the house can migrate through the membrane into the batten space above and escape through the tile laps and the ridge. That reduces the risk of condensation in the loft and on the timbers.
They are lighter and more flexible than felt, come in wide rolls that are quick to lay and resist tearing and UV better. Many are rated for exposure while the roof is open. They are widely used in modern construction and loft conversions, where warm, insulated roof spaces are common and ventilation is harder to provide.
Breathable membranes are classed by how easily vapour passes through them. A low-resistance type lets vapour pass freely and is suited to roofs in which ventilation is limited, such as insulated sloping ceilings. A high-resistance type is less permeable, more like felt, and is normally used with proper eaves and ridge ventilation. The manufacturer's instructions and the relevant standards, including BS 5534, set out how each type should be used. Our page on BS 5534 and tile fixing covers the wider standard.
| Feature | Bitumen felt | Breathable membrane |
|---|---|---|
| Passes vapour | No | Yes, in controlled amounts |
| Stops liquid water | Yes | Yes |
| Weight and handling | Heavier, stiffer | Light, flexible, wide rolls |
| UV and ageing | Becomes brittle over time | Generally more stable |
| Loft ventilation | Eaves and ridge ventilation needed | Depends on membrane type |
| Cost | Lower | Higher |
Condensation and why it matters
The main case for a breathable membrane is moisture control. Damp air from the house rises into the loft, and wherever it meets a cold surface it condenses. With felt, condensation can form on the underside of the felt itself, drip onto insulation and timbers, and over years cause mould and rot. With a breathable membrane, more of that vapour passes through and escapes. This is especially helpful in roofs with a lot of insulation, where the roof space is colder than it used to be, and in rooms in the roof, where there is no loft void to ventilate.
But a breathable membrane does not cure all condensation. If the ceiling below lets moist air leak into the loft, through unsealed hatches, downlights and pipes, moisture is the real problem, and the membrane will only cope with so much. If the loft is blocked with insulation pushed into the eaves, the ventilation path is lost whichever underlay is used. Our answer on condensation in the loft explains how to tell where the moisture is coming from.
Condensation, mould and the signs your underlay is failing
An underlay that has reached the end of its useful life leaves clues in the loft. Look for felt that is torn, hanging down between rafters or lying in loose, crumbling pieces. Look for daylight or draughts where laps have opened. Look for dark water stains on the felt and timbers, or black mould spots on the underside of the roof boards, which suggest condensation. Dust and grit in the insulation or on the loft floor may point to missing or torn underlay letting tile dust through. A roof without any underlay can show pale streaks where wind-driven rain has run down the rafters.
These are signs of a problem, not a diagnosis. Water getting in from outside and moisture getting out from inside look similar, and a roofer can tell them apart by pattern. Condensation is usually general, across the cold surfaces of the roof, and worse on cold mornings. A leak is localised and follows rain. Both are worth correcting, and both are dealt with differently.
Which suits which roof
For a standard cold loft on a pitched roof with good eaves and ridge ventilation, either felt or a suitable breathable membrane works. Breathable is the more forgiving, and for most re-roofs we now recommend it. For a loft conversion, an insulated sloping ceiling or a roof with limited ventilation, a breathable membrane of the correct type is the better choice, installed with the airflow that the manufacturer specifies. For a low-cost outbuilding or a roof where the budget rules, a reinforced felt may be the right answer. On a period roof with a lime-mortared or unlined underside, the underlay choice interacts with how the roof breathes, and advice from the council's conservation team may apply.
How the underlay is laid
However good the material, installation decides how well it works. The membrane is rolled out horizontally from the eaves upward, with each course overlapping the one below, at the overlap the manufacturer specifies. It is draped slightly between rafters so water runs down into the gutter and does not pond. At the eaves it is dressed over a support tray or an eaves carrier so that water drains into the gutter and does not back up under the first course of tiles. At hips, ridges and valleys, special laps and pieces are used. Battens are nailed through it, and the nail holes are small enough to seal around the fixings. Counter-battens may be used on warm roofs. The tiles then go on, held by the battens, which also create the ventilated space above the membrane.
Errors are common. Overlaps may be too small, the membrane may be stretched tight, gaps can be left at the eaves, or it may be laid the wrong way up, with the printed side down. Membranes cut round pipes and chimneys need careful lapping and sealing. Our overview of roof timbers explained shows the structure that the underlay sits on.
Eaves, ridge and the airflow path
The underlay does not work alone. Air needs a path to move through the roof space, entering at the eaves and leaving at the ridge, and the underlay has to be arranged so it does not block that path. At the eaves, an over-fascia vent or a soffit vent lets air in; a tray or support keeps the underlay lifted so it does not sag into the opening. Insulation must stop short of the eaves so it does not smother the vent. At the ridge, a vented ridge system or ridge ventilator lets air out, with the underlay dressed beneath it. If the roof has a breathable membrane that allows vapour out through the whole surface, the airflow path matters less, but it still helps with batten-space drying. Whichever route is used, the design should come from the manufacturer's guidance and the standard, not from guesswork on site.
What this means for a re-roof
When a roof is re-covered, the underlay should be renewed. It is cheap compared with the tiles, and the roof cannot be inspected again for decades once the covering is back. We recommend a breathable membrane of the correct type for most houses, and we tell you which we are using and why. If your roof has no underlay, which is true of some older houses, the re-roof is the chance to fit one. If it has old felt that has perished, renewing it removes a common source of leaks and condensation. For the full service, see our pitched roofing page.
How we assess your roof
From inside the loft, the underlay shows its condition: torn, sagging, missing or stained. We check ventilation at the eaves and ridge, and note any signs of condensation. The report includes pictures taken in your loft and a written quote at a fixed price that names the membrane. A 10-year workmanship guarantee is set out in the paperwork. If you want the underlay in your loft looked at, the number is 01256 213956 and a WhatsApp photo of the felt will do to start.
