Look at a slate roof from the street and you see slate. Look at one from the loft and you see something quite different: rafters, battens and, on many roofs, a dark membrane stretched between them. On older houses you may see no membrane at all, only the backs of the slates and daylight at the joints. Between those two pictures lies a long list of choices that decide how well a roof copes with wind-driven rain, fine snow and the damp air of a lived-in house.
Underlay is the sheet fixed across the rafters before the battens are nailed on. Sarking is a trade word with two meanings: boards fixed over the rafters to give the slates a solid deck, or the membrane laid over those boards. Both exist to catch what the slates let past, and both change how the roof breathes.
This guide covers the types of membrane, what boarding adds and takes away, the ventilation figures from BS 5250, what an older unlined roof is telling you, and the points to put in front of a roofer before a quote is accepted.
What lies between the slates and the rafters
A slate roof is built in layers. At the base sit the rafters, the angled timbers carrying the lot. Over them goes the underlay, then the battens, the narrow strips of timber the slates hang from, and finally the slates themselves, each lapping the course below. The slates shed nearly all the water. The layer beneath handles the rest: the rain that is blown upward, the fine snow that gets through a joint, and the condensation that forms on cold surfaces inside the roof.
On a roof with sarking boards there is one more layer. Boards run across the rafters, edge to edge, to form a continuous deck. The membrane, if used, goes on top of the boards, and the battens and slates follow. Boarding is more familiar in some parts of the country than others, and it is a choice, not a requirement. Plenty of sound slate roofs have none.
Why older roofs have no underlay
Many Victorian and earlier slate roofs were laid straight onto battens with no membrane. Some carry torching, a lime mortar rendered on the underside of the slates to seal gaps. Others simply rely on tight laps. The method worked because the loft above was cold, draughty and well ventilated by the gaps themselves. Houses were not sealed tight, and the roof dried out through the same gaps that let a little weather in.
That changes when the house changes. Double glazing, draught-proofing, loft insulation and a bathroom fan all add moisture and take away leakage, so a roof that coped for a century can start showing damp. This is why the layers beneath a slate roof deserve thought whenever it is stripped, and why a re-slate is the natural moment to decide on them.
Membranes: the two families and what separates them
Underlays fall into two broad families, and the difference is how readily water vapour can pass through. Marley's guidance on BS 5250 uses the terms high-resistance and low-resistance, and they matter because they set how much ventilation the roof needs.
High-resistance underlay
Traditional bitumen felt belongs here. It holds back both liquid water and vapour. Used under slate, it needs a clear air path above it, because vapour that cannot go through the membrane has to leave the roof space by ventilation. Marley gives the figures for this type of underlay on a cold roof: a gap of 25 mm running the length of the eaves where the pitch is 15 degrees or under, falling to 10 mm above 15 degrees. Slate roofs sit above 15 degrees, so the 10 mm figure is the one that usually applies.
Where the pitch exceeds 35 degrees, or the span runs past 10 m, or the roof is a single slope, Marley adds a further continuous 5 mm of ventilation at high level. That catches many steep Victorian roofs and most lean-to slopes.
Low-resistance underlay
Low-resistance membranes, often called breathable, let vapour pass. Marley says a tight ceiling below and a covering that lets air through can make the small leakage in ordinary eaves construction, about 3 mm, enough by itself, but only where the third-party certification terms are met in full. Those conditions are specific. A membrane maker's certificate sets out what the roof must look like for the claim to hold, and the question to ask is whether yours meets it.
Neither family is better in itself. The right choice follows from the roof's ventilation, the ceiling below it and the manufacturer's certificate, which is why we survey before we specify.
Boarded roofs: what sarking adds and what it hides
Sarking boards do several jobs. They make a stiff, continuous deck that spreads loads and resists the push and pull of the wind. They give a second line of defence if a slate cracks or slips. They block the fine, driven snow that can find its way through gaps in an open-laid roof. And on a roof being rebuilt they give fixers a firm surface to work from.
The cost is not only money. Boards reduce the ventilation paths the roof had before. If the old roof breathed through open joints between the slates and the battens, laying boards and a high-resistance membrane over them shuts those routes, and the damp air has nowhere to go but out through whatever ventilation is designed in. A common arrangement is boards, then a membrane, then counter-battens running up the slope and battens across them, so air can move in the gap above the membrane. The exact build-up depends on the membrane maker's instructions.
The hidden-leak problem
Boarding also hides things. On an open roof, a leak shows from the loft: you can see daylight, wet timber or a drip. Under boards, water can run along the deck and travel some distance before it is seen. The wet may reach the timber without showing. For that reason a boarded roof benefits from a membrane that is detailed carefully at every valley, abutment and penetration, and from inspection after storms.
Weight and cost
Boards add materials and labour, and a little weight to the structure. They are not a reason to change the roof covering, but they belong in the price. A slate re-roof is typically £100 to £180 per m² in MyBuilder's guide (updated 21/08/2026), and where boarding is included it is one factor behind the position within or beyond that range. What a boarded roof comes to depends on access, the roof's complexity and what the survey finds.
Ventilation: where the damp air goes
Ventilation is where underlay choices stand or fall. Warm, moist air from kitchens, bathrooms and the people in the house rises into the loft, meets cold slate and timber and condenses. A roof that cannot clear that moisture goes damp from the inside, and the signs look like a leak. Our guide on telling condensation from a leak sets out how to tell the two apart.
A cold roof, the usual arrangement on a slate house, has insulation at ceiling level and a ventilated space above. Air enters at the eaves and leaves at the ridge or through high-level vents. A warm roof puts the insulation at the rafters and works differently, so the figures above do not apply to it in the same way. Which one you have changes what a membrane can do.
Eaves, ridge and the insulation trap
The eaves gap named in BS 5250 only works if it stays clear. Insulation stuffed tight into the eaves, a blocked soffit vent or a fascia that has been fitted tight can shut it. Eaves vents and tray supports hold insulation back and keep the gap open. At the ridge, vents or ventilated ridge pieces provide the high-level path where one is needed. Slate roofs often have ridge tiles bedded and fixed mechanically, per BS 5534, and a roofer can choose a ventilated version.
A site on the Sussex coast brings its own considerations. Sea air is damp, and a wind-driven gale can force rain upward under the slates. A membrane should be lapped, taped where needed and carried properly into the gutter at the eaves. Near the airport at Shoreham and along the seafront, the exposure also calls for careful fixing of battens and slates, not just the membrane.
Spray foam in a slate roof is a different story: see our guide on spray foam on a slate roof.
Reading an unlined or ageing roof
Many of the slate roofs we are asked about have never had an underlay, or have a felt that perished long ago. The loft can tell you a good deal. Daylight showing at the joints means the roof is open-laid or torched in places. White dust or flakes of lime on the insulation can mean torching is breaking up. A felt that is brittle, torn or sagging between the rafters, or one that holds a pool of water, has finished its life.
Stains under the felt, mould along the timbers and soggy insulation suggest condensation, and rust marks on nails point to wet that has been sitting. A musty smell on a cold morning with no rain suggests the ventilation is poor. None of these needs to be a leak in the slates; the cause may sit entirely within the roof space.
What a sound repair looks like
For a few cracked or slipped slates the roof can be repaired without touching the layers below. Where the underlay has failed and the slates are sound, a roofer may strip and relay: lift the slates and battens, fit the membrane, re-batten and relay the slates, with new ones to make up any losses. That is the point at which the choice of membrane and the decision about boards are made. Our guide to replacing slate with tiles covers the case where the answer is a different covering altogether.
Where the roof has been stripped back to the structure, Approved Document L requires the whole thermal element to be upgraded once more than 50% of its surface is renovated. Stripping back to the rafters is a renovation of this kind, so insulation may come into the job whatever the membrane.
Do not walk on an old slate roof to check it; see walking on slate.
Detailing, fixings and the questions to put on the quote
The best membrane fails if the detailing is poor, and most underlay faults happen at the edges. The key points are straightforward. Laps should run with the fall, with each higher sheet over the lower, and each lap kept as the maker specifies. The membrane should be supported at the eaves so it drains into the gutter, not behind it, and it should not sag between rafters to form a trough. Around chimneys, valleys and roof windows the membrane needs to be dressed into the flashings, with lead lined and welted as the detail requires.
Battens matter too. They hold the slates at the right gauge, and where they are fixed through the membrane the nail holes close around the shank, which is why nailing through a counter-batten is a common way of keeping the membrane watertight. Timber should be treated and nails of a type suited to the exposure. We choose fixings on survey and do not assume what the last roofer used.
Questions to ask
- Which type of underlay is proposed, and does the maker's certificate cover this roof?
- Is the roof to be boarded, and if so, how is air to move above the membrane?
- What eaves and ridge ventilation will the finished roof have, and is the insulation kept clear of it?
- How will the membrane be dressed at chimneys, valleys and the eaves?
- Does the price include VAT, scaffolding and taking the old material away?
The written quote comes after the survey, with your own roof photographed and a single total at the bottom. If the membrane is the weak point, you will see that in the pictures. For how the options sit together in money terms, see the guide to slate roof cost, and for the wider picture start at the slate roofing page.
