The short answer
Sarking boards are planks or sheets laid across the rafters, with the battens and slates fixed on top. They form a second layer under the slates, give the covering a firm base and stop wind-blown rain reaching the loft. Where they are sound, they make repairs simpler. Where they have rotted, they add timber work to the job.
What sarking is and where the word comes from
Sarking is a trade word for boarding that sits directly on top of the rafters. The slates are hung from battens, and the battens are nailed over the boards, usually with a layer of felt or membrane between the two. The idea is old: a continuous timber skin gives a slate roof a second line of defence behind the slates themselves, which are only ever overlapped, never sealed.
The word is common in Scotland and the north, where driving rain and exposed slopes made boarding standard on better buildings. In southern England it turns up less often, but it does appear, especially on larger Victorian villas, chapels, schools and any roof where the original builder wanted extra protection or a flat surface under thin slates.
How to tell whether your slate roof has boards
The loft is the easiest place to look. If you can see the undersides of the slates and daylight between the battens, there is no boarding. If you see a continuous surface of planks running up the slope with the rafters visible beneath them, that is sarking. Some boards are tongued and grooved, others are plain square-edged planks with small gaps, and newer work may use plywood or oriented strand board.
From outside, the clues are weaker. A roof with boards often sits slightly higher at the eaves because the build-up is thicker, and the slates tend to lie flatter. A surveyor can also tap the slope or look under a ridge tile or at the verge. Please do not go on a slate roof to find out for yourself, because slates break underfoot and the roof is slippery when damp.
Our survey photographs will show the underside of the covering wherever we can get safe access to it, so you see what is there before any price is agreed.
How boarding changes the way slates are repaired
Boards change the repair method more than most owners expect. On an open-battened roof, a slipped slate can be fixed with a metal tingle, a small strip that hooks behind the slate above. Where the slates are nailed to battens over boards, the nails have a solid timber base on one side of the batten but the underside is hidden, so inspecting the fixings is harder. Our page on why a slate roof leaks explains how nail fatigue starts, and boarding can hide the early signs.
On the other hand, boards are a benefit when a leak happens. Water that gets past the slates usually runs down the felt and drains at the eaves rather than falling straight into the loft. Damp may sit unseen for a whole season.
When we lift slates on a boarded roof, we check the timber under the problem area as well as the slate above it. Water tracks, and the damage is often a metre or more from the visible fault.
Rot, felt and what to expect when the slates come off
Timber that stays damp rots. On a boarded roof, the worst spots are valleys, the lower courses at the eaves, around chimneys and behind gutters that have overflowed. A rotten board loses its grip on the batten nails, so a patch can sag and the slates above it slip, even though the slates are in good condition.
Old roofs usually have bitumen felt, if they have any underlay at all, and the felt is often brittle or torn. When we strip a boarded slope, we can see exactly where the felt has failed and which boards have gone soft. Replacing a few boards is a modest job. Replacing a whole slope of boarding, with new membrane and battens, is a full re-roof and is priced as one.
Do not assume the boards are the problem if you see a stain. Often it is a flashing or a hairline crack in a single slate. The survey sorts out which, and the lightest fix that cures the cause is the one we price. The page on torching covers the older lime plaster layer you may find on unboarded roofs, which does a related job from the inside.
Ventilation, insulation and solar fixings
Boarding stops air moving through the roof the way it does in an open-battened one, which matters for condensation. A boarded roof needs a planned route for air, usually at the eaves and the ridge, otherwise warm moist air from the house settles on the timber. Marley's cold-roof guidance, based on BS 5250, sets out eaves ventilation of 25 mm for pitches of 15 degrees or less and 10 mm for steeper roofs with a non-breathable underlay, and your roofer should be working to the product's own instructions.
If you plan to add insulation between or over the rafters, boards limit where it can go. Foam sprayed beneath boarding is a particular worry, because it can hide rot, and as of November 2024 a number of lenders declined mortgages on foam-insulated homes while others judged each case. Put the question to your mortgage provider before agreeing to it.
Solar panels need rails fixed to the rafters, not the boards. On a boarded roof we find the rafters first, which is easy from the loft. Our guide to solar panels on slate goes through the fixing methods.
What it means for cost and the next step
Sarking does not make a roof dearer on its own, but it adds variables. National guides price slate re-roofing at £100 to £180 per square metre, according to MyBuilder's slate roof cost guide updated in August 2026, and that figure assumes ordinary preparation. Rotten boards, extra felt and new battens move you up the range. Scaffolding for a two-storey house, which MyBuilder puts at around £850 to £1,200, usually applies whether or not there is boarding.
Suspect boarding and slipped slates together? Send pictures taken in the loft and across the slope. We will say whether a patch cures the trouble or the boards mean stripping. Our reslating guide walks through a full job, and the slate roofing hub lists the rest of our slate work. Our survey ends in a written quote, and ten years of workmanship cover, in writing, stands behind the finished job.