The short answer
Sarking boards are timber boards laid across the rafters, the angled timbers of the roof frame, with the underlay and battens laid on top. Some slate roofs are built this way, and the boards then give the covering a solid base. Whether you need them depends on what the roof has now and what the survey finds: sound boards stay, rotten ones are replaced.
Boards under a roof are hidden until the covering comes off. That is when we can see whether they can be kept or must be renewed.
What sarking boards are and where they sit
Look into a loft and you see rafters running from ridge to eaves. On a boarded roof, planks lie across the rafters, tight together, forming a continuous timber deck. The underlay is laid over the boards, then the battens, then the slates or tiles. Roofers call the boards sarking.
How is a boarded roof different from a battened one?
On an unboarded roof, the battens, thin timber strips, are nailed straight to the rafters and the covering is hung from those. On a boarded roof the deck is a second layer of timber below the battens. It spreads loads, gives a firm surface for the underlay and adds a little more to the roof's make-up.
Both are used, and neither is better in every case. We find out which you have when the survey photos are taken from the loft, and those pictures also show the condition of the timber, so your price rests on what is really there, because the underside of the boards or the rafters is visible from inside. Our guide to rotten roof timbers shows what bad timber looks like, and our roof replacement page shows where the deck fits in a whole-roof job.
Do you need them under slate?
If your roof is boarded now, the boards are part of how it was built, and a new covering is normally laid over a sound deck. If it is not boarded, there is no rule that says it must be after a re-roof. The decision is a design one, made with the survey in front of us.
Should I add boards if I never had them?
Adding a deck changes the roof. It adds weight, because boards are heavy timber, and weight matters under the Building Regulations: the Planning Portal asks for approval and a structural check when a new covering weighs 15 per cent or more than the old. A deck also changes the way the roof ventilates, because the underlay now sits against solid boards. BS 5250 ventilation guidance for the underlay type then has to be followed, covered in our answer on roof vents on a new roof.
We would not add boards because it sounds better. We would add them if the rafters or the covering call for it, and we explain why in the written price.
Repair or replace: judging the boards
When the old slates are stripped, the boards are on view for the first time in decades. The boards are checked board by board, from above and from the loft.
How can you tell a bad board?
Dry, grey, slightly stained boards are often fine. Soft, crumbling, black or visibly rotten ones are not, and neither are boards that are split or have shrunk apart. Pressing a screwdriver into the timber tells us quickly: sound timber resists and bad timber gives. Beetle holes need judging by how widespread they are and whether the timber is still strong, and a few old flight holes in dry wood are not alarming on their own.
Where the damage is local, only those boards are cut out and new ones are fixed to the rafters, because replacing the lot when most are sound is waste. Where the damage is widespread, or the rafters beneath are affected, a larger section or the entire slope is re-boarded. Common causes are long-term leaks around chimneys and valleys, and condensation. We photograph what we find, show you the pictures and agree the extra before it is done, so there are no surprises in the final price.
What boards mean for the roof's fixing and fire safety
A solid deck gives nails and clips something to bite into, which helps slates sit flat. It also hides the rafters, so any later work in the loft is easier to plan if you keep photographs of the roof as it was opened up.
Does changing the boards need approval?
Replacing a few boards like for like is repair. Changing the make-up of the roof is different. The Planning Portal also asks for approval where a new covering would perform quite unlike the old one if fire broke out, so tell your building control body if the new roof will not match the old.
If you are not sure which type you have, a photo from the loft hatch of the underside is enough for us to start. We then come to the roof, survey it and show you your own photos. Somerset Building Control can be reached on 0300 303 7790 for any regulation question about your boards or the structure beneath.
Board repairs in brief
Can new boards be fixed over old ones?
Boards laid over a sound deck add weight and thickness for no good reason, and they trap the old timber. The better route is to renew only the boards that have failed and keep the sound ones. That keeps the weight where it was and lets us check the rafters beneath.
Will boards stop a slate roof leaking?
No. Boards give support, but the weather barrier is the covering and underlay that sit on top of them, plus the flashings at chimneys and valleys. If those fail, water reaches the boards and rots them. Fixing the leak path, whether a valley, a chimney flashing or worn underlay, is what protects the deck.
The full picture: Roof replacement and new roofs in Taunton ยท More short answers