The short answer
Old stone slate roofs were usually sealed from underneath with torching, a lime mortar pushed into the gaps between slates and battens. A breathable underlay is a modern alternative, and some relays use both. Which suits your roof depends on the building, its age, whether it is listed and how the loft is used.
How a stone roof stayed dry before underlay existed
A traditional stone slate hangs from its batten on an oak peg. Big stones sit low on the slope and tiny ones high up, which gives the roof its familiar graded look. No sheet lies beneath them. Instead the underside was sealed by torching: a lime mortar, often mixed with animal hair, pushed in from inside the loft so that it fills every opening where slate meets batten.
Torching does a different job from a modern underlay. It stops wind-driven rain and fine snow from blowing up between the slates, and it steadies each slate so that the pegs are not worked loose by gusts. Because lime is vapour-open, it lets moisture pass in both directions. That is why many old stone roofs stayed sound for generations with nothing more than mortar under them.
When a breathable underlay makes sense
A stone roof that is being stripped and relaid gives a one-off chance to decide. A vapour-permeable underlay, the sort sold as a breather membrane, offers a second line of defence if a slate cracks or a peg lets go. It also keeps the loft cleaner, because no mortar crumbles down from the underside when the building moves.
The catch is weight and fit. Stone slates are thick and uneven, so the membrane must be laid with enough slack to droop between battens without being pinched by the slate above. Fixed too tight, it can hold water against timber. Fixed with care, it works quietly for years. Before recommending one we look at the battens and the rafters, and a stone roof relay often needs new battens at the same time.
Signs that an existing lining is failing are easy to spot from inside. Look for daylight at the slate joints, flakes of old mortar on the loft floor and fine spray marks on timber after a gale. A brown tide mark on a rafter, or damp insulation under a single course, suggests water is getting past the stone. None of this means the whole roof is finished, because patches of torching can be renewed and a failed strip of membrane can be cut in.
Torching, underlay or both?
Many conservation-minded relays keep the lime, because the aim on an historic building is usually to keep the roof working as it was built. On a newer stone-slate roof, or on an extension built to match an older house, an underlay alone is common and often perfectly sound. Some roofs end up with both, with new torching applied after a membrane is laid, though that is a case-by-case judgement.
Where the loft has become a bedroom or study, the choice also touches ventilation and insulation. BS 5250 covers how roof spaces are ventilated, and the type of membrane decides how much airflow is needed at the eaves. A breather membrane over a well-sealed ceiling can need less ventilation than a non-breathable one. That is a design decision for the whole roof, which is one reason we go into the loft and not just up a ladder.
If you cannot tell what your roof has now, a torchlit photograph taken inside the loft tells us a great deal.
Listed buildings and conservation areas
If the house is listed, changing what lies beneath the stone can count as an alteration that affects its character, and listed building consent may be needed. Even in a conservation area, a council officer may have views on the roof. Before any strip begins, check with the planning team at Cotswold District Council or Swindon Borough Council, depending on the address.
Work that restores the existing appearance seldom calls for planning permission, though the phrase exactly as before carries weight. Swapping lime for a membrane alters a hidden detail rather than the visible finish, but a listed building officer may still want to hear about it first. We tell you which questions the council tends to raise, and we set out the answers plainly next to the quote. If the property sits in the North Wessex Downs or the Cotswolds, the same checks apply, and the decision rests with the council.
What shapes the cost of lining a stone roof
Torching and underlay are priced on survey as part of the whole job, because the area and condition vary from roof to roof. What shifts the price is more practical. Lime torching is skilled, slow work done from the loft, and the hours depend on the area and how much old torching can stay. An underlay is quicker to lay but needs new or checked battens, and any rot found in the rafters has to be dealt with first.
Stone is very heavy, about 1000 kg per 10 square metres on traditional guidance, so the structure needs checking whichever lining you choose. Our page on stone roof structure covers what we look for. The survey is followed by a quote on paper, illustrated with shots of your slopes and loft, so you know the plan before committing to anything.
What to ask before the slates come off
Ask what lining the roof has today, and whether the plan is to keep it, repair it or replace it. Ask who will lay the slates back, and whether oak pegs and graded courses will go back as they were. Our overview of Cotswold stone roofing sets out our approach, and the page on stone ridge tiles covers the top of the roof, where lining and ridge stone meet.
If you are still weighing stone against another covering, the comparison of stone and Welsh slate is worth reading before you decide what goes over whichever lining you choose.
Finally, ask for the plan to be written down. A clear description of lining, battens, fixings and finish stops confusion later, and it lets you compare any two proposals fairly.