Question answered
What is torching on a slate roof?
The short answer
Torching is a layer of lime mortar, traditionally mixed with animal hair, pushed in from the loft side between the slates and the battens. It was used on roofs built without an underlay. It helps stop wind-blown rain and snow reaching the loft and helps hold the slates in place. It cracks and drops away with age, and loose torching on its own does not prove a roof is leaking.
Torching is the grey lime layer you may see under old slates when you look into the loft. It was the original seal against wind-blown rain on roofs built with no underlay, and it still matters.
Where torching sits and what it does
A slate roof keeps water out by overlap, but the overlaps are never perfectly tight. Old roofs were built with no underlay beneath the slates, the sheet that now catches whatever gets past them. Instead the slates hang from battens, the timber laths fixed across the rafters, and the gaps are sealed from the inside.
Torching is that seal. A tradesman in the loft presses lime mortar, usually mixed with animal hair for strength, into the joint where each slate meets the batten beneath it. The result is a rough grey crust on the underside of the roof. It cuts the draught, keeps fine spray and snow out in a gale, and grips the slate lightly to the timber so it rattles less.
Roofs with this layer are often called unfelted. The word comes from roofing felt, the old name for underlay, and tells you the roof depends on its slates and its lime rather than a membrane.
Why torching fails
Lime is soft and breathable, which suited timber that moves with the weather. It is not permanent. Over decades it dries, cracks and loses its grip, and the roof flexes a little in every gale. Pieces fall as crumbs and dust onto the loft floor or the insulation below.
Moisture speeds that up. If water reaches the torching through a slipped slate or a failing nail, the lime softens, and the batten behind it can rot. A dripping roof can therefore have two problems at once: the slate that let water in, and the torching that was damaged by it. Our page on why slate roofs leak covers the first.
Repairs done with the wrong material add to the trouble. Cement mortar is hard and does not move as lime does, so it cracks away from the slate and can hold water against the timber. Lime is the right material wherever torching is being repaired.
What loose torching tells you
With a torch in the loft, look at the underside of the slates. Grey crumbs on the insulation or the boards mean the lime is shedding. Daylight showing through is common on old roofs and says little about a fault, though it tells you a little rain and a lot of draught can get in.
Loose torching alone does not mean a leak. The roof may have stayed dry for a century with patchy lime. What matters is whether there are stains, damp patches on timber or slipped slates alongside it. Keep your feet on the loft boards and off the roof and ladders; the view from inside is enough to describe the problem to us.
We trace the cause from inside first, then check the slope above. That is also how we judge whether failed torching is simply old age or a sign that nails are failing, which is covered in more detail under nail sickness.
Renew it, leave it, or add an underlay
Sound torching can stay. Patches that have fallen can be re-torched with lime in place, which is a modest job and suits heritage roofs that must be repaired like for like. On a listed building, the national park authority's position is that small in-situ repairs replicating the original need no consent, though altering historic fabric does.
Where the slates are coming off anyway, a different route opens up. A roof being stripped for re-slating loses its old torching as the slates are lifted, and it is normally relaid on new battens with a breathable underlay under the slates. That gives the roof a second line of defence it never had. On a listed building the underlay choice is agreed with the conservation officer first.
Foam is not a substitute. The Energy Saving Trust does not recommend spraying foam beneath a roof that is damaged or leaking before the fault is cured, and foam over torching hides the very layer you need to inspect. The page on spray foam on a slate roof explains the risks.
Where torching fits in the bigger picture
Torching is one part of the old system, along with the ridge. A slate ridge was bedded in mortar and is now fixed mechanically; see ridge tiles on a slate roof. If torching has failed across the roof and the battens are poor, the question becomes repair or re-slate rather than patch.
On cost, the sums are the same as for any slate work. MyBuilder's guide of 21 August 2026 gives a typical UK range of £100 to £180 per m² for a slate roof, and slate against tile cost compares it with other coverings. Local re-torching is priced on survey, and you receive pictures of the underside along with a written quote for one overall sum. For the whole subject, start at slate roofing, where repair, re-slating and maintenance are all set out.
Torching: two more questions
Is torching the same as pointing?
No. Pointing is mortar on the outside of the roof, at ridges, verges and flashings. Torching is lime on the underside of the slates, where it cannot be seen from the street. Both are lime on a traditional roof, but they do different jobs and fail in different ways.
Do I need to replace torching if the slates are re-laid?
Not always. When slates are lifted the old lime usually breaks away, so most re-slating adds battens and an underlay in its place. A listed or conservation-area roof may be re-torched instead, which is agreed with the council's conservation team before the work starts.
Tell us about your roof
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