The short answer
Torching is a layer of lime mortar, often mixed with hair, pressed into the gaps under slates from the loft side. On older roofs with no felt or boards it stops wind, driven rain and snow blowing up between the slates. It cracks and falls away with age. Where it is loose, a roofer can renew it in lime, or replace it with underlay and battens if the roof is being stripped.
What torching actually is
Before felt underlay became standard, many pitched roofs were battened and slated with nothing beneath. Slates overlap, but they do not seal, and wind can drive rain or fine snow up through the joints. Torching was the answer: a lime mortar, usually with animal hair mixed in to bind it, pushed in from inside the loft and smoothed against the underside of the slates and battens.
You will see it as a grey or off-white layer lining the underside of the roof. It covers the gaps between slates, fills around the battens and cuts down draughts. It is not decoration, and it is not a sign that someone cut corners. It is how the roof was built to stay weatherproof.
Plenty of Victorian and earlier houses around Swindon still have it, particularly where the roof has never been stripped. In an old farmhouse or a village cottage it can be the layer that has kept the loft dry for generations, and removing it carelessly strips away part of the roof's original build. On those houses the mortar and the slates have aged together, and the roof still works as a single unit.
How to tell torching from something else
Take a lamp into the loft and study the roof from beneath, moving slowly along each rafter bay. Torching is hard, crumbly, pale and rough. A modern membrane is a flexible sheet, usually black or silver, stapled across the rafters. Boards laid over the rafters are timber. Our page on sarking boards explains how those differ, because boards leave a very different pattern of light and shadow in a loft.
Daylight showing between the slates, or a heap of grit and fragments lying on the ceiling joists, tells you the torching is failing. A handful of crumbs is not alarming, but whole slabs of mortar on the floor suggest large gaps above. Please do not scrape the rest away to see what is behind it, since that leaves the roof less weatherproof, not more.
Photograph what you find, with the torch held to the side so the texture shows, and send it to us. A handful of loft photographs shows how the roof was put together.
Why torching cracks and falls
Lime mortar is brittle, and it is also slightly porous, so it breathes with the building. Roofs flex with temperature and wind, and over decades the bond between mortar and slate or timber loosens. Damp from leaks makes it worse, and so do vibrations from nearby traffic or building work. Hair mixed into the mortar can rot if it stays wet.
Birds and insects sometimes pick at it too. Once a piece falls, wind comes straight through the gap and the neighbouring piece loses its support. Many old roofs have patches of sound torching beside patches that have gone entirely, which is quite normal. What matters is whether the failure is limited, which a repair can manage, or widespread, which suggests the whole roof needs attention.
Loose torching rarely causes a dramatic leak by itself. A roof can lose a good deal of it and still shed rain, so the problem tends to be gradual rather than sudden. It lets in draughts and fine spray first, and you may first notice grit on the loft floor or a cold patch under the roof.
Repair, renew or strip and re-slate
Where the loss is local, perhaps one rafter bay above a landing or a bedroom, the usual repair is to chip back loose mortar and re-torch the area with a lime mix, working from the loft. Lime suits the job because it moves with the roof, where a hard cement mix would crack off again. On an older house, repairing in lime also keeps the character of the building.
When the slates are due to come off for another reason, such as nail sickness or leadwork, there is a choice. The roof can be stripped, the battens renewed, a breathable underlay laid and the slates put back, which removes the need for torching. Alternatively the torching can be put back after the slates are relaid. Our page on slate hanging covers how wall slating is fixed, which is a different job but sometimes found on the same house.
Solar panels, maintenance and what to ask
If you are thinking about panels, the roof will be disturbed, so check the torching beforehand. Our guide to solar panels on slate explains what fixing to a slate roof involves, and torching that is already loose may fall when brackets go in.
Regular upkeep helps. Our slate roof maintenance page lists what a yearly look should cover, from slipped slates to blocked gutters. A loft inspection every few years for fallen mortar costs nothing. Check at the same time that nothing stored in the loft is resting against the underside of the roof, because boxes pushed up against the slates can knock torching loose and hide the damage from view.
When we survey a roof with torching, we record the roof from inside the loft as well as the outside, and the written quote says whether we propose a lime repair or a fuller job. Our advice is the smallest job that genuinely mends the fault, and workmanship comes with a 10-year guarantee in writing. Please do not seal the underside with expanding foam, which hides faults and causes trouble of its own. See slate roofing for the wider service.