Lime mortar, torching and pointing on stone roofs

People use one word for several different pieces of work, and the confusion can lead to a quote that covers the wrong thing. On a stone roof there are three separate lime jobs.

Torching

Torching is applied from the inside. The roofer works from the loft and presses mortar into the gaps between the undersides of the slates, filling the space where one course meets the next and where the slates bear on the battens. It is a draught-proofing and stabilising layer. It is not the waterproofing: the stone sheds the water, and the torching deals with whatever the wind pushes past.

Pointing

Pointing is applied from the outside, where mortar is raked out and refilled between stones. On a roof this means the joints of stone ridge pieces and the edges of a verge, the angled edge at the end of a gable.

Bedding

Bedding is the mortar a ridge stone or hip stone sits in. It is the heaviest mortar job on the roof because it carries weight and takes the weather. Since the 2014 revision of BS 5534, ridge and hip pieces also take mechanical fixings, so the lime acts as the seating and weather seal.

A stone slate roof has no sheet underlay in its traditional form. The slates are hung on battens and lap over one another, and the roof relies on the stone for rain and on the lime for everything else. Whether a roof should carry a membrane as well has its own answer: should a stone roof have underlay?

It holds the slates still

Pegged slates hang by gravity. Wind can lift the tail of a slate and let it slap back on the batten, and over decades that movement wears peg holes and cracks stone. A bed of mortar behind the slates cushions them against the batten and ties each course to the next, so the whole slope behaves more like one skin than a pile of loose pieces.

It keeps the weather out of the loft

In a gale, fine rain and powdery snow are driven up between the courses. Torching fills those routes. Roofs without it often show the same pattern: dust on the loft floor under the eaves in summer, and damp patches in winter at the foot of the rafters.

It breathes

Lime mortar lets moisture pass through it as vapour. That matters because the loft of an old house is not sealed, and a stone roof needs air movement to dry out. A hard cement layer would shut that route and trap damp against the timbers.

Inside the loft, with a torch in hand, we note whether the torching survives, whether it has crumbled, and whether someone has swapped it at some point for something harder than the stone around it.

Lime mortar is lime and sand, mixed with water. Beyond that, the choices change the result, and the right mix on a roof depends on what the existing mortar is made of and how exposed the work will be.

Lime putty or hydraulic lime

Lime putty is slaked lime stored wet, and it sets slowly by taking carbon dioxide from the air, a process called carbonation. It is soft, flexible and very forgiving of movement, but it needs time and protection while it cures. Hydraulic lime contains compounds that let it set partly by reacting with water, so it hardens faster and tolerates damper conditions. Hydraulic limes are graded by strength, and a stronger grade is not automatically a better one for a roof: the mortar should be weaker than the stone around it, so that any movement shows up in the mortar, not in the slates.

The sand

Sand gives the mortar body and decides its colour and texture. A well-matched sand makes a new repair sit quietly beside old work. Sharp, well-graded sand binds better than rounded sand, and washed sand avoids bringing salts and clay onto a roof where they would do harm. On visible work such as ridges and verges, we test a small sample against the existing joint before committing to a full mix.

Hair and other additions

Traditional torching mixes carried animal hair, which helped the mortar hold together when pressed into awkward gaps. Some modern specifications use fibres for the same purpose. Whatever goes in, the aim is a mix that grips the stone, flexes slightly and does not outlast the slates it protects.

Proportions of lime to sand vary with the lime, the sand and the job, so we do not quote a standard ratio. We take a sample of the existing mortar where we can, and match to that.

Cement is harder, stronger and less permeable than lime. On a brick wall that may be tolerable; on a Cotswold stone roof it is a common source of damage, for three reasons.

It is harder than the stone

A stone slate is a relatively soft limestone that moves a little with temperature and moisture. A rigid cement bed cannot move with it. The stress goes into the stone instead, and slates crack along their beds or around the peg hole. The mortar stays whole while the stone fails around it.

It holds water in

Cement mortar is far less permeable than lime. Water that gets behind it cannot dry out through the joint, so it stays against the stone and the timbers. Over a few winters that wetness can feed rot in battens and rafter ends, and freezing in the damp stone lifts the face off the slate.

It is hard to undo

Cement bonds to stone so strongly that removing it often takes the surface of the slate with it. A roof that has been repointed in cement is usually far more work to put right than one that has been left alone. We rake out cement carefully by hand where we can, and we accept that some slates will not survive the process.

On a listed house, replacing a lime repair with cement, or the reverse, can count as an alteration affecting the building's character. Ask the council's conservation officers before any work begins, and read our guide to listed stone roofs.

Good lime work is slower than it looks. The steps are simple, but each one affects how long the finished repair lasts.

Preparing the joint

For pointing, the old mortar is raked out to a sound depth, loose material is brushed away and the joint is dampened. Lime mortar bonds badly to a dry, dusty surface, because the dry stone sucks water from the mortar before it can set. For torching, the loft side of the slates is cleaned of loose debris and old crumbled mortar, and the surfaces are lightly wetted for the same reason.

Filling

Mortar is pushed in firmly and in layers where the joint is deep, so that each layer can take up the shrinkage of the one beneath. Torching is pressed upwards into the gaps between slates and battens and struck off so that it does not smear across the peg holes. The pegs must be left clear and their holes unblocked, since the slates still depend on them to hang correctly.

Finishing

Pointing is finished slightly below the face of the stone, not proud of it, so that water runs off the stone instead of pooling on a ledge of mortar. Overfilled, smeared joints look neat for a week and then crack. After the mortar firms, the surface is brushed to expose the sand and match the old work.

Looking after it while it cures

Fresh lime needs shelter from strong sun, drying wind, heavy rain and frost until it has taken up. On an open roof that means damp hessian or sheeting, and sometimes working one section at a time. The cold-weather side of this is covered in our guide to stone roofs in a Stroud winter.

Outside torching, the places lime mortar is most visible on a stone roof are the ridge, the verges and the hips. Cotswold District Council's technical guidance describes the traditional ridge as a plain angled piece of dressed stone, each one laid in a mortar bed along the apex.

Re-bedding a ridge

When ridge mortar has perished, the joints open and rain runs in beneath the stone. The usual repair lifts each ridge piece, cleans out the old bed, relays the stone on fresh lime with a mechanical fixing alongside, and points the joints. The stone is numbered or sketched before it comes off so that every piece returns to its place. For the stone itself, see our guide to stone ridge tiles.

Verges

At a gable the slates can be bedded on a mortar course set on the wall head, or left as a plain overhang. Mortar on a verge takes the worst of the weather because it faces up into the wind. It is the joint most likely to need attention, and it should not be mistaken for a leak at the slates beyond it.

Cost context

For scale, MyBuilder's 2026 slate roof guide gives £300 to £600 for new or re-set ridge tiles and £200 to £400 for repointing a ridge or hip. These are national figures for tile and slate, not stone. Yours comes once we have surveyed, covering scaffold, materials, lime work and labour.

Lime failure is rarely the whole story. Crumbled torching and open ridge joints are signs that tell us where to look next, and they are often the first evidence of a larger fault.

Reading the underside

From inside the roof, we look at the torching along the full slope, at the battens and rafters beside any damp patch, and at daylight showing between slates. Where torching has fallen away in one area only, the cause is usually a local leak or a slipped slate nearby, and a patch repair makes sense. Where it has gone across a whole slope, the slates are likely to be loose too, and that points toward stripping and re-laying. Our guide to re-laying a Cotswold stone roof describes that job.

When to leave it alone

Old torching that is intact, even if rough and discoloured, is doing its job. There is no benefit in removing sound lime and replacing it for the sake of tidiness, and disturbing it can loosen the slates it holds. Our advice is to repair what has failed and leave what works.

Paperwork

Lime repairs on a small part of the roof normally sit inside the Planning Portal thresholds (less than 25 per cent of the building envelope and less than 50 per cent of the roof), so Building Regulations approval is not usually needed. Like-for-like work that leaves the appearance unchanged is not normally development; a listed home, or a conservation area with an Article 4 direction, is the exception to check. Where anything is notifiable, we handle it. The workmanship guarantee runs for 10 years, in writing.