What is torching on a stone roof?

An old stone roof has no underlay in the modern sense. The slabs overlap, but the joints are never perfectly tight, because the stone is thick and uneven. In a gale, rain and fine snow can be driven up through the gaps and into the loft. Torching is the traditional answer to that, and it was done with whatever the mason had to hand: lime, sand, and often hair from the farmyard.

Working from inside the roof, the mason presses lime mortar into the spaces between the underside of the slabs and the battens, forming a rough seal that sits below the covering. It does not carry water off, since that is the slabs' job. It is a second line of defence against driven rain and draught, and it also steadies the slabs against rattling in the wind. Our page on underlay below a stone roof covers the modern alternative that some roofs have been given.

From the loft hatch, torching looks like grey or off-white mortar daubed along the battens and between the slabs, often rough and thick in places. Some contains animal hair, a traditional binder that helps the mortar hang together. The mortar sits in the joints, so you can still see the underside of the slabs on either side.

Not every old stone roof has it, and some have since been lined with felt or boards, which hides what was there. Fine grit and dust on the loft floor or on the insulation is a good sign that torching above you is breaking down. It is no cause for alarm, but it is worth recording, because it means the seal is thinning. Take a picture of the underside by torchlight and send it to us. Without going up, we can often tell what the roof has, and whether it needs attention soon or can wait for the next planned repair.

Lime mortar is soft by design, and it ages. Decades of temperature swings, wind vibration and the occasional damp spell dry it out, crack it and let it fall. A roof can carry on shedding water perfectly well while its torching goes, as the slabs above are doing the real work, so crumbling mortar is seldom the first cause of a leak. What it does mean is that the roof has become draughtier and more open to driven rain than it used to be.

The telltale is gritty dust in the loft, sometimes with small pieces of mortar lying on the ceiling boards. Movement can also be behind it, since a slipped slab shaking in the wind can jar the mortar loose. See our answer on why stone slates slip if you spot both signs together. Falling torching in one corner of the loft can also point to a nearby slab that no longer sits snug.

Torching is renewed from inside the loft, usually while the roof is being repaired or re-laid, since the slabs are easiest to reach once lifted. The old loose mortar is brushed out, the battens and slab undersides are inspected, and fresh lime is pressed in by hand. Lime stays flexible and lets the roof breathe, which keeps the timbers drier. Our guide to lime mortar on old roofs explains the reasoning.

Cement mortar is the wrong material here. It is hard and brittle, it traps moisture against timber, and it can crack the stone as the roof moves. Lime is what we use, and the quote states it, with one total price confirmed after the survey. Where only a patch has fallen, the repair can be local and need not touch the rest of the loft. Dust sheets go down first, and the grit is cleared away when the work is done.

Some roofs have been sprayed on the underside with foam insulation, which buries the torching and the back of the slabs. The RICS consumer guide, from March 2023, cautions that badly fitted foam can lead to damp, and that stripping it out might damage the structure beneath. Mortgage lenders differ too. In November 2024 a conveyancing trade site reported that some would refuse a loan on a house with spray foam, and that others look at each case on its merits.

On a stone roof, foam also hides the pegs, battens and slabs that need inspecting, and the RICS guide says it must not go into a listed building without prior consent. Our spray foam insulation removal service covers assessing and removing it where needed. If you are unsure what lies under your slabs, ask us to look, because a quick view from the hatch settles most doubts, and for the wider picture of stone work see our Horsham stone roofing page.