torch

To torch is to fill and finish the joints between slates on a lath-and-plaster roof from above, working lime-hair mortar into the gaps to create a weathertight seal and smooth surface. The work happens on the underside of the slate layer, between the individual slate pieces where they rest on wooden laths. This is distinct from pointing done on exposed brickwork; torching addresses the hidden interface between overlapping roof slates and their supporting structure.

The mortar used is a coarse lime mix, typically lime putty combined with animal hair (usually cow or goat hair chopped to 25 to 50mm lengths) to reduce cracking as it sets. The hair acts as a reinforcing fiber and delays shrinkage enough to prevent the mortar from separating from the slate. A slater working from inside the roof space, in the attic or void between rafters and ceiling, pushes the mortar up into the joints using a hawk and trowel, working methodically across the underside of the slate field.

Torching is performed only on traditional slate roofs laid on timber laths with no sarking board or underlay. The practice dates back centuries in Britain and Northern Europe; it prevents wind-driven rain from entering the roof void by blocking the gaps that would otherwise exist between overlapping slates. On modern roofs with breathable membranes or solid boarding, this technique is unnecessary and would trap moisture rather than shed it.

The condition of torching is a critical indicator of roof age and durability. Hair mortar can last 80 to 120 years if well-laid and protected from freeze-thaw cycles, but once it begins to fail, erosion accelerates rapidly. Loose or missing mortar allows water and wind penetration; some re-torching work on period roofs involves carefully removing failed mortar and re-filling with matching lime mortar composition. This is skilled, time-consuming work and one reason why older slate roofs command high maintenance costs.

The term torch derives from the Old French torche, meaning to twist or turn, though the exact semantic path to roof-filling work is uncertain. It may relate to the twisting motion of applying mortar or to an older sense of rough plastering. In modern trade speech, torching has largely fallen out of use except among heritage roofers and conservation specialists, replaced by more generic terms like pointing or bedding.