What torching is and where you see it
Shine a torch at the underside of an older tiled roof from the hatch and you may see a rough grey-white layer pressed against the back of the tiles. That layer is torching. It fills the gaps between the tiles and the battens or laths they hang from, running across the slope in line with each course.
The mortar is applied from inside the roof space, with the tiles already in place. Because it sits underneath, nothing about it shows from the garden, and a roof can lose most of its torching without any change on the outside.
Torching is not what keeps rain out. The tiles do that by shedding water, one course over the next. The torching stops fine, wind-driven rain and snow from being forced up through the joints between tiles, and it cuts draughts. On a roof built without an underlay, it did that job alone.
It is a general feature of older tiled and slated roofs, not of one type of tile, and the same word is used wherever the method was used.
When it fails and what you notice
Mortar ages. Dry roof spaces, movement as the roof heats and cools, and small leaks that soak it and let it dry again all make it crack and loosen. Eventually pieces drop, and the first sign is often grit or white dust lying on the ceiling boards, on stored boxes or on the loft floor.
This is what a roofer looks for when torching is suspected:
- Loose crumbs and dust in the loft, or lumps lying between the joists.
- Daylight showing through the roof where torching has gone from between tiles.
- Powdery snow or fine rain finding a way in after a storm.
- Tiles that rattle, since nothing now wedges them against the battens.
Dust landing in a thin line along one ceiling edge often marks the slope it is falling from, which helps the survey. Loss of torching is not an emergency by itself, and a roof can serve for years with patchy torching. It becomes a reason to act when it comes with slipped tiles, damp patches or draughts.
Replacing it, and the ventilation question
When a roof is re-covered, the old torching comes away with the tiles and is not usually reinstated. A modern re-roof fits an underlay over the rafters, under the battens, and that layer takes over the job of stopping fine rain and snow. The roof is then built to BS 5534 practice rather than relying on the mortar.
Where a roof is repaired rather than stripped, the torching can be renewed in patches from inside the loft, in a mortar suited to the building. A listed house brings the mortar mix and the method into the consent discussion, and the page on listed house tile repairs covers it.
Ventilation is the thing to hold in mind. An old roof breathes through its gaps. Seal those gaps completely, or press insulation hard against the underside, and moisture from the house can collect in the roof space. BS 5250:2021 is the current moisture standard for buildings, and under Approved Document C a cold deck roof is ventilated to meet the requirement. A roofer works out the airflow before the torching or the underlay goes in. A re-roof is the easiest moment to settle ventilation, because the battens are off and the eaves are open.
Underlay as the modern substitute
The modern way to do what torching did is the underlay: a sheet fixed over the rafters before the battens go on, which catches whatever the tiles let through and sheds it to the gutter. Two kinds are in use. One is a breathable membrane, which lets water vapour from the house pass out through it. The other, the older kind, called HR underlay, is not breathable and needs ventilation at the eaves to carry moisture away.
Marley's eaves opening for a cold roof with this underlay is 25mm along the eaves where the pitch is 15° or less, and 10mm where it is steeper.
If you are asking about torching because someone has found it failing, ask what is happening on top of the rafters and at the eaves. The answer should cover the underlay, the battens and the ventilation, not the mortar alone. Either way, the roof timbers should be dry and the airflow clear before the covering goes back.
For the layers around it, see roof underlay and battens and the eaves ventilation gap. The whole service is on the page for pitched roofing.
