In the days before roofing felt, torching or lime mortar was used on the underside of tiles or slates to keep them in place and to prevent strong winds from getting under the tiles and lifting them. Over the years this torching can crumble and break, normally falling with a thud on the floor of the roof space during the middle of the night! Whilst this may be alarming, unless there are subsequent leaks then missing torching is a little like torn roof felt – not a problem in itself but it may contribute to other problems with a roof.
Whilst torching may be seen as a little old-fashioned it is still used today in heritage properties, as an alternative to modern breathable membrane. The torching on heritage roofs contributed to securing the clay tiles on roof, in the days before nibs were added to hold them on the wooden battens.
When a roof is being renewed for other reasons then torching is often replaced with breathable membrane not least because most roofers do not have the skills to do torching and current building regulations direct them towards other methods.
Renewing torching is a specialist task. Roofers who can do this are normally to be found working on heritage properties. A mix of lime mortar mixed with horse hair is often used for this task.
Note: torching on pitched roofs is not to be confused with torch on felt used for flat roof construction.