Before the advent of roofing felt roof tiles and slates were cemented to roof battens (thin horizontal timber strips) from the underside. The purpose was to both seal any gaps between the roof tiles/slates and so help avoid rainwater penetration, and help to stick the covering down so it could better resist storm damage.
Over time this cement or “torching” will crack, become powdery and falling away. Not only does this make your attic dusty it makes the roof more prone to rain and wind damage. It is a sign that all is not well.
The torching can be replaced but this is laborious and costly. It is oftena sign that reroofing will be needed before long, incorporating a modern breathable membrane lining which does same job and also helps to avoid the build up of potentially dmaaging condensation in the roof space..