Older roofs with no felt just slates and morter. Over time the morter comes away from the slates leaving gaps and holes in the roof. The loose morter can be taken off the dust and cob webs cleaned away and sprayed with ridged foam. The roof is not only insulated but stronger. Open cell foam forms a strong bond with the slates keeping them in place and stops them slipping and moving .(SLATES THAT ARE BROKEN OR CRACKED SHOULD BE REPLACED BEFORE SPRAYING). Ridged foams are not water proof.