A coat of lime mortar was applied to the exterior of a nailed or pegged slate roof to prevent the slates from lifting and to keep out wind-blown rain and snow.
Rhoddwyd haen o forter calch ar y tu allan i do llechi wedi'u hoelio neu wedi'u pegio i atal y llechi rhag codi ac atal glaw ac eira a chwythwyd gan y gwynt rhag mynd i mewn.