Painting poor quality slate roofs with bitumen is sometimes called ‘Turnerising’.   I don’t know the origins of the name or where it started.

When slate nears the end of its useful life it starts to delaminate.   A quick and cheap way of trying to keep the roof watertight was to paint it in liquid bitumen,   It was a bodge repair and should never be done on a Listed Building – or indeed any building.   Rain can get in under the cracked bitumen and this accelerates the damage to the already damaged slate.

Removal of the bitumen is not a viable option.   The whole roof will normally need to be stripped and renewed.