If this was happening outside I would suspect the damage was caused by water penetration and the weather generally. This is happening inside a 150 year old stone built semi detached cottage on the adjoining (attic) gable. There is a dissused joint chimney stack vented from the top and the adjoining roof tiles are bridged (cement bedded) with ridge tiles. I'm not aware of any evidence of water penetration elsewhere in the house. Looking at it you would think it was okay, but poke it with a finger and it crumbles?