I've sold our house and the engineer for the purchaser came to view, he said that there was a 6in gap in the attic under the tiles on the semi dividing wall running from the apex down to the soffit, this had been fill with fibre glass insulation material. He would not accept fireproof boarding or fire proof foam, he wanted it filled with morta but the builder said that it was too wide to do this, the the builder cemented in small blocks which is much better job but more than 2 hrs work that engineer had estimated.
What is the norm for this because fire can still spread via battens for the tiles. We also had another Engineer 2 weeks previous who never found a fault