One for the fire engineers: I have a historic building in which we're trying to achieve a degree of compartmentation. The wall is sufficiently thick and in good condition, but it needs extending to the roof. The roof is constructed with trusses, over which has been laid 1" boards, over which are tiles and/or lead. Is it sufficient to butt up to the boards and then fill the gap with intumescent mastic? Or do we need to do something with the boards and extend right up to the tiles? I've had a quick look at the Red Book and nothing is jumping out at me. Ian