Abstract: The invention provides an assembly for accommodating service lines which comprises a length of ducting, a block of structural cellular material which fills the internal cross section of the ducting over at least a part of the length of the ducting, said block having cell walls which define cells which extend in the length direction of the ducting and on the cell walls a coating of intumescent material expandable to close the cells under fire conditions. With the block positioned where the ducting passes through a fire-resistant wall, the spread of fire through the ducting is usefully opposed. Using thermoplastic ducting, which has heretofore been contra-indicated, results superior to those obtainable with steel ducting are achieved.