I am building a loft room in my bungalow with full building regs. approval. I have installed a 200mm x 100mm RSJ, above the ceiling, into which the 196mm x 48mm floor/ceiling joists are notched. The plans indicate that I should use 12.5mm fireline board for the ceiling below the RSJ, but they do not state that I should replace the whole of the existing ceiling with fireline. Should I? What I would prefer to do is remove 600mm of the existing ceiling (9.5mm plasterboard with 2mm to 3mm of plaster & artex) each side of the RSJ and replace it with 12.5mm fireline board and then to leave the rest of the existing ceiling in place and overboard the whole ceiling with another 9.5mm plasterboard and plaster skim. Will this all provide the necessary 30 minutes fire protection to the RSJ and ceiling/floor joists? Thanks.