Diagram 33 ADB & 26 BS9991 shows the compartment wall going to the underside of the roof. To pick up on the earlier post - is it acceptable to use an alternative i.e double sheet the ceiling where the FFL is below 4.5m and incorporate the 20m cavity barrier in the roofspace?
I also have a sprinkler system within each apartment, the roof space is not accessible to the residents with a locked hatch within the communal area under the control of the factor for the common areas?
Not sure where the 'double sheeting' of the ceiling comes from, to be honest? Firstly two layers of ordinary plasterboard isn't going to give you an hour's F/R & in any case it only really works in risk terms if you have a F/R ceiling membrane (to the same rating as the compartment walls) in all the top storey accommodation (not just the corridor)?
Cavity barriers/curtains providing the same function as compartment walls are problematic because they're (generally speaking) not designed to be self-supporting, so strictly speaking the supporting structure ought to have the same F/R rating as the compartment wall - which will not necessarily be straightforward to design (especially for 'gang-nailed' trussed rafter roofs, which can collapse really quickly in a fire).
I know there's the 'reasonably practicable' test to be done where we're talking about risk assessing existing construction, but for new build or refurbishment the recommendations are hardly onerous...