Niel: Not a builder, but would guess you should install plastic sheeting to prevent future leaks from dripping onto the ceiling board. Especially since the roof is a deck which will eventually spring a leak.
You never use plastic as a permanent solution, as it doesn't breath, and could cause rot. It has to be something loike building wrap or similar that breaths. It probably will need some form of venting though, so the roof space doesn't get condensation .