If possible, get any plaster that needs to be applied above the roof line in place before the finish roof is installed. If that is not possible, be sure to protect the roof from any dripped plaster. Once on a roof, especially a roof with texture like shingles, shakes, or clay tile, plaster is all but impossible to remove and the roof itself may need to be replaced in sections.
The picture above is an example of how to properly protect a roof while applying plaster. Do not assume you can keep the roof free from debris with careful detail. Instead, assume you WILL spill plaster and protect the roof accordingly. It is an even better plan to schedule the plastering after the roofing felt has been installed but before the finish roof has been completed. As with many other aspects of construction, a little forethought goes a long way!