The ceiling peeling after a new roof is due to moisture. Rain, even though the roof was tarped, dampens the insulation and plaster. Then with the first heating cycles of your furnace, your house is drying out. Moisture, being what it is, is trying to equalize itself and being greater in the attic, it’s trying to come in. A good indicator of this is when the paint peels clean down to the bare plaster.
Usually the moisture comes from the lack of circulation in the attic, or too few roof vents. What happens is the moisture trapped in the attic condenses when the temperature outside drops. More vents in the attic help prevent this.
So before you work on the ceiling, correct the source of the problem, or at least prepare to do so.
Patching the peeling on the ceiling can be easy if you keep your patience. A rushed or skipped step here may be very difficult to repair later.
Peel all the lose paint off the area using a scraper, putty knife, or even a small pocketknife.
Wash the area with a mild solution of T.S.P. (trisodium phosphate) and water. Use about one tablespoon per gallon of water, rinse well and let dry.
Prime the bare area with plaster bonder or a shellac primer, like Bin Primer.
Using a six-inch putty knife, the wider the better, apply a layer of drywall joint compound around the peeled area. Keeping one edge of the knife on the paint you can feather the compound to the bare plaster. A professional will need two coats to do a good job; you will need three. Don’t try to do this in one coat; rushing this will only leave the repair visible. My favorite tricks here are to keep the coats of compound thin, and to use a sponge to clean up excess compound in between coats eliminating sanding.
Now you have a smooth spot in a ceiling with a slight orange peel texture from years of painting. Take the sponge, used for cleaning in between coats, and lightly dip it in the compound. Now tap the repaired area (it should look good by now) with the sponge until you emulate the texture of the old ceiling. The thinner the better.
Prime and paint your ceiling and move on to the next project.