If the wood firring strips that the plaster was on are fairly flat and level, across the plane (check with a straightedge) then you can just go over them. Use sheetrock screws long enough to go up into the actual framing though. Don't rely on screwing to the wood strips. The correct way to do it is to rip down all the little wood pieces, put 1x3 strapping 16" on center. I would bet good money that the joists above are irregular and the strapping (firring strips) will have to be shimmed to form a flat plane. I would tack all strapping up in place, using only one 6 or 8 penny shiny smooth shank nail every other joist. Then you pry down the high spots, putting a shim in the gap. The shiny smooth nails are so it is easy to pry to level. If you used airgun ring nails you couldn't pry to level. Once you pry down and shim high areas then you can shim all gaps and put at least one 2.5" ring nail or screw in each joist/strapping intersection. Then screw up new sheetrock. You can use a molding - quarter round or cove or cornice for where the ceiling meets the way to avoid all the flat taping and mudding there. 1/2" minimum on ceiling - 5/8 best, to avoid sagging. 3/8" will sag. Use screws not nails and don't let screw heads go past paper or rock will fall eventually. Usually (sometimes) ceiling falling is due to leaky roof deterioratiing the plaster. I usually say "ceiling trouble? check your roof shingles". FYI: if you get any more loose plaster - walls or ceiling - they make cool plastic washers (sometimes metal), perforated, about 1.5 to 2" diameter, that a sheetrock screw can go through. These help to anchor the plaster. Perforations allow s r compound to adhere w/mechanical bond. Washers act as large screw heads to hold loose plaster. I always use these. They should have them at home despot or local sheetrock/plaster supplier. With the old plaster - generally "when in doubt, rip it out". If you re-do a room, for example, don't try to save half a wall of the stuff, etc. you will spend more time dealing with the old plaster than doing all over new.
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