Have you considered putting spacers on the joists and putting a whole new layer of drywall?

joint compound and fiberglass tape.

Be sure and insulate while you are at it.

tim45z10 yes sorry, just edited my OP. We're using filler pieces and new drywall sheets. I was just wondering what to use between the drywall end that meets against the plaster.. I wasn't sure if joint compound would be robust enough or if I need special cement (?) filler. We actually are also installing new insulation too! :) The old fell out and was gross.

It could be that the wire (expanded metal) was only a corner reinforcement. I usually pull the rest of it out if that was the case, it is just a source of trouble going forward. It's possible that they wire-lathed the whole ceiling, but it's not likely. For patches like this I spray bonding agent (Link or Plasterweld & other brands) Wipe off any drips of Link, they are hard to get rid of when dry. Then use a setting-type drywall compound (aka Hot Mud; Durabond 90-minute or 45-minute set time) Which dries faster and with less shrinkage than mud that simply sets by evaporation. Don't try to fill deep/wide gaps in one shot.

Once you have the surface filled level, use a wide fiberglass repair tape over the repair zone, and then you can skimcoat with normal green-lid mud after bedding the mesh in the Durabond. Hot mud cures very hard, it has some Plaster of Paris in it, so it is very hard to sand, but ridges can be scraped off.

The bonding agent really does the trick here. I have some drywall/plaster ceiling joints that have not re-cracked over ten years. FWIW, Link is close to the same stuff as white elmers glue, a PVA adhesive.

Casey

Not to be captain obvious here, but two layers of 1/2" drywall = one 1" layer of plaster.

You can use normal joint compound..

Sombreuil, the wire lathe definitely spans the entire wall. It's terrible. It's been in every spot we've cut into.. grr. We just got an angle grinder with diamond blade last night though and it's (fortunately) cutting the edges smooth/flat.

Are you saying you use the spray bonding agent (Link or Plasterweld) where the new drywall meets the plaster, then put the Hot Mud on top of those 2 ends? Do you wait for it to set before putting the fiberglass repair tape on it?

Vith, instead of using 2 layers of drywall we're just using filler pieces and one layer of 1/2" drywall. I was nervous about the joints though, where the end of the drywall board meets the end of the plaster. I didn't know if paper tape + normal joint compound would be strong enough to hold the 2 materials together...?

If you want to take advantage of the self-stick properties of the fbg tape, it has to be dry or it won't do. But if you lay down a bed of fresh mud, you can trowel the tape into it. You lose the advantage of seeing exactly where the joint is; but the 6" wide tape should have it covered.

Casey

So should I mud then use fbg tape or fbg tape first, then mud?