Hello,

New member here. I hope this is in the right place. I am trying to establish the best way to attach a fairly heavy cornice molding to a number of walls in a 200 year old building. The walls are lath and plaster. I am attaching a solid strip of poplar (1 1/2" tall by 1 3/4" wide), then a c shaped bit of blocking over the solid strip, then 4 pieces of trim that comprise the cornice.

I don't have room behind the lath to use toggle bolts and the plaster is very brittle and susceptible to cracking. I've installed plastic zip its with yellow PVA in pre drilled 3/8" holes and I've also used PL in some places with the same 3/8" holes. I'm just starting this install and I don't have a ton of experience. Any suggestions about how to attach the trim would be appreciated. Thanks in advance.

Hey there. Sounds like you’re on the right track. Who covers plaster damage according to your contract? There’s a decent chance of some spalling or cracking occurring, and even if it’s not due to something you’ve done, there’s a decent chance that they’ll try to blame you for it if any plaster falls off that ceiling in the next 1-3 years. Get it in writing and get it before you touch that ceiling again my friend.

I usually just rip some cdx plywood at the an angle that will reliably fit behind your spring angle, even if you have to roll it up or down, a little short in width, full length. Run long screws through into the top plate and/or studs. That way, you’re most nailing into plywood backing, not so much the ceiling or walls. It may be balloon framed that you won’t find a proper top plate like a newer stick framed wall.

Catching all the ceiling joists is not enough?

Oops, didn't mention studs first. You many not have any joists running perpendicular to the wall?

It helps to know what the construction is behind the wall finishes; in a 200 year old building support and nailing is usually sparse as the builders of the era often spaced things as needed rather than a standard 16, 24, 32" o.c.. Larger, commercial sawmills were just coming into being at the time and milled lumber was often a long way off.
You said there's no room for toggles behind the plaster lath?. It may be you're getting tangled in lumps of brown coat, or, since you're working at the juncture of the wall and ceiling, you're hitting backings for the lath and/or plaster; long screws may be the answer.
Point I'm trying to make is gluing to or setting plugs into just the plaster or browncoat can come back to bite you if it cracks or loosens.
I would suggest carefully drilling small holes- 1/8"- with an extra long masonry bit where your cornice work will cover them to discover whats behind and how far away from the finish it is and use fasteners accordingly.

I would drill holes for anchors (plastic, whatever) gingerly, then insert anchors with either expanding foam or PU to hold the anchors then affix your cornice as needed. The PU or expanding foam will firmly anchor your trim/backer as well as likely stabilize the plaster lathe. Hopefully you've got either time and materials on this one or a ton of fat.

PU would be the easiest. Drill a fastener hole with ginger pressure as not to disturb the plaster lathe, squeeze a good shot of PU in the hole, insert anchor, move on. By the time your done with a run you'll be ready to go back and start affixing your backer.

Whatever you do, don’t rely on adhering to the plaster or plaster/lath assembly only.

Old riven lath can be relatively thin and nails are of variable manufacture which can mean irregular heads and length. I’ve seen situations where the lath and nails are performing well but I’ve also seen a good deal of lath with badly split ends and very wide spacing which can lead to a failure of any molding attached only to it.

The best approach IMHO is attaching the canted plywood nailer behind the trim and screwing it to the framing assembly whether it be standard stud and joist or “plank” construction.