Lathe and plaster merged into sheetrock.
 

Or maybe vice versa would make more sense as that would imply that the sheetrock is new and the lath and plaster old (duhhh).

Nice house in Marin, I'm guessing 20s or 30s - lath and plaster - studs are 3.75 x 2 - knob and tube wiring. I'm knocking out one wall to merge the master bedroom in Unit A with the bedroom in Unit B. This is about the nicest 2 unit house I've ever seen, not a duplex - unit A is a deluxe 2 bedroom while unit B is a deluxe studio.

I'll have to heal over the stub where the old wall and ceiling were and I damn sure don't want to shorten an apprenticeship at lath and plaster from 2 years to 3 days.

My first instinct is to use paper and hot mud over the joint tween the plaster and the rock, and sand and re-coat like I was enjoying myself.

Seems to me that it's going to be very hard to make the wall and ceiling look seamless. Any chance of some kind of moulding at the joint to hide it?

It sounds like a good opportunity to make a master bedroom suite, put a pair of french doors in there........
If the plaster is smooth and not too rippley then you shouldn't have a problem fixing a gap of that size. You should be able to skim coat the repair area and have it be pretty much completely hidden.

I floated the idea of putting a modest arch sort of transition at the joint. I checked the attic and the wall isn't load bearing - well it was a tiny amount - the roof rafter above it had a 3 foot small post resting on the top plate (not a steep roof) - the rest had one brace going from center of rafter over to the plate at the top of the common wall, the one that will stay in place - dining room on other side. So I added a brace. If I had needed to put in a triple 2x12 or something to support major weight, I'd have been off the hook for the seamless merge.

Oh well, should be interesting and I'll take pictures.

Fine Homebuilding, January 2010 has an article for fixing damaged plaster walls that is pretty comprehensive

I've done some of this working on an 1860's wood frame townhouse-

When I took down walls, I used a circular saw set for the depth of the plaster, and cut the plaster into squares. This way the plaster can be removed and then you can remove the lath (if its wood) so you can save it to fix holes. Dont just go at it with a sledge, as you will break plaster keys elsewhere and end up with loose plaster on the walls and ceiling

Where the walls meet (the stub) there will be no lath, and this is where after removing the corner studs you can use the old lath to fur out the new gyp. board. Build up to the plaster surface with gyp. board so the mud is minimal. Paint the edges of the plaster with Plaster Weld- it is pepto bismol pink and acts as a primer to the plaster so the plaster does not suck all the moisture out of the mud. Use fiberglass mesh to bridge over the whole area and skim coat.

Dude, I hope you are getting those materials tested, old plasters can be full of asbestos, 20's, 30's vintage is lousy with the stuff. As long as it is undisturbed, it is considered safe, but once you disturb it......

If you can afford it putting a half inch of drywall over the whole ceiling makes a lot of sense. Patching the trench will be very hard to make nice. The expansion and contraction will probably crack the patch too.

It might be easier to meld LATH and plaster to sheetrock than lathE and plaster. Not quite so lumpy.

That's a good call, I'll look into it.

Excellent advice. I have boxes of older issues of Fine Homebuilding but the trick is having the index. I seem to recall that's in the last issue every year. At any rate, I'll look for that one at a library.

It looks like I'll need to get a sheet of 1/4 and 3/8 sheetrock so as to patch in where needed.

If it has wood lath I would expect to find horse hair in it instead of asbestos.

i think the architect guy speaks truth.

additionally - if i'm understanding the project - even if the ceiling can look smooth, there will be a slight hump, which will show up as a dip on each vertical wall. either demo it all, or put new drywall over the whole thing.

Before the '20's, they used hair to bond the plaster better in the finer homes, horse hair, human hair from barber shops and salons, dog hair, etc, and it was pretty safe. Right around 1916 or so they started mixing vermiculite instead, and mined vermiculite is really nothing more than asbestos fibers. The asbestos fibers had the same effect as hair on the plaster, made it bind better and not crack as much. From the '20's until the advent of drywall, they used asbestos like crazy in the plaster - hair was a limited supply product while vermiculate is easy to get, pushing all the contractors of the time to switch to it, and contractors were motivated to use it so they wouldn't have to eat the costs of fixing cracks and they could use it as a "fireproof" selling point. It's use was not ubiquitos, but it was used extensively in the more extensive homes, especially if the plaster was subjected to some sort of decorative treatment.

You can tell a lot just by looking at it - if the plaster was hair based, you can actually see the hair in it - long fibers, or you will see short black fibers - possible asbestos. If you see nothing at all, get it tested anyway, because there was plenty of pure white vermiculate used in those days too.

Since you are doing the work with a real respirator and proper filters, it should not cause you problems. One of the issues of testing materials is that if the test comes up positive, then you legally have to admit knowing about it when you sell the house, and follow all regulations when dealing with it.

The other issue is you'll have lead paint too-

The wood frame Italianate I owned/had two kids in/ abated 44 lead paint violations after the oldest tested high for lead/ was all original plaster, 8 piece door and window casings, plaster molding at ceilings.

If you have little kids in the place/coming to visit, I'd be more concerned about proper lead paint treatment than asbestos.

I use a welder's respirator that I really like. Very lightweight - carbon filters - you can't smell paint through them.

I found out today the house was built in '32 or '34. I wouldn't be surprised if it does have asbestos here and there. The heating ducting in the basement is asbestos clad. People used to be wild about the stuff. Holy crap, the stuff we've surrounded ourselves with.

Lead paint, leaded fuel. Better living through chemistry.