I don't know, I had to do this as a long-term "temporary" solution after tornado water-damage caused part of my kitchen ceiling plaster to fail.

(I didn't have any power that week, so I never checked the USG website! Sometimes it's better to not know things.)

After I cleaned up the mess, I stabilized the edges of the holes (big ones, 3 X 6 feet) with plaster washers. Then I used roughly fitted pieces of sheetrock attached directly to the lathe to fill in for the missing plaster and bring the patches out to the depth of the nearby intact plaster sections. I slapped up some joint compound to fill in between plaster edges and sheetrock edges. Then I hung the sheetrock, taped the seams as normal and moved on. Seems to have worked OK for more than a decade. I don't like sheetrock, but it's not high on my list of things to replace.

(Before closing up the holes, I snapped chalk lines to indicate where the joists were - and did it again as I was hanging the rock. My 19th c house doesn't have perfectly-evenly centered framing so I'm used to having to do that.)

I suppose if your plaster was really bumpy, you'd need to fur it out to have a level plane. Is yours? Check especially at the edges where ceiling and wall meet. Take a long straight-edged piece of lumber and see how completely flat your ceiling is. A little dodginess at the wall/ceiling joint could be finessed with some crown molding.

L

I'm no professional sheetrocker nor I have I seen USG's website, but I have done exactly what you're asking about in at least 4 houses. So...

Wood 1x4 or 1x3 strapping works fine. You can use a laser level, but an old fashioned chalk line works, too, to ensure a level surface.

IMHO 4'on center for the strapping is too wide. I would consider 2' oc a minimum and 16" is even better if you're supporting the weight of a lot of plaster that has pulled awy from the lathing or is likely to.

Attach the lathing right through the plaster into the ceiling joists at 8 to 12" spacing with heavy duty screws at least 2.5 " long. Plaster washes may be useful in places where the plaster is hanging down significantly.

Heaving around 4x8 sheets is the hardest part of the job. Lots of help and 2x4 braces make it easier, but not as easy as a rented drywall lift.

I know the "pros" might cringe at my method, but 10 years ago when my wife (at the time) and I bought the "house we we gonna raise our kids and grow old in" (yeah like that happened - lol!), I had to do this in what became my son's bedroom. It was an 1860's farmhouse and most of the ceilings had been sheetrocked over the plaster, but for some reason my son's was never done, and the plaster was cracking, falling down, etc.

I just screwed 1x2 pine faring strips the whole length of the room 2 feet apart....didn't bother trying to hit studs or anything...just used like 2" long deck screws and assumed most of them would wind up in the lathe behind the plaster. Then screwed the drywall into these strips.

I am still at the house almost daily picking up and dropping off the kids...probably 8 yrs later and the ceiling still looks like the day I did it.

The problem is if the plaster then separates it is very likely to load the drywall enough to cause sagging, and maybe even simply falling.

Drywall is not designed to hold ANY weight, and even 1/2 inch drywall on 24 inch centers has been known to sag (and look terrible).

You might consider simply pulling the plaster down, along with the insulation.

If you put 2x2 pieces on the bottom edge of the old joists you can quickly create a new flat surface for drywall, and then just install it.

A laser level makes leveling the added wood go quickly and eliminates shimming.
You make the plane of the new wood as low as the lowest spot in the old wood.
The new wood sticks down from the old joists at any higher spots.

You might need some blocking around the edges of the ceiling to attach the drywall, but it is just a detail job to make sure you have framing to attach the drywall.

If the joist are wider than 16 inches on center you can use thicker drywall or add some new joists.
You could even use 1x strapping perpendicular to the old joists to establish a plane for the drywall (though then you have to shim the strapping to make sure it is flat).

the one thing my flipper did that I liked was put up armstrong ceiling tiles (tin ceiling type) that were spraypainted copper in the kitchen. Much better than the 1980's PO who did STOMP PLASTER on the ceilings. (this is like popcorn but looks like frosting on the ceiling)

Why everyone insists on removing the LATH as well as the plaster is beyond me--if forces you to do the job in layers of sheetrock. All you need to do is remove the plaster (easy to do, just run a square-edged shovel where the lath and plaster meet). Next, if the lath isn't level, add small shims over it to give a flat surface, then put up sheetrock. You can see the joists, so no need to worry about missing them with screws, and no need to worry about the insulation above the lath either.

Thanks everyone.

It turns out the ceiling is in worse shape than I thought. Only 80 percent of the lathe is still there, and the surface is out of level by 4" in some spots due to settlement down the middle of the house (this is also true of our floors below...ugh!)

So we are basically framing a new ceiling. The new runners will be attached to the original ceiling joists every 24" or so. Then we furring it in the perpendicular direction with smaller pieces every 16".

A lot of work!

"Why everyone insists on removing the LATH as well as the plaster is beyond me--if forces you to do the job in layers of sheetrock. All you need to do is remove the plaster (easy to do, just run a square-edged shovel where the lath and plaster meet). Next, if the lath isn't level, add small shims over it to give a flat surface, then put up sheetrock."

Lathe is rarely flat enough to leave behind drywall, and the joists may not even be flat enough either.

Shimming is a slow time consuming process, and still often results in a less than flat surface.

By stripping back to the joists it is simple to sister new pieces of 2x in a plane to create a surface that is flat.

Saving old insulation is rarely worth significant effort, especially if it can easily be replaced.