I think my news for you is bad; you probably have celotex or "beaverboard" that was used as the base for plaster. It always ends up sagging as you describe. The only true remedy is to remove it and start over. They never should have used it, and were probably talked into it because it was cheaper than the alternatives, Rocklath or woodlath with three-coat plaster atop.
Celotex just simply lacks the integrity to take the weight of plaster, and on the ceilings it would have warped of its own weight.
The upside is, now you get to do any wiring, plumbing or insulation you may need to attend to.
Casey

That was not what I wanted to hear, I cannot afford to gutt everything down to the studs. The plaster boards must have sagged shortly after installation when the finished plaster was applied. They seem very stable now. There are no stress cracks in amy seams and the walls are as solid as concrete. The only hint I had to the construction was a small hole where I could see the layers of material - hard brown "cement" then white limestone plaster.

It is a late depression era house, so cost cutting would make sence. I even found stamped plaster walls under the wallpaper in the downstairs bath. The lower half of the plaster walls were stamped in a 4" square pattern to simulate tile. Those walls were suprisingly very flat!

You can tell more of what may be happening by seeing if the center of the panels is flat and the joints have sunk, or if the center of the panel is bowed out. A two foot level is about right for this. If it lays tight the length of the panel but shows dips from panel to panel across the joints the original plaster job was done to quickly.

This was one of the first purposes gypsum panels were used for.
They often measure only 16 or 24 inches wide by 4 feet long.
The stud side was covered with thin paper, and the face side was bare gypsum.

After the gypsum was nailed up, a scratch coat of plaster was applied (often with sand mixed in), then an 1/8 inch layer of finish plaster.

The finished thickness is usually around 3/4 inch (sometmes a full inch) for the while stack up.

I have seen the outlines of the seams in many houses finished this way. They did not wait long enough for the scratch coat to harden and shrink before applying the finish coat, resulting in long term shrinkage at every seam.

It is pretty easy to skim the seams with Easysand to bring them back to smooth, and if you mix the Easysand about as thick as peanut butter it will not have any appreciable shrinkage as it hardens.

yes brickeyee, the "waves" are fairly level if a straight edge is placed horizontally at the top of the hump. If i place the straight edge verticaly I can see the valleys. I am certain the base is not the cane fiber beaverboard, but a hard cement board like sheetrock and its about 24" wide. From the attic I can get to some of the ceiling below, and can see the sheets with the paper on the back, but there is no paper on the front.

That's part of living on old houses. Strip the wallpaper, paint it and live with it until you can afford to gut a room and redo them.

If you don't mind losing the space, you can put 3/4 drywall over the top of the wavy walls, and add new baseboards. Shim where necessary as you install the drywall.

"Strip the wallpaper, paint it and live with it until you can afford to gut a room and redo them. "

When you look up the price to replace even two coat plaster with the same material you will likely think very hard about ripping it out.

Skimming the joints back to level is far less expensive and will preserve the desirable aspects of plaster walls, like sound isolation.

You could use 1/4 inch drywall, but by the time you are done removing trim, installing and finishing the drywall, and then putting the trim all back up, skimming looks very inexpensive.

I lived in 100 year old house that had been relocated to a new site. The moving process had caused the walls to buckle and crater in places. The family that moved the house just did basic patching and lived in it that way for 30 years. When we moved in (we were renters), we chalked it up to part of the house's story, and learned to live with the imperfect walls too.

One day we found pics of the house moving process with French captions on the back. Apparently they ran into trouble with some wires, and had to leave the house in the middle of Main Street overnight until the power company could come out in the morning. It was funny to think of our house blocking traffic.

Perhaps having a constant memento of the depression will inspire you somehow (to thrift, or appreciation of better times). I just made red beans and rice from Jeff Smith's frugal gourmet. He explained the history of the dish, how it was the food of slavery and hard times, and we should eat it and remember this. We did. (and it tasted very good too!).

Sorry for the long digression--houses with quirks can really get me going sometimes.

I understand that older homes have quirks and that's what gives them part of their character and charm. I have just never seen walls that had so many! I don't have any desire to rip out the old plaster walls since all the walls are stable and sound. Brickeye may be right about the old plaster having cured unevenly after it was applied making the seams of the wall panels visible. Some walls are wavier than others. The worst are the first walls visitors see in the house. The eggshell finish paint I used there probably made the grooves more noticeable.

It seems to me that skim coating the most visible walls to even them out would be the way I should go. The rest I can live with. Has anyone found that certain colors are better than others in hiding wall imperfections? I noticed the original colors in the house (1930's) were coral in the living and dining room, blue in the kitchen, greens and pink in the bedrooms. Definitely not the antique white used by the previous owner! Has anyone noticed if certain colors are better at hiding wall imperfections?

Brickeyee, should I sand the painted walls or use a bonding primer before using the Easysand plaster to skimcoat the wall?

My 1935 house has the same "problem." On every ceiling you can see the 16"x4' seams. It is also like that on all of the upstairs walls and many of the downstairs walls. We did a kitchen remodel a couple of years ago and it was interesting to see that the some of the walls were plaster and lath and some were either the gypsom or beaverboard described by others. I thought maybe they splurged for plaster and the living and dining rooms, but there didn't seem to be much method to the madness. I'm not sure whether its the plaster coat or the boards themselves but they are literally rock solid. I have to use a drill to hang anything, and many times the drill bits break.

The grooves don't bother me as much as the peeling paint. In a few of the seams on the ceilings the paint literally falls right off. I'm not sure if I should blame it on a bad paint job or the materials in the ceilings.

I also have the faux-tile that ken tn speaks of. It went all around the kitchen from the floor until about five feet up. It was in the bathroom and the half bath as well. In the kitchen the previous owner just plastered over it, but you can still see the pattern in places. I'm not sure if it was a stamp or something premade, but it is definitely not 4" ceramic tile.

I don't know that the finish or paint colors have made much of a difference. It's very visible on the flat white ceilings and very visible on the various colors of satin walls. I did find many of the same old colors, lots of teal, green and coral around too.

" On every ceiling you can see the 16"x4' seams."

Sounds like two coat plaster.
Skim all the seams.

If the plaster is in good shape a light misting with some water should be all you need to get Easysand to bond well.

Finish plaster does not suck the water out of repairs like the underlying scratch coat does.