If the plaster is in good shape, the easiest thing to do would be to just cut and patch where you need to fix the plumbing and electrical. Unless you are moving a lot of plumbing around, it shouldn't be too hard. (I know, famous last words....) You don't need to drywall over perfectly good plaster.

Thanks! So you think if the plaster's good it should be able to hold the tile? There's several layers of paint on the plaster, some holding on better than others. I'm wondering I can prepare the surface to make sure the thinset sticks, without risking damage to the walls.

For the holes, should I patch/repair the holes first with more plaster, drywall or is there something else you recommend?

Also, what about waterproofing? There's no shower enclosure but a couple areas of tile could see the occasional splash of water - do you think I still need to put up cement board or something?

Sorry for the barrage of follow-up questions..anyway, I appreciate the advice!

I'm not a tiling expert, but there are some really great pro's over on the Bathroom forum. You might want to cross post there.

Just as a historic point of fact, people were tiling WAY before drywall or cement board. You can certainly tile over plaster as long as it is in good shape and you remove any flaking paint etc. I suspect you would want to sand it down use a modified thinset for best bonding.

As for waterproofing, I guess it depends on what you mean by "occasional splash of water." Are we talking more like what a backsplash might get or are there going to be any joints where water might sit up against?

First off, I'm a HUGE don't tear down perfectly good plaster fanatic.

However, that ends at the bathroom door. The main reason is mold. Old houses don't generally have mold problems because they leak air like a seive. In modern times "air infiltration" is a dirty word (phrase?). However stopping the air infiltration, gives mold a perfect habitat to live. A nice warm, damp area with a bunch of tasty lathe to munch on.

If you insist upon it, go top end ventilation with a forced 20 minute timer on the fan. Meaning don't wire the fan and light to separate switches and get a fan that runs for 20 minutes after the light switch has been turned off. There is no such thing as a water proof tile job and plaster is a sponge.

The best solution is gut the whole thing and cement board all the walls. Your tile work will look much better and go up worlds easier due to the fact you are mounting them on a truely flat surface. As for the rest of the walls not being tiled, mold & mildew resistant plaster skim coat over the cement board looks and feels like the real thing.

You get the same mold resistance from green board, but it just doesn't feel right. It looks like it could be a lathe and plaster wall, but as soon as you touch it you can tell it isn't.

Since this is your only bath though, I'd probably settle in the middle if I were you. Remove all plaster and lathe where you are going to tile and put up cement board. It will cost you time on the front end in the demo stage, but demo is much easier than varying thinset depth to get a nice looking tile job. Patch any holes you make for plumbing/electic with cement board (if possible, depends upon the original plaster job), skim coat it, and cover it all with a high quality mold/mildew resistant paint and don't forget to a decent ventilation system.

This is a link to a long thread I started. Skip most of it, but scroll down the sombreuil mongrel's Jan 13 post and brickeyee's Jan 21 post, both regarding materials to use when patching old plaster. I followed their advice and the results look and are performing great (in spite of me!). Also there is a bit of info about calcimine in that thread; I followed sombreuil's tip for that too and got the alkyd paint. If you have a peeling paint, there may be a layer of calcimine in there somewhere; better to deal with it now instead of when the tiles are falling off.

I know it seems like a lot, but really, it's just a matter of using the right materials instead of grabbing whatever happens to be first on the shelf at HD. The labor is about the same (actually, the plaster bonding agent saved me time and crumbly messes), and you are greatly increasing the odds that your work will last the test of time.

Here is a link that might be useful: patching plaster