Add a porcelain bathroom fixture to existing tile

I am going to give you a very short answer to a very involved topic. Ceramic and porcelain are basically similar products. There are very good and very cheap versions or each product. When you go to your supplier, ask about the hardness rating. Softer cheap tile usually has a redish tint on the back side and is made from softer clay, typically in Mexico. Some of the better grades come from Turkey, Italy etc. and have hardness ratings of 5 or above and have a gray or whiteish back. Most ceramic tiles have a thinner glaze finish than Porcelain. I usually prefer a porcelain glaze on a good hard base in shower and bath applications. Rather than hitting the box stores first, go to a few tile specialty tile stores where a good knowledgeable sales person can give you a good education on the differences. If and when you feel comfortable with your knowledge about tile, find a professional flooring liquidator in your area where you may save anywhere from 40 to 80% on overages, returns or closeouts. I recently purchased 600 sq feet of Turkish porcelain grade 5 commercial tiles for 60 cents a square foot that retailed for over $8 a foot, at a local flooring liquidator. (overages from a hotel job) Huge savings.

I'm a little concerned by the fact that "this one has had issues," but it sounds like this part of the job is being done correctly.

Just a couple of definitions, quickly:

• Drywall (aka sheetrock, plasterboard, wallboard, gypsum board) is gypsum plaster covered in paper, and is used on most walls and ceilings in dry areas.
• Cement board (aka CBU) is used in wet areas like showers; drywall - even the green or blue stuff - will eventually disintegrate in wet areas.

I'm assuming that the 4" patch at the top of the shower walls is cement board, same as the lower part.

It's standard to use fiberglass tape over joints between cement board to minimize movement between the boards, and apply thinset and tile over that area as usual. You might fill the gap with a little extra thinset while tiling, but you don't use drywall mud in there since moisture will just weaken it.

The fact that the cement boards aren't flush to each other shouldn't be a problem unless the gap is more than 1/8". Even a little more than that isn't going to cause problems, depending on the size of the tile used.

The most important thing to be aware of when a new shower is being installed is preventing water from getting into the walls. For this reason, a vapor barrier (plastic sheet) should go between cement board and the wall studs, and extend down over the lip of the shower base (or pan). This allows any water that gets into any joints between cement boards to run down into the shower base and down the drain.

The cement board should extend down to about 1/8" above the lip of the shower base - there's no need to overlap it, since the plastic is doing so. The bottom row of tile should extend over the bottom edge of the cement board, mostly covering the lip of the shower base, then silicone caulk should be applied to seal that gap.