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I'm in the middle of remodeling my bathroom, and am about to tackle putting up the shower walls. My house was built in the 1930s, and thus most of the walls are lath slathered with mortar that holds the plaster wall.

The bedroom is on the other side of the show, and I would like to keep its plaster intact. My problem is that I would like to install shower niches (for shampoo, soap, etc.) in the wall. However, the mortar holding the plaster on is brittle, and if it comes off, so does the plaster. I was planning on putting a piece of plywood for the back of the niche. However, I'm worried I'll lose at least a 1/2" because the of the mortar pressing against the back of the plywood (not to mention potentially being destructive to the wall behind it).

Does anyone have any suggestions or experiencing dealing with this type of configuration?

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    What are you planning on finishing the shower surround with? Plywood and plaster are usually not things you want in the shower. Are you tiling?
    – DA01
    Jan 3, 2013 at 4:14
  • I'm planning on doing: 0.5" cement board, redgard, then tile. The plywood was just to build out the niche, and provide something that I can seal and tile.
    – Abe Schneider
    Jan 3, 2013 at 13:19
  • Ah, well, that should work. Note that getting a good seal on the corners of the niche can be tricky. They actually make 'niche boxes' that are plastic boxes that provide a more water-tight container. Personally, I built niches into my last tiled shower and regretted it. They ended up being a place for water and soap scum to collect and ended up being more of a pain than useful. In hindsight, I would have preferred to just install a higher quality shower caddy.
    – DA01
    Jan 3, 2013 at 15:18
  • Yeah, I've been doing a bunch of research on building the niches. It looks like a lot of sealant plus fiberglass tape is necessary. As for the water and scum build-up, in theory sloping the bottom and top help with that. I currently have a shower-caddy and think it's more of a pain to clean etc. It's always possible I'll change my mind...
    – Abe Schneider
    Jan 3, 2013 at 16:00
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    yea, the theory is that it'll drain. In our case, the bottles and soaps and such trapped enough water even with the slope that we'd have mildew. If you do decide to go this route, I'd suggest using a solid-surface material for the bottom of the niche (rather than tile it) to avoid having any grout joints on the bottom.
    – DA01
    Jan 3, 2013 at 16:02

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What I did in that same situation was tear the lath/plaster down to the nearest stud and then install drywall. It is very likely that the thickness of your lath/plaster will be thicker than drywall (1/2"). So what you will need to do is shim below the new drywall to even it out with the old covering. What I did was use 3/8" plywood as substrate then drywall over it. Then just regular drywall finish (tape+mud) to make the transition between the plaster and new drywall.

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That mortar is from the "lathe and plaster", and the plaster that sticks through is called a "key". And yes, it is fragile. Sometimes plaster was strengthened with horse hair, but either way it has been there a long long time.

There are plaster repair kits that include glue for toughening up plaster. As a cheap alternative, I have soaked keys in concrete bonding adhesive, which seems to be doing the same job. PVA "white" Glue and water also has been used by others. Mist the plaster keys in water first, so the glue can soak in good and deep.

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