A Cabinet as a Shower Wall?

Crazy as it sounds, this question is real: In a tight-space remodel, how to build cabinets so their back side can be tiled and serve as the wall of a shower. Read and learn. May 6, 2007

Here's a new one for me. In cramped NYC bathroom renovations, an architect is requesting that I build cabinets which will double as a substrate for a build-up of cement-board, thin-set, and ceramic tile for a shower stall!?! Every fiber in me screams against doing it, but the architect is a very good client and I want to give it every shot that is reasonable (if there are any), or if not, give good reasons why not.

The reasons against include:
1) Flex in substrate (cabinet side!) will lead to grout cracks, moisture penetration... Even if I use Extira, not a happy detail.
2) Cabinet side will be extremely unbalanced, creating more movement, more cracking.
3) As a shower stall, and the side that will be sprayed upon, it will be exposed to all kinds of hydraulic effects, as well as repeated high humidity.

Some remotely possible precautions:
1) A thin impermeable moisture barrier?
2) Steel angle reinforcing for rigidity?
3) A "spoil wall" of an Extira panel, which the cabinet then slides against, fastened for some rigidity?

Has anyone dealt with any related condition before? Has anyone invented a round thing called a wheel?

Forum Responses
(Cabinetmaking Forum)

From contributor R:
I would wrap the back and sides of the cabinet with a sheet of shower pan rubber and then I would make sure the grout was unsanded, and that the grout line was 1/16'' wide max. Then sealed with a 2 part epoxy sealer. Instead of the shower pan rubber, I might use melt down rubber roofing on the exterior of the cabinet.

From contributor P:
Why couldn't you just build a stud wall and turn the studs sideways, same as in an old plaster wall? Only be an extra inch and a half or so.

The tile installer can use a moisture and isolation membrane underneath his Hardee board or Duroc or whatever you use up thataway. That plus a good silicone caulking will pretty much take care of any leakage I would think.

From contributor E:
I don't think this should be too bad. A good tile installer will make sure no moisture can wick through. After all, many tiled showers are attached directly to green moisture proof drywall with no Duraroc. Is this cabinet going up to ceiling or a standard cabinet height? As far as grout goes, ask them to use a two part epoxy grout. Spectralok is water and stain proof and never needs sealing. It should not crack.

From contributor J:
Schluter makes a product called Kerdi that will make the shower totally waterproof. It can be used over drywall, so I'm sure your cabinet will be safe.

From contributor A:
There was a time in history when they didn't use cement board. It did not exist. Unfortunately it was common in the Northeast to bond tiles directly to plywood. The shower in my house was done in the early 50's. I have no idea when the grout line failed and water got into the plywood. But here we are in 2007 and my wall is slowly rotting, yet the tiles are still hanging on for dear life. It would be better for this thread if you could provide more details.

From contributor W:
Thank you all for responses - you've almost changed my mind. Now I'm leaning toward trying it with a membrane. I'm going to look into Kerdi. Turning studs sideways or using 1 5/8 metal studs has already been ruled out as too space-consuming.

Below is a section. As you see, there is the added complication of a niche.

Click here for higher quality, full size image

Plan... obviously full height.

Click here for higher quality, full size image

From contributor O:
Kerdi is nice to work with. I haven't used it in this exact application, but I believe it would actually go on top of the CBU, so it would not be pierced by fasteners at all.

From contributor A:
Nice thing is that your cabinet is more or less buried. I've done a few fiberglass sailboat interiors over the years. On the racing boats, they try to save weight wherever possible. Bulkheads on regular boats are typically marine plywood glassed into the hull. To save weight they switch to fiberglass skinned balsa cored panels (the come in 4x8 sheets). It's very cool stuff. Half the weight, twice the stiffness, four times the price.

These panels would be ideal. However, you've got one small project. If I were in your boat (shower stall), I would build the cabinet side or wall (if you can put another sheet good on the inside of the cabinet) out of 3/4" marine grade plywood. Then I would skin the outside of the plywood (under the cement board) with one layer of 10 oz fiberglass cloth with epoxy resin. Wrap the cloth right under the bulkhead where it will land on the floor.

This whole procedure might take an hour at best, cost $25, and be totally waterproof. I would encourage the tile guy to bond the cement board to the bulkhead with a waterproof construction adhesive. I would sleep soundly at night with the glass/epoxy method.

From contributor T:
Epoxy resin sounds like a fair solution, or FRP or a phelonic panel towards the wet side. There are also cement boards that are watertight but I forget what brand. One of the easiest I think might be the fiberglass resin and mesh after the boxes are installed. After you get the boxes sealed, you could epoxy a few nailers into place for the tile backer to be attached to.

From contributor Y:
Cementuous backer boards need to be fastened with large head 1 1/4" screws at specific centers at edges and in the field. I would be very clear that you're not to be held liable for any future problems.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor G:
I've been working on my own shower that is in or near wood. Kerdi is a basis for things.

I did a couple of tests, one with PL Premium (a polyurethane construction adhesive) and one with a contact adhesive meant for outdoor carpet and Kerdi. It was especially challenging since the material I was gluing the Kerdi to was aluminum sheet.

In a small test swatch, both seemed to work fine. The PL Premium took at least three days to cure, as the two covering surfaces didn't contain water and were not porous.

In a larger application, it became apparent the solvents in the contact adhesive were swelling the Kerdi. I would not recommend using Kerdi with contact adhesives for this reason. Since I didn't want to replace the section in question, I ironed the Kerdi smooth (using an old cotton sweat shirt to protect the iron). Since I am going to be applying T&G cedar (impregnated with epoxy (CPES actually)), I don't think the few weaker bonding sections are going to be a problem.

The PL Premium was used on one corner strip (from the Kerdi shower kit), and it seems to work fine. I also used it for the interior corners which come with the kit.

On the Kerdi side of things, I think the instructions are not quite in the correct order, and are a little vague. But that isn't a wood thing.

I am going to route a "pool" in a 10 foot 2x6 so that I can partially immerse the 8 foot cedar T&G that I am impregnating. I am using Smith's warm weather formula at slightly lower than recommended/expected temperatures so that I get longer curing time. Being cedar, I am told I need 2 applications (at least) to work properly. Once in place, I am going to clear coat with something. No UV, so I don't need to protect the epoxy from that.