(bet I had you going, huh?)

It will work, I simply don’t know how good of an idea it would be. You aren’t supposed to install redgard over plywood in a wet area such as a shower. Wood has excess movement, whether or not it’s covered with redgard. So I honestly don’t know how stable that would be long-term. The wood will be waterproof, BUT the expansion and contraction inherent in wood may cause the tile to become de-bonded over time.

Don’t get me wrong – your design sounds very cool. But I would probably opt to find a proper substrate with which to cover with the redgard like cement backerboard. Maybe take a sheet of the backer and attach to it cubes of backerboard to it rather than the milled wood. Cut squares of backer into 1 1/4″ squares and build them up layer by layer by gluing them together with liquid nails or epoxy. Once you get the shape you want coat the entire thing with redgard and then tile it.

It would be best to install the kerdi first, then coat the niche with redgard over onto the kerdi rather than having the kerdi overlap the redgard. I would build the shelf out of backerboard covered with redgard as well. As much as you possibly can use a substrate which water will not affect and does not have inherent structural expansion and contraction like wood does.

I want to make a complex niche with my mosaic tiles, I want the top to have a 3 dimensional pixelated effect to it, picture legos. I’m doing the high-tech look everywhere else, so I think it’ll look awesome. I’m going 3 inches deep, I want to have 3 levels of different shapes, so in the end I’ll have a pixelated looking arch along the top and from the back forward.

My plan is to take a sheet of plywood, get some milled 1.25×1.25 wood and cut it into cubes. Then off that sheet of plywoood, build up one layer at a time, gluing the pieces together. Now cool, I figured out how to make the shape, this would be my tile backer. Problem is – wood isn’t waterproof, I can’t just put tiles on that and be done with it. It’d be hard getting Kerdi to fit in all those groves, and it’d be even harder to put cement board bits in there. I was thinking I could just be really, really, really generous with liquid membrane, then tile over that and overlap the Kerdi from the shower over the niche. Would that be risky?

Second question, I want the shelf to be exactly one tile all the way across. That’s an inch tall, kind of tall, but I think it’ll look better than any alternative I could come up with. The tiles are quarter inch thick, so with the thinset and liner in there, I’d have about 3/8ths of space for the structural support of that shelf. Once again, I could use some wood and liquid membrane it?.. or any other suggestions?