mongoct:

Thanks for your detailed explanation. I will lay the large slate tiles in the shower and check for rocking.

As far as using slate in the first place -- we have had this slate tile sitting in our basement for a few years with no opportunity to use it. We are now trying to incorporate some of it into our master bath shower, with hopes of using the rest in other portions of the bathroom in the future. I almost feel obligated to use it since it is essentially "free" and has a nice color/texture. But, I don't want to use it in the wrong spot (eg shower floor) if it will cause problems down the road.

cut the tiles you have if they are too large

Thank you for your very detailed explanation, mongo, especially the part about checking for "rocking". When I (eventually) get to redo my master bathroom, I was wondering how I would know the right size of tile. This was very helpful.

As a general rule, 6x6 tiles are about as large as you can do in a conventionally sloped shower. Even then, you may get some issues. It will cost more to have a pro do the quadrant slope that Mongo mentions, (it's a bit trickier to accomplish) but it is a great look for a modern bath. Another choice would be to slope the entire floor in one direction only and use a trench drain. You could use one single slab of (flamed or honed) stone for that, but it would be a headache logistically.

GreenDesigns:

The shower pan has already been poured. I've laid some tiles down on the pan and there is significant rocking, so I am going to go for the 'traditional' 4x4 or smaller tiles.

Regarding the trench drain, I wanted to use a trench drain and build a curbless shower, but backed off due to the cost of the trench drain and floor height issues. Just read a post by Bill Vincent stating that Laticrete has a less expensive trench drain in the works, and realized that my floor is naturally pitched allowing for a trench drain without any structural modifications. If I hadn't already poured the pan, I would go down this route. But, now it's too late.

i have a 3/4" square glass mosaic tile, would it prove to be too slippery to utilize as the floor in a walk-in shower?

Not slippery at all. The grout will provide traction

Slate.....OK....lets talk slate on a floor. My neighbor put down slate on his entire first floor and has regrets. It's uneven, hard to keep clean, hard to walk on.....ever tried walking on an uneven surface....it's very unpleasant. Now imagine putting that on a shower floor.

I do my own tile work and used 2" slate on a pinwheel pattern on a bathroom floor. When walking on it barefoot you can "feel" the slate tiles....they're different than the ceramic....almost bumpy.

Cutting slate. The cut edges will be sharp. You would need to file ALL the cut edges to use them on a shower floor. The labor alone to do this "right" would be huge!

Even with a traditional pan, there IS a way to use larger tiles, but the pan has to be planned with that in mind, and the drain pretty much MUST be centered. The way it's done is the pan is made up of 4 flat planes, broken by a diagonal line from each of the four corners of the drain to each of the four corners of the showers. A continuous joint is then made along those four lines, and what you end up with is four flat "panels" on the shower floor, and the ability to use a larger tile, being that you're using flat surfaces to set your tile on.

What about a single straight grout line at the peak between the two showers, and then slope each shower from there?

I'd go as small as reasonably possible. Smaller = less chance of slipping. I have 2" in my current tiled shower ... and I'd like 1". I would not go bigger than 4" personally.

Ironwood-- I hate it when that happens!