Originally Posted by Zeus

cmac - I'll add to the advice already given...

I would definitely use a membrane underlayment of some sort to act as an uncoupling layer. I think Kerdi is more for waterproofing...Correct. It has NO anti-fracture capabilities.you may want to use Ditra instead?Ditra is very costly - after all the costs for it and setting materials are tallied to install it. It is very thick as well - as contrasted with Compotite Gold. There are numerous membranes and waterproofers available these days - Ditra has been marketed to death.

Slate can effloresce,Untrue. Slate does not effloresce. It contains nothing that would "effloresce" either. so I would give it as much time as you can before you seal it prior to grouting. If you seal and grout too early, you'll get little bloom marks. I would let it dry after setting for a couple of days if your schedule permits to allow all moisture to evaporate and let the slate fully dry.Not so, if properly using a breathable sealer - one that allows for moisture vapor transmission. Then seal and grout.

Something to perhaps advise your client of -

Depending on the color, the choice of sealant can make a huge difference in look re: wet vs. dry look. The gloss sealant can really darken the tiles and some don't like the wet look.

Also - if you are using a formal color, it will scratch incredibly easily and the marks will really show. I laid a very high quality black slate tile in my previous home and it looked fantastic when done. Very formal. It quickly went from formal to rustic however, as gravel, sand and pebbles from everyday traffic into the home took its toll. We used a semi-gloss sealant and every nick showed up. Not a big deal as we liked the rustic, worn look as it suited our decor. If your client's residence is more formal or urban though, it's something to consider. You would likely have to reseal it periodically to keep it looking formal.You are mixing and confusing topical floor finishes and sealers - without regard to penetrating floor enriching and darkening sealers in the above statements. If the slate scratches - it will do so regardless of the floor finish or sealer applied - topical or otherwise. I also have no idea of what you mean by, "formal color?" There are no "formal colors" of slate, as pertains to nomanclature. Slate is slate, irrespective of color.

Just my $0.02.