cbu is a better substrate – and you can use it if you want (and it will help you sleep at night), no problem at all. Any penetrations through the kerdi needs to have silicone filled into the hole first, then the screw. That’s true with drywall or backerboard.

The maximum size you want on the shower floor are 2×2′s. Anything larger will be slippery.

Not sure about the dog flames and unicorn farts – I’ll test it in the garage this weekend if I can get approval from the wife.

I absolutely agree about new construction! Well said.

You want to silicone the vapor to the back of the backerboard to create a ‘dam’ around the valve. When water runs down the barrier it will hit that bead of silicone and run around it rather than into the wall cavity. As you install the tile you want a bead of silicone around the valve sandwiched between the tile and backer – same reason. Those coupled with the escutcheon seal prevent any water from getting in there.

I’m starting a shower remod and boy do I have some questions.  I have demo’ed the walls back to the studs (16″ OC) and will be building a shower where a tub once sat.  I still need to move the back wall out to enlarge the shower footprint.  My vision is to install 12″ x 12″ marble tiles  with small glass or tile mosiac accents over Kerdi membrane and a drywall substrate.  Smaller marble pieces on the ceiling and over a mortar shower floor.  And this brings me to my first question, I know that the manufacturer recommends a drywall substrate under Kerdi, but really? I’ve been married for 20 years so I obviously know how to follow directions, and I am confident that I can create a watertight barrier.  But, I could “what if” this to death, when the frameless glass surround is installed the mounting screws will breach the Kerdi, if water gets through to the drywall… game over!
Why wouldn’t a c.b.u. substrate be a better medium? 

The new back wall to be framed will be 2X6′s, again 16″OC.  I am going to build a niche in this wall and for aesthetic reasons would like it to be wider than the approximate 14″ between studs. Framing is no problem, but with a plywood backing (5/8″ or 3/4) can I run this niche out to 20-22 inches?  My concern here is flex and instability due to the unsupported width.

You steted in other print that you liked Marble, as do I.  Is marble suitable for the floor tile if I keep the pieces small or can it be too slippery?  How often does it need to be sealed? 

And finally… if a dog bursts into flames, will a unicorn fart put it out?  I’m just sayin’ 

Thanks a bunch, 
    Dumas