On Tuesday, April 8, 2014 9:20:04 AM UTC-4, Mayayana wrote:
You want to build up the plywood backer by 1/4"

behind the light fixture? What's wrong with a piece

of 1/4" plywood? Or even loose tiles lightly glued?

Unless I've misunderstood, I can't see why you're

thinking about any kind of mortar.



Probably because it's a small area, sounds like just a few
tiles, he has to set the tile into something anyway, so why
not use the product that is going to hold the tile to fill the 1/4"?