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.
One factor that could affect it, though: Most
vanity lights will wire inside the fixture, with just
a wire coming out of the wall. However, if you
have a junction box then you should either move
it out or fill the gap around it with non-flammable
material. (Maybe tile and/or thinset.) Plywood
shouldn't be exposed to possible sparks in that
1/4" space between the old box and the new light
| The rundown: Old building, Bathroom fixtures mounted to plywood and
| tile cut around their footprints (why? I have no idea). I will be
| replacing fixtures, but mounting on top of tile this time, so I'll
| need to fill in discrepancies in the footprint. I have not been able
| to find matching floor tile, but I do have a couple pieces that should
| be sufficient for visible areas.
| Someone recommended that I use quick-setting concrete to fill in for
| 1/4" thick tile -under- the fixtures (not visible). Any
| recommendations on brand/type of concrete? It would be great if it set
| in a couple hours.