So I ripped up the 3/8 plywood subfloor in my bathroom and installed 1/4" hardibacker using thinset and the correct backerboard screws, my floor still seems a little uneven, around 1/8 of play maybe a little more. I guess its easy to spot as I plan on laying 24"x12" tiles and they dont sit flush on a dry test. Will they be ok once I bed them in thinset or do I need to apply some self leveller first. This is my first attempt at tiling a floor, I have also installed an underfloor heating mat which covers approx 1/2 my rooms square footage.
At what point did you install the heating mat? Generally 1/8" is doable. The thinset should level it out, though self leveling compound isn't outside the realm of reason when dealing with tile that large. Generally you'd want to put your heat mat on top of the backer board and cover with a leveler anyway.
The heating mat is installed on top of the backerboard using thinset. So I guess I either SLC or leave it and try to tile. My concern with SLC is how to prevent it from going through the holes in my floor for the plumbing and heating vent. I did buy some last night but did not pluck up the courage to use it.