How should I prepare subfloor for backerboard installation in bathroom tiling project?

Question

A couple questions about installing tile in my second floor powder room. There is tile in our kitchen on the same floor.

1. I assume because of the kitchen tile on the same floor that my engineered floor truss joists are strong enough to handle tile. Safe assumption?

2. I replaced about half the subfloor sheathing due to rotting. The new sheathing looks much nicer and flatter than the old sheathing I left on. How important is it for the subfloor to be absolutely smooth before I lay the thin set + concrete backerboard?

3. I left gaps between the new and old subfloor sheathing (and the walls) of about 1/4 inch (slightly more in a few spots). Do I need to fill these gaps with anything before I lay the thin set + backerboard?

Answer 1

1) Safe assumption. Your floor trusses are carrying the load of the entire room. A little extra tile weight isn't going to matter.

2) You want a relatively level surface but the thinset itself will have enough flexibility to allow your final floor to be perfectly level, provided you take the time to make sure each tile is placed level. If your variance is more than 1/8" you'll need to apply more/less thinset based on the relative height (lower subfloor - more thinset) in order to ensure you can get level. If you want to remove all doubt, a thin layer of self-leveling concrete poured first (seal all holes!!!) will give you that level surface you want.

3) I personally wouldn't have put any gaps between the subflooring as long as the material is the same. However the thinset will fill your gaps - and you don't want to fill the gaps around the outside wall because those do serve a purpose for expansion.

One other note - this is a bathroom floor right? PLEASE apply waterproofing to the floor before you proceed. Water will penetrate grout (unless it's epoxy) and get trapped, and if you don't waterproof the wood then it'll rot.

Also - related to my comment above - you don't need to use backerboard. Backerboard is for wall mounted tile, to be used instead of drywall (it doesn't rot). However, as indicated in the comments below, using it is fine and may in fact be recommended as it's a better surface for water protection than wood when under a floor.

Answer 2

If you have single-layer plywood, you might want to consider Schluter Ditra membrane. It serves as crack isolation and waterproofing:

Designed specifically for ceramic tile and dimension stone installations, DITRA serves as an uncoupling layer, waterproofing membrane, and vapor management layer that accommodates moisture from beneath the tile covering. Further, DITRA performs all these functions while still providing adequate support/load distribution for the tile covering. The combination of these four essential functions allows for the successful installation of tile over a wide range of substrates, including plywood/ OSB, concrete, gypsum, heated floors, etc.

I see from your followup comments that you need to build up some height. You can lay another layer of plywood prior to adding waterproofing, but confirm that you need a full 3/4 inch increase in the subfloor prior to waterproofing, mortar bed, and tile. Schluter does have a thicker version of Ditra called Ditra-XL that is intended to make the overall thickness of Ditra-XL + mortar + tile match up well with 3/4 inch hardwood.

(I have no association with Schluter, but I'm using it in my current bathroom remodel, and my research led me to strong reviews of it at the John Bridge Tile forums, you can find a reference library.)