Slate tile over wood subfloor

[bobandsandi]

We have a small mudroom/bath with 3/4 t&g fir subflooring. we beefed up the joists (2x6 fir joists max 14"oc, and max span 6'), already did the deflecto calculator and says we are good for either tile or stone.

The floor is out of level and has many low spots. We will be transitioning to 3/4" wood flooring in kitchen. Question is how best to prepare the floor for 18x18 slate tile?

My understanding is that it is more important that the floor is flat than level.
Should we fill the low spots before or after installing the plywood subfloor, and how best to do that? And then what is recommended to install on top of the plywood?

There is a stacked clothes washer dryer in the room that will create vibrations so I'm very concerned about cracking.

Thank you so much for your time.

[cx]

Welcome, Sandi.

Are you sure the 3/4" T&G flooring is the only layer of flooring in that room(s)? It's not laid over some other subflooring?

If your existing subfloor is substantially un-flat, you would do best to remove it all and do your repairs at the joist level. Sistering new joists beside the existing with the new tops in plane would provide your best fix.

There's really nothing you can effectively do between your existing subfloor and the layer of plywood you must add for your tile installation. Any flattening would need to be done atop the plywood if you use a membrane-type tiling substrate, or atop the CBU if you go that direction. Neither will be as effective as bringing the joist tops into plane.

My opinio; worth price charged.

[bobandsandi]

Thank you for your quick response CX.

Yes the 3/4" t&g is the only subfloor. We have missed any window to level the joists; this is an area with very little crawl space access, lots of plumbing pipes and the floor has just insulated.

We were thinking of marking the low spots and using self leveling concrete or some other substance (??) to fill them to bring the floor flat. Any recommendations regarding this approach?

[cx]

OK, if y'all are unwilling to remove the existing subflooring, your only available next step is to install a minimum of nominal half-inch exterior glue plywood directly over your existing board subfloor.

After that, any flattening - keep in mind that leveling is not required unless you want the floor to be level, tiles don't care - depends upon your choice of tiling substrate.

If you plan to use a CBU, you must do all your flattening after that is installed directly over your subfloor with the requisite mortar bed per manufacturer's instructions.

If you plan to use a membrane as your tiling substrate, you must do all the flattening over the plywood subfloor prior to installing the membrane.

There are a number of cementitious patching materials out there made by various of the setting product manufacturers and your selection would be based upon availability and the dept of the needed filling. No self leveling product makes any sense in your application unless you actually plan to level the floor and that requires a good deal more than just filling the low spots.

My opinion; worth price charged.

[bobandsandi]

Thank you again. We'll go with flat, not level.

Do you recommend the CBU or membrane method for our particular situation?

And -- should we install a break in plywood underneath the clothes washer to try to contain the vibrations?

[cx]

1. Yes.

2. Don't unnerstan the question.

[bobandsandi]

1. Does the answer "yes" to question #1 mean you don't have a preference for either backerboard or membrane for our situation, or that you do have a preference but don't want to share it?

2. From what I've read it is recommended to leave a 1/8" gap between sheets of plywood, and cover the gaps with tape. I was thinking to help isolate the vibrations of the washer/dryer, it might be a good idea to have the w/d sitting on its own piece of plywood, with a 1/8" gap between the surrounding sheets. Just wondering if this makes sense or would be a waste of time.

[cx]

1. Just mean you gotta do one or the other and it's your choice. CBU is less material cost but more labor intensive; membranes are more expensive but easier to install.

2. If your second subflooring layer is properly installed, I see no benefit to that.

My opinion; worth price charged.