Subfloor issues

[Eggman73]

I am preparing to lay large format porcelain tiles. My problem is that the subfloor is uneven/wavy, along with being only 5/8" OSB. My thoughts are to pour on a self leveler to get it flat and level, followed by a layer of 3/4" OSB for strength, then 1/4" hardiebacker. Any thoughts/suggestions?

 

[C.J.]

I don’t like that idea. 5/8” has flex and you’re prolly gonna be pouring the leveler thin. Then you’ll be nailing or screwing through the leveler? Too much possibility of the leveler cracking up.

Where and why is the subfloor uneven? Is it because of dips between the joists due to the 5/8” subfloor? Are the joists in plane with each other or are some high and some low? Got any pics with a level or something straight laying on the subfloor for reference?

 

[Eggman73]

For the most part, the joists are uneven. The floor is high in the (almost) center and continuously dips towards the ends. On the one side, it dips down to 3/4" lower over a run of about 16', starting at the high spot. The other side dips about 1/2" over a 10' run. I was considering adding varying thicknesses of plywood to make up the majority of the difference and then using the leveler followed by 3/4" plywood once I have a flat and level surface. Any other thoughts on how to fix this?

 

[Mark Brown]

Leveler never goes under plywood, that is a recipe for disaster. Fix what you can with sanding and shims, roofing felt, shingles... whatever you can. If it's really bad you can rip 2x4 wedges the length of the area you want to level and then install on those as if they were the joists, lay them on the joists FYI.

Then after a good 3/4 layer of ply if something is still off would be the time to level the floor. I cannot see how it would be warranted but then again who knows.

 

[Mark Brown]

Me again... if you were going to pour a self leveler then there would be no legitimate reason to use hardi-backer, not that I can find a real reason to ever use that stuff but something like Liquid Backer Board from Ardex could kill two birds with one stone and be a lot less work. The best part is you can use it over exterior rates osb panels.

 

[rugaddict]

Osb is junk—- especially for tile—-leveler on top is even worse—- the only safe flooring is carpet