Leveling an OSB subfloor for ceramic tiles

Shawn49424

I am getting ready to lay ceramic tiles in my bathroom and was about to begin prepping the floor for install when I found the floor in not level. The floor is 3/4" OSB and is convexed in the center. Over a 4' span, the floor slops a minimum of a 1/4" as you move away from the center of the floor. Other than removing the OSB and starting over, what would be a good fix for leveling the floor? My plan was to use 1/4"-1/2" cement board underlay over the OSB. Another problem I have found with the bathroom floor is that the wonderful builder of the home decided the need for fasteners was nonexistent. The OSB is only glued to the joists. I had already planned to screw the OSB down, but I don't think that is a fix all...HELP

oh'mike

Install your 1/4" Durrock or Wonder Board into a bed of thinset and nail or screw it as per the instructions---then use a self leveling compound to get the floor flat----

I do not recommend Hardi backer for this job--the stiff board may not conform to the contours and leave you with springy voids---

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Seattle2k

Which manufacturer approves their SLC to go over CBU? I'm not doubting you. I'm just not familiar with any that do. Usually, SLC, then thinset, then CBU.

JazMan

You've got it all backwards.

1. Which manufacturer approves their SLC to go over CBU?

They all do. The SLC doesn't know it's over CBU, just apply the primer and go.

2. Usually, SLC, then thinset, then CBU.

I've done it that way in a pinch, but only when the SLC was still plastic and even then I was rolling the dice cuz no real good way to know how far to drive the fasteners. You don't want to let it harden and then fasten, kinda breaks it all to pieces. SLC goes over CBU, not under. Another reason I like Ditra, which is installed once the floor is as you want it.

Jaz

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Seattle2k

Ah crud, you're absolutely right! I was going off documentation for Henry's floor patch products...and even then, I was being too exact. Their floor patch products state they'll bond to concrete, but don't literally state "cement backer board". However, I see the leveler (544) does state it will bond to "cement backer board".

Thanks, I feel a little dumb now!