Redoing a master bath. Right now it has OSB Subfloor down. The floor varies in how level it is some parts are 1/4" to 3/4" off.

I was planning on putting down 1/4" HardieBacker board then pouring self-leveling above this so the tile floor is level.

Is this the correct process? Should I look into a different solution to get the floor level?

  • Is the floor wavely or a flat slope? Floor finish covering don't care about flat slopes, you might if it is too much. Dips, ridges, waves do need leveling. Would place levelling stuff before more flooring, imagine 1/4 inch will not like dips.
    – crip659
    Apr 13 at 14:10
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    Is the room such that you can lift the subfloor and repair the framing? That's the right fix, IMO.
    – isherwood
    Apr 13 at 14:52
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    Tim, my primary concern is putting wet stuff down on OSB. You'd want to put down poly first. I don't consider running screws through the leveler to be a problem, and I'd rather bond tile directly to backer board, but as I said I'd rather avoid this situation altogether. Please update your post to provide more context, and add photos if possible.
    – isherwood
    Apr 13 at 15:02
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    @Isherwood thats a good idea for sistering the joists to get them level and then just installing new subfloor on top then the self leveling wouldn't be needed.
    – Tim Hoste
    Apr 13 at 15:08
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    If you need to attach the backer board through the SLC, then use long enough screws to go all the way through the subfloor and into the joists. That way, even if the SLC cracks a little, the screws still have solid purchase in the joists. You'd probably want to do that anyway. I'd agree with @isherwood that leveling the joists would be the best option and think he should write that up as an answer.
    – FreeMan
    Apr 13 at 16:04

1 Answer 1


You want your floor flat - levelness doesn't matter too much.

I assume you have a transition in the doorway to handle any changes in room to room level.

The professional answer to this question is to flood the entire bathroom with enough SLC that it slightly covers the high portion of the floor. This is the most materially expensive option but the easiest, least labor intensive, most fool proof and highest profit (for a contractor). You'd need to use the primer for the SLC you choose.

An option, as isherwood points out, is to pull the subfloor and sister the joists and get the joists perfect then re-install the osb. If your subfloor has been glued with construction adhesive and is nailed down and is under your walls, I recommend against this approach in a small room.

The more labor intensive hybrid approach is to find the high spots, find the low spots and do a combination of filling and sanding. I suppose you could also supplement this and remove a high area and shave down the joist and re-install some osb instead. It depends on a few things.

How big is the bathroom?

how thick is your osb?

are your joists i-joists or standard spf? or do you have psl lvl etc.

are you planning on LFT / what size are your floor tiles?

why are you planning on 1/4" backerboard over the osb?

I've done 3/8" plywood on top of osb and then added two lifts of SLC (with primer in between). You can drill through SLC without cracking it. Are you doing a crack isolation membrane in between your tile and your subfloor?

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