The pier and beam foundation under the corner of the bathroom was rotten and collapsed. At some point concrete was poured over the entire bathroom. Hammered through the concrete and installed new pier, joist, and plywood sub-floor. There are some pretty major (and deep) gaps where the plywood meets the concrete. How should I approach filling these gaps with the goal of applying self-leveling underlayment and flooring (probably tile).

Thanks for any advice!

 

Your goal should be to make a contiguos floor, not a patchwork of different bases. It is not uncommon to have cocrete mud beds ubder tile. It also is fairly common for failure due to water or rot. You should remove all concrete and rebuild from there.

Give us more info on what the floor make up is. What size joists, on center spacing and unsupportes span. How thick is the original subfloor and how did you perform your patch?

BTW. Welcome to the forums!

 

Floor make-up:

Joist: 2x4
Spacing: 10"
Max span: 5'



Really hoping not to demo the rest of the 'mud'.

 

Is your repair 2x4's or is the whole floor, even under the concrete, 2x4's. 2x4's are not strong enough to be used in a flooring system of any kind let alone one with tile involved.

-Two different substrates will move (expand and contact) at differing rates. Your tile will fail as a result.
-You can not bond tile directly to plywood, it needs an intermediary surface like cement board or ditra. (or in your old case, concrete)
-your rebuilding of the patch to level with the concrete is therefore incorrect even if you could proceed with your rebuild.

 

Just the repaired section uses 2x4s.

Welp, bummer- thanks for your help.

I was hoping to plug the gaps somehow, pour out some self-leveling underlayment, and viola! Maybe layer of uncoupling membrane and tile or....?

 

It is always good to do your research before you begin a project.

Self leveling compounds can not be poured directly on plywood. They require expanded wire mesh/lath be put down first to hold the SLC. Again, the plywood height on your patch will not work. It also is way under built to support any kind of flooring long term.