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I am planning to install 5/8 in. plywood underlayment for some floating 1/4 in. cork plank flooring. Unfortunately, there is a hump in the floor spanning three joists, based on my measurement and estimation. So I need to flatten out that hump somehow for the flooring.

I believe our joists are 2x8 in., 16 in O.C. Those three joists appear to be ~5/8 in. higher than the remainder of the joists in the room. The subfloor is 5/8 in. plywood.

I'm wondering how best to handle the hump. Should I cut and rip the subfloor to get access to the floor joists and sand/plane 5/8 in. off of each joist? Should I sand the subfloor down to virtually nothing? Should I just install the underlayment plywood overtop of the subfloor, then sanding the underlayment down to virtually nothing?

My worries for each scenario:

  1. Would shaving 5/8 in. off of a 2x8 beam risk its structural integrity too much?
  2. Would sanding the subfloor down to virtually nothing in the high spots basically destroy the subfloor and it's integrity?
  3. If I sand underlayment down to virtually nothing, how would I attach the extremely thin portions of the underlayment to the full-thickness subfloor that would most likely splinter/split if screwed with wood screws?

I don't think I want to use self-leveling compound for the entire room in case I want to rip the flooring out of the room in the future.

How would you all recommend dealing with this hump?

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  • "The tile sits ~1" higher than the level of our wood flooring." - What does a level say would happen at the tile transition if you shimmed the entire floor where necessary? Put a piece of 5/8" and 1/4" on the hump and the the other end of the level on the tile. If you still need more; thumbs up.
    – Mazura
    Commented Feb 13, 2021 at 22:21

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You should probably understand the root cause of the hump (settling? rest of the floor going down for some reason? built that way?).

Assuming it is dimensional lumber (not modern i-joists), I'd only consider cutting the subfloor up and planing the joists. You won't change the strength materially by lopping off a max of 5/8". You won't have to mess with a paper-thin layer anywhere (which is its own curse).

As an aside, I'm wondering why you opted for 5/8" underlayment. That's thicker than typical, and will make a bit more of a step if everything else is based on the subfloor. Typical underlayment is more like 1/4".

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    Our kitchen, which is next to this room, had its backing board and tile installed on top of the underlayment that we just removed from the den. The tile sits ~1" higher than the level of our wood flooring. The kitchen tile has curvature, so we figured bring the floor up to the tile level, we could just have two reducers leading into the den and kitchen, as opposed to trying to figure out a curved reducer between the den and kitchen.
    – rossy__
    Commented Feb 13, 2021 at 21:15
  • Fair enough... there's nothing wrong with 5/8" underlayment...
    – Aloysius Defenestrate
    Commented Feb 13, 2021 at 21:21
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    root cause, +1. I've never sanded 5/8" off a floor to get a hump out. You either jack the rest of the house up to where it should be, or use shims.
    – Mazura
    Commented Feb 13, 2021 at 22:40

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