I have 1/2 plywood subfloors in my house and I'm preparing to put a second layer of 5/8 OSB subfloor before installing hardwood flooring. In one of the rooms, there are two big areas, about 15' by 8' each, where the subfloor is sagging between 1/4" and 3/8". What's the best way to level these areas before putting down the OSB?

  • 1
    Best would be to fix what is sagging. Either build up/replace or shim the joists.
    – crip659
    Nov 27, 2023 at 20:30
  • Wood creeps over time. A beam across midspan of the joists can be easy if the space below the floor permits. If the joists are very solid and have just sagged with age (this is unlikely), then spacers are adequate.
    – popham
    Nov 27, 2023 at 21:38

1 Answer 1


A 5/8 sheet of plywood can vary in thickness from 5/8 ( 7/8 to 3/8 ) in my experience. Sometimes the manufacturer misses a layer or an extra layer gets added so 1/4 to 3/8 out is really expected at the rough framing stage. That ignores the variance of joist hangers, dimensional wood, beam growth, expected mid span deflection, etc.

You can use a combination of 3/8" plywood and 1/8" door skin to bring the dipped areas up to the same level as your subfloor. I'd use something like PL subfloor to span between the doorskin/plywood and the surface you are getting to. SLC tends to be brittle and will crack when you throw your other layer on top. If the two layers end up having any kind of allowance between them you can end up with a noisy floor. The subfloor stuff is more of an elastomeric glue that won't crack but can fill minor voids. Avoid cementitious fillers between layers of wood.

Depending on if you like a solid floor or a floor with some give you can also use a minimally compressive underpad. A lot of the engineered hardwood manufacturers will spec a maximum compressibility to an underlay. You could also use rigid foam or cork if you can get it in the thickness you are after.

  • You need a better sheet goods supplier! If somebody sent me 5/8” +/- 1/4, they’d be picking it up again. Nov 28, 2023 at 15:54
  • 1
    @AloysiusDefenestrate I just never thought to even look. It wasn't the whole sheet either so you just end up with a small area of the sheet which is low or high... generally the sheet is 5/8". It wasn't every sheet either so if I knew to look for it maybe I would have sent it back but if you get an order 50 sheets of plywood it becomes difficult to check each sheet... sometimes the individual layers used in the ply have different thicknesses and that also results in differences. Not sure how common the 1/8+- is on a sheet of plywood. Nov 28, 2023 at 18:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.