Leveling OSB Flooring for Hardwood Install

That is one of 2 reasons why I would never buy a house with OSB floors. The other is.... pour some water on it and watch it grow.

Is the ceiling below open? If so, I'd put cross bracing in place between joists. First jack the osb back in place, nail in the cross support. I'd also consider an overlay of 1/4" plywood on top.

A 1/4" sag is quite a bit.

That's almost to the point where many folks would consider tearing it up and replacing it with "real" plywood.

If you are trying to do it on the cheap, you CAN "seal" the OSB. "Sanding sealer" might work. Insofar as the OSB is porous, the sanding sealer would penetrate. I have become a fan of sealers/primers that use denatured alcohol as a solvent: they make a good base, clean up is easy, and they DO penetrate.

Using denatured alcohol as a solvent makes it dry quickly. And then you slap on your portland cement based leveling compound. The sealer will keep the water away from the OSB until the compound sets and the water is gone.

Sure can. Just slop on a gallon of cheap poly varnish (will cover a

Congratulations, You knew all along what the right way to do it is. Leveling compounds can be more of a nightmare than people realize except in relatively small areas. Unless you are absolutely sure that the floor is perfectly level wall to wall except for the sags, then rip rip out the OSB and get flooring grade plywood down. When the OSB is gone, an angle grinder will take care of stubborn screw remains and adhesive and let you true all the joists so that your new floor will lay beautifully flat. A further advantage is not having to trim door bottoms and dealing with other mismatched levels from the surface build up. HTH

Joe