Leveling OSB Flooring for Hardwood Install


After removing carpeting in the family room to replace with hardwood flooring, the OSB has sagging between the joists. The sagging is around 1/4" to 3/8" worst case. The hardwood installer said he can't warranty the install because of the floor not being level.
Leveling compound specifies not to use on OSB. Can the OSB be sealed to use leveling compound?
Is there some other alternative? Replacing the OSB with plywood would be a major effort as the OSB is screwed and glued to the joists.
Appreciate your advice,
Otoe
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
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.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Sure can. Just slop on a gallon of cheap poly varnish (will cover a 25' square room). That will give you better looking sagging floors.

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.