On Aug 20, 9:58*am, Otoe wrote:
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?

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.

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.

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