Starting with a flat subfloor is essential for a good wood floor installation. Older homes often have areas where the subfloor is irregular. Refasten any areas of loose subflooring. It is sometimes advisable to renail the entire subfloor using ring or screw shank nails. Renailing can also be needed in new construction where the subfloor was left exposed to the weather.
A great wood floor installation begins with proper subfloor prep…
Generally the subfloor should be flat to within 1/4″ or less over 10′. Sand any small high spots flat. Small depressions can be filled with layers of thin plywood. Cut the plywood to progressively smaller pieces (like a contour map) then feather the edges by sanding. Plaster based floor patch is not recommended for a good wood floor installation.
If the floor joists have sagged in an old house removing the subfloor and sistering new joists to the old ones can be a good solution. Adding stiffness to the framing is better than reducing stiffness.
If you are not doing a large area and the plywood is sound you can use 2x4s turned on edge. Cut them to follow the floor contours and create a flat top. We have used a metal rail system and a router to trim the tops of the 2x4s to a flat plane. Install the floor as you would over a sleeper system. This is still a lot of work but worth the effort for a successful floor for the longterm.