Wood – How to fix this broken floor

This is an educated guess only because I can't really see or feel what you are describing. This may be old floor wax that has had something damp left sitting on it. Often in older homes folks would use old fashion floor waxes that will discolor or turn white with age, water, friction or heat. You may first try using a little mineral spirits or a mild cleaning agent with a polishing compound, such as Bar Keepers Friend with a soft cloth on a small spot to see if it will remove the white residue. If in fact this floor has been treated with a wax product, the entire floor would have to be stripped of wax, perhaps buff sanded and restained, then protected with a few coats of Urethane. Don't attempt to apply urethane until you have determined what is causing the white waxy residue. Putting any coating such as urethane over a waxed floor will bubble, peel, and create a mess to clean up. another test you could do would be to gently scrape a very small section of the white stain with a putty knife. If the material comes off easily it may be wax. it may be old varnish, but that would not feel waxy, but not out of the question. Let us know what you find, and we can go from there. Good Luck

Plywood to build up the wood floor height is an appropriate way to address this problem. For use under carpet some regions may allow use of other products for this purpose such as particle board but plywood is still the best.

Installation of the plywood also gives an opportunity to firm up the floor and work at eliminating any possible squeaking in the old floor. It is best to screw down the plywood into the original floor joists. You can probably find where these are by looking where the old boards were nailed. It can be handy to apply some masking tape to the bottom of the adjacent walls and marking the location of the joist center lines so that after you lay down the plywood it is easy to locate the joints and snap chalk lines across the plywood for a screwing guide.

You may still need to do some special work at the plywood to concrete transition zone because there may still be some minor height variation after the plywood is installed. This can be handled by troweling in a layer of an appropriate floor leveling compound and feathering it out away from the transition zone.