I have a house that was built in 1980 and utilizes a subfloor of a compund I have never seen in a house, before or after this house was built. The material is a gray, almost pressed paper looking material that is a full 1-1/2" thick. The problem is that over the last 32 years, the subfloor has drooped a bit between the floor joists, and the joists themselves may not be the straightest, either. Nevertheless, we are determined to put an engineered flooring in and I am at An impass. I purchased and laid 3/8" plywood over about 250 feet of flooring (1 bedroom and 2 hallways and a bathroom) and began laying my flooring. The plywood did not take out enough of the differences from the peaks to the valleys to make the flooring install correctly. I realized this after installing it in the bedroom, and I pulled it back up to address the subfloor.

The subfloor has up to a 3/8" difference across different parts of the floor. I am considering using a self leveling compound, but posts I've read state that it's not the best on big floors and the bedroom itself is about 150' and it needs it all over the floor. Additionally, There is still some flex where the plywood did not completely adhere to the original subfloor and I fear the self leveling compound will break up over time.

I am at a standstill and completely clueless as to what to do. I have already committed several sheets of plywood down with glue and nails, so ripping out the original subfloor and replacing it is a bigger task.

Any thoughts or directions would certainly be appreciated.

Thanks,
Mark