In a previous post I mentioned that I had my living room down to the studs and subfloor. Well there is a main beam running through the house and it looks like the main beam was most likely nailed together unevenly, so when the subfloor was placed over it, it wasn't exactly flat compared to the rest of the room. I have a carpenter doing our windows and this was his theory as it doesn't appear the house is settling around it (1940's built). We originally had a layer of .25" plywood with parquet glued to it. I cut out the .25" and parquet to expose the .50" subfloor. My plan was to add another layer of exterior grad .25" plywood (glued and nailed) on top of the subfloor, and shim any gaps with asphalt shingles. A floating engineered wood floor would then go on top of the .25". Any thoughts or concerns about my method?