We have a room approx 13 x 13 with existing red oak hardwood flooring in a 70 year old house. The room has a partial dip in the floor that runs approx 6 (length) x 1 (wide) roughly in the middle of the floor.

My question is what is the best option to deal with the dip and re-floor the floor with unfinished 3/8" red oak flooring to be then refinished. Should we

a) use a compound to fill the dip and the glue and nail the new hardwood
b) use a compound to fill the dip and glue 1/2" plywood sub-floor and then nail hardwood
c) use no compound at all and glue 1/2" plywood sub-floor and then nail hardwood

Suggestions most welcome and appreciated.

Shim the dip with layers of roofing felt. tapering each to get where you need.

Unless it is a real good dip.

Going over existing, you will need to run the wood opposite of the existing wood.