I have a 110 year old farm house which I recently bought. The whole house was covered with carpet, and I ripped it all out. Underneath was 1/2" particle board, which I ripped out. Underneath that was the original pine floors. The floors were only ever finished on the edges, and the middle of each room was unfinished (assuming they had a rug in the middle). There are a lot of nail holes from where the particle board was nailed in. There is no sub-floor under the pine floorboards, they sit right on the joists. There are a few spots missing where air registers or something used to be. The floor sags badly in the center of the house (approx 1.5 inches), which had been leveled out by the particle board so I didn't realize this when I bought the house.
I am considering either refinishing the pine floors, or replacing them with solid oak floors. I would really prefer oak because I like the way it looks and I am worried about the durability of the pine. I have dropped things on the pine and it dents very easily. I am also worried that if I get a dog, it will scratch the floor up. I have had a dog before with oak floors, and it did not scratch.
I have found some salvaged oak floors from an old house that I am considering using. I could either remove the existing floor, put down a plywood subfloor, then lay the oak floor. Or I could put the oak floor right over top the pine floor, which I think would be easier. I need a way to make the new floor level. The particle board which I removed had plaster of paris poured underneath it in the low spots of the floor. I have also seen products advertised, which look like a rubber sheet that goes down under the new floor which is supposed to reduce creeking.
I just posted a similar problem.
After removing the carpet, I just refinished the pine. Works, but becomes more squeaky over time as some of the tongue and grooves crack, then deflect under bodyweight. And the dog does scratch them up- adds a nice distressed look .
Thinking you need some plywood and levelling compound between the flooring layers, but maybe someone who really knows will chime in. Good luck.