In this house there is no sub-flooring, instead the tongue and groove flooring was laid right on the joists. It's not in the best condition so I'm putting down a layer of OSB which will stiffen up the floor and give me a single, level layer throughout the entire house. The process is simple, the material is trimmed to fit, cutting any holes for floor registers and nailed or screwed to the existing floors. I also deal with an interesting problem of a tree growing where a tree shouldn't grow.