I am installing laminate flooring on the 2nd floor in my home. In the back bedroom I discovered the subfloor was both unlevel and wavy. Also in some spots it would depress about 1/8" under weight. I tried to screw it down tighter, but the "studs" underneath didn't seem to want to hold a screw tightly. Finally I pulled up the subfloor to find out what was underneath.

Under the floor are pre-fab OSB i-joists. None of are level with each other. I'm trying to figure out a way to get them level so that I can install a new subfloor. I was thinking about trying to sister 2x4s alongside them, but I don't know the best way to attach anything along the sides.