stevie22 literally told you what to do, and yet you've just asked the same question anyway??? "If your joists are solid leave them alone and pack them up. You say you're 95mm low? Your 2x4s will measure 95 so that fills the gap: either lay them along the joists or cross them. Crossing easier as you can make sure your centres are spot on to suit your boards. This really is a no brainer."