in the upstairs part of our back extension the floor slopes away from you as you walk into the room all the way to the back window; it is like walking downhill! I need to level the floor but what do I use? I was thinking of buying some of that 8 X 2 chipboard from wickes and laying that as the stuff down at the moment seems paper thin, how do I raise the flooing up from the joists though? I am a DIY virgin!