I live in a 17 yr. old property where the upper flooring was laid using 8 by 4 sheets of what I call blockboard, probably not its proper name. The upper walls were then fitted on top. What would have been the thickness of this boarding as I need to screw it down in alot of places as it creaks badly when walked on. If I know its thickness I can then get the right length screws. I did contact the builders of the property (Bloors) but they couldn't be bothered to reply.