I've just had to cut out and replace a section of chipboard type
floorboarding that had started cracking up and pulling away the T&G (for
some reason the join line was not ON the joists, but just off them. On
cutting out the failing section I found that the floorjoists had a crossed
couple of battens between them.
http://www.astd60.dsl.pipex.com/UnderFloor.jpg

Is this typical for upstairs in a 1980's house?

OG