having thejoint above the gap is bad and really the joint needs moving onto a solid base if there are any other joints on the gap they may well fail in the future. as a last resort when all else fails and the gaurantee is about as usefull as a choclate teapot u could drill a small hole in the board and insert xpanding foam(spraycan with flexi nozal) and fill the gap keeping floor at required hight whist drying and fill hole
mick
February 2007