you can use fball Styccobond B93 that will do the job the only problem with glueing over underfloor is that if you have a problem with pipes the floor will take some getting up also it will not be useable again
all depends on what flooring you are putting on it solid would doesnt need it as you can secret nail to the floorboards below laminate or engineered boards i would recommend 6mm ply screwed at 6inch centres
The easiest way to do this is to put a board on top of the last row then take another board and mark it from the wall onto the other board as per image [img]http://img2.timeinc.net/toh/i/a/interiors/flooring-00.jpg[/img]
