Aluminium plates are designed and made to be placed on timber joists and then secured by screws or nails to the joist. The plates have 16mm preformed grooves in them, which are generally set at 200mm centres to house the 16mm pipe running centrally up and down the joists, which typically are at 400mm joist centres.
The plates are for heat distribution only and are not structural. They normally cover approximately 80% of the floor area and should never touch each other, as they expand when heated and can create noise. The plates are only laid under straight runs of pipe, not the loop bends.
A standard installation for this system assuming the joists are deep enough would be to fix battens to the sides of the joists (about 70mm from the top of the joist) then a 50mm rigid insulation is cut and placed onto the battens, the pipe work is then clipped directly into the aluminium plates.