SIPS floor over concrete plinth

[WWilts]

Installing SIPS panels (floor, walls, roof) for 3mx 2.5m garden room.
Advice is to allow the SIPS floor to be ventilated underneath.
Concrete slab in place already.
Height limit overall 2500mm above ground level (which can be altered within reason). Wall panels 2400mm. 

Q. What is the minimum gap below the floor SIP for ventilation purposes?
Would pieces of cement board under the ground frame/sole plates suffice to raise the SIP floor slightly? Is cement board sufficiently waterproof for this use?

Detail would be (ground up)
Concrete slab
DPM
Cement board
Frame & SIP floor