First you must make sure the roof is strong enough to support 55 lbs per square foot. Next you will need to install a watertight EPMD rubber membrane over the top of the roof deck for protection against water infiltration in place of roofing materials. You will then install “sleepers” or flat 2x6’s laid flat every 16” on top of the roof surface. This will provide the surface for you to attach the decking.
There are proprietary pedestal systems that provide a very stable, level and serviceable deck over a roof membrane, though they are relatively expensive compared to conventional methods.
In whatever manner a deck is constructed over a roof, maintenance for cleaning and repairs of the membrane below is recommended. Deck “panels” should be constructed that can be lifted with minimal effort or damage. The roof below should be cleaned and inspected at whatever intervals is found necessary for each deck. A deck located under pine trees will need much more maintenance to clean out debris than a rooftop deck above a high-rise.