Building a deck over a flat roof is very similar to building a deck over a concrete patio. These decks are often referred to as “floating”, as they are not secured in place as a stand-alone structure. They are merely “placed” on top of another structure.
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 flat roofing system over the top of the roof deck for protection against water infiltration. You will then install “sleepers” or flat 2×6’s laid flat every 16” on top of the roof surface. This will provide the surface for you to attach the decking.