Occasionally a project is presented where a deck will be built over a flat roof of a house or a garage.  First you must make sure the roof is strong enough to support 55 lbs per square foot.  Next you will want 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 on their profile every 16” on top of the roof surface.  This will provide the surface for you to attach the decking.  In most cases your “sleepers” will float on the surface of the roof with only the weight of the deck to hold it in place. 

Make sure the deck slopes slightly to promote drainage towards gutters or at least away from the house.  Always check with the local building inspectors and obtain a permit before building a rooftop deck.