There are some other issues associated with putting a deck on a roof.
Even a "flat" roof is sloped slightly so it will drain. You'll have to
allow the new deck to follow the roof slope or shim the support points
to level the deck. Don't place members on the deck that could dam or
otherwise obstruct water flow as water could build up and flood the
roof. Water weighs a lot and it is common to collaspe roofs if the
water builds up. The added weight of the deck will also make the roof
sag a little more and potentially increase the ponding effect (and
Additionally, depending on where you live, a roof is not sized to take
the additional load of a deck and the added people/furniture associated
with the deck. Typically, a roof is designed for 12 to 20 pounds per
square foot additional live load (not including the existing material
load). A deck could weigh 10 psf plus the live load of 40 psf, well
you can quickly overload the roof. If the roof was designed for a
heavy snow load, then the 40 psf live load won't be there when the big
Lastly, high winds actually try to lift up items off a roof, especially
around the edges and corners. Not tieing down the deck could damage
the roof during high winds and cause a public hazard if it actually
clears the roof parapet.
You may want to contact a local structural engineer is address the load
capacities of your roof before preceeding.