Hi...Live in Toronto, ON, Canada...
Have a flat roof on top of my garage.
Rough size is about 450 square feet. (It is not sqaure or a perfect rectangle) Would like to build a deck up here.

Before I call a contractor to quote me on it, I am just wondering if someone could give me a rough estimate of money....US or Canadian. I am thinking pressure treated, with privacy lattice 6' high on 3 sides.

Any rough ideas? I would have a contractor build it...don't want my own first attempt at deck building to be this high off the ground.

Thanks,
Derek

 

First off, if you build over a flat roof over what is apparently a double garage, how do you plan to repair the roof afterwards? Flat roofs are notorious for leaking.
I built a self-designed enclosed boathouse with an overhead deck on it in 1995. I built-in a slanted roof of aluminum roofing sealed at the edges with silicone caulk and roofing cement (except low side drain edge, of course). Then I built a 12' X 24' deck suspended above that. That is o.k. for a boathouse, but I wouldn't recommend it for a garage.
If you really want a deck above that garage, you will have to have it built up over a new a-pitched roof of aluminum roofing (won't rust), or some other permanent, maintenance-free material. Or build the deck above with about a 4' "crawlspace" under it for roof repairs. Where there is a will, there is a way. Ask around about good contractors, and let the best that you can find help design what you want.
My wild guess is that this project, done right, would cost you about $35 US per sq. ft., turn-key, at least. Good Luck!