Hi all. I know this post is a little old, but I found it in a Google search and after reading it; I thought I could shed a little light on the questions.
I'm not a builder, but I am a professional licensed roofing contractor. As you can imagine we get alot of calls about roof decks, and not to build them, but to fix the leaks that the roof decks cause. A roof deck is not a bad thing! Placing a $10,000 deck on a cheap or old roof is a very very bad thing. I'll try to share some of what I've discovered so you or others don't make the same mistakes I've seen. I couldn't tell you how many home owners I have had to give the bad news that their couple year old deck has to be destroyed to replace the roof, or that the simple 15 minute repair will require a partial rebuild of their brand new luxury roof deck. An ounce of prevention is worth a pound of cure.
First anyone considering a roof deck in the Chicago area, or anywhere for that matter, should be aware that a "regular" roof is not designed for a deck nor for a garden. The roof must be properly designed and built to receive the deck. What does that mean? Well, don't even consider placing a deck on an old roof. The new roof should be a premium roof, no 10 or 12 year rated roofs. By the way your roof is a 12 year rated roof, a smooth silver coated modified bitumen, and I do see some installation error in the photograph. A high density cover board should be installed beneath the roof, and installed without fastener. PVC is a premium roof membrane, and is my roof of choice beneath a garden or deck. I prefer a premium membrane because there is less chance it will fail any time soon. People always buy the bling and over look the roof, the roof is more important than the deck! I can and have written multiple page articles about the proper design of a roof for a deck, this is just the summarized version.
Next is the deck. The construction of a roof deck is not the same as a patio deck, or a deck on the ground. The roof deck should be built modular, so that is is easily taken apart. This is because the roof WILL eventually leak. The only difference between a good roofer and a bad roofer is the amount of time it takes fort he roof to leak, but we roofers have a saying and that saying is; "Water always wins". So the easier it is to take apart the deck without destroying it will save you money on cost of ownership later on down the road. This includes the use of screws, not nails, and includes being built modular in such a way that it can be taken apart, just think of the Chicago Bulls stadium floor. They also play ice hockey (Chicago Black Hawks) in that arena. The floor is taken apart constantly and put back together constantly.
Then there is structure, load capacity, was the roof built to support the weight of the deck? What about code issues? Chicago permit approval requires engineering prints and approval. Chicago code requires two entrances/exits to the deck for fire purpose.
Where the roof meets the deck, the roof should never meet the deck! Always have pads, typical walk way pads are sufficient to ensure that the deck never touches the roof. The weight of the deck can and will cause punctures to the roof. There's alot more to it than that, alot more than I can share in this post; but if anyone has questions feel free to reply as I have subscribed to this thread.
Reliable American Roofing