Let's see -

1. Will your structure support the weight? Wet soil in planters can get pretty heavy.

2. Will you get sufficient sun for growing things? If the sun is partially blocked by trees/buildings, you may not get enough sun to grow things.

3. Is there adequate drainage (unless you plan on growing cactus)...?

4. How is the area accessed? Do you want to haul planters and soil up a ladder?

5. Where is your water supply coming from? If you can manage some kind of rain barrel, that would be a great source for free water. Just remember, a gallon of water weighs about 8.34 pounds. The smallest water containers I have are 50-gallons and the largest is 212-gallons. That's a lot of weight! We hand water from water barrels, and I carry two 2-gallon watering cans for much of the watering. I also have a solar-powered drip irrigation system, but I still need to haul water to fill the feeder pot.

Just some things that came to mind....

-Grainlady

The roof is fairly easily accessible and I can have water from the house through a hose, I imagine. I could probably also organise the drainage down the gutters, I suppose. There is lots of sun. The real problem seems to be the weight and what you would need to do to support the roof.
thanks for your trouble
bob

There are a fair number of green roofs in cold climates. They are more popular in Europe and some areas of Canada. So the project would definitely be doable project. First you will want to contact a architect about the structure and possibly the construction depending on how handy you are.
What were you thinking of growing on the roof?
Most are planted with a hardy sedum. They require water for a year or two to get established but after that.