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top three things to keep in mind:
Load (wet soil is very heavy - this can damage your rooftop. More so than any meandering roots. KNOW what your roof can handle in lbs. per sq. ft. We found this by referencing blueprints)

Drainage (from the container to the rooftop and off the rooftop. Sitting water will damage your roof)

Plant choices that are drought and heat tolerant (you might want to look into Steppable line of plants vs. grass)

a more comprehensive guide from chicago gov:
http://tinyurl.com/2habsz

Also keep in mind that you want to some sort of drain screening system to prevent soil or other sediments from building up in your roof drains. A partial or total blockage of one or more drains due to sediment buildup could turn your lawn into a muddy wading pool at best or worse, cause an incredible flood disaster.

In general, green roofs do not use soil as the planting media. You would be using a lightweight mixture of expanded shale and other minerals and organic matter. Each green roof manufacturer has their own proprietary mix, so you'll need to do some research on weight, nutrient loads in runoff water, filtering abilities, etc. You will want an 'extensive' green roof, with the planting media less than 6" deep, as the roof structure was not designed for heavier loads.

With an extensive green roof that will be walked/played on, I would suggest using native grasses which can switch from drought-tolerant to moisture-tolerant (they're called CAM grasses, with the other grass types being C3 and C4) rather than succulents, which will not stand up to pedestrian traffic. Trees and shrubs will not do well in an extensive green roof, unless they are in pots above the soil level.

Keep in mind that if you want your roof to look green all summer, you will have to irrigate it. Green roofs are not a catch-all solution to every problem of water quality, runoff reduction, and aesthetics, so you'll have to make some trade-offs.

Also, many of the modular green roof systems are not designed to be walked on, so you should probably go with a monolithic membrane and a single layer of soil. You should definitely consult with a green roof expert, who can give you much more advice and tailor a green roof for your specific situation.

I hope this helps!

Also, the Lady Bird Johnson Wildflower Center at the University of Texas at Austin has been running a research project comparing different types/manufacturers of green roofs for the last couple of years. I'm not sure if they've published any of the research yet, but it's one of the only green roof research projects out there, so it's worth looking into.

We have a green roof and have been growing vegetables for the past two years. We have a drainage layer and root blocking membrane on top of the roof, below the soil mixture. The rootbloc works as a chemical barrier - when a root comes into contact with it, the root slips and spreads laterally instead of vertically. We are even growing grapes which have woody vines and so far don't have any problems. We chose Henry brand products. We recently harvested 43 pounds of tomatoes in one day! Here are some pictures of the garden: http://www.flickr.com/photos/pedalpowered/sets/72157621970823615/

You can't just plant grass. Roofs are too hot and it won't live and will be too heavy. The light soil needed is also loamy and doesn't hold water really well. You need drought-resistant plants. Finally, there are about 7 or 8 layers to a greenroof. I documented the layers and construction of our greenroof here:
http://www.flickr.com/photos/81911961@N00/sets/72157605854175233/

the root systems of native grasses are several feet deep - too deep for an extensive green roof.

'Native grasses' don't necessarily mean the deep-rooted Great Plains prairie grasses like big bluestem and indiangrass. What is considered 'native' varies by region and sub-region, and some do indeed have shallow root systems. The best thing to do would be to get in touch with the agriculture department of your local college or university, or cooperative extension, and ask them about native plants for your situation. Generally, natives are best suited to the climate and microclimate of your region, and are less likely to need extra irrigation, but you may find other species that will work (but do your research to make sure they're not invasive). A good green roof designer should know appropriate plants for your area.