Sedums are among my favorite plants. I'd like to suggest something though -- consider growing a border of hens and chicks (sempervivums) around the edge of your roof. You could even create something like a checkerboard by using blocks of sedums alternating with blocks of sempervivums.
Here are some websites with groundcover type sedums available -- just search under "sedum"
http://springhillnursery.com/
http://www.bluestoneperennials.com/b/bp/index.html
You can frequently find low growing sedums at local nurseries in 3 inch pot size. Many of those pots have more than one plant in them. Sedums are EASILY propagated by just taking a stem cutting and inserting it in some slightly damp potting soil until it forms roots. Once you have them, you won't be able to get rid of them easily. You could buy the flats now and propagate some additional plants this fall. Then keep them protected over the winter to plant next spring.
Hens and chicks are supposed to protect your house from lightning strikes! You can find large pots of them at most nursery centers.
Don't worry too much about getting "named varieties" -- just find one you like and buy a few flats of it for now. I also suggest waiting until spring to start the roof. It's pretty hot right now and even sedums need some moisture!
I love the sedum that looks somewhat like evergreen leaves. Don't recall the name but it's easy to find.
Sedum spurium 'Dragon's Blood' turns red in the summer. Another favorite one is the [/I]sedum sieboldi [/I]which is hardy in zone 5 here in wet Michigan. Here is a picture of it:
http://www.softel.elblag.pl/~kattka/gal8/photos/photo_98.html
Here is another excellent website showing all kinds of possibilities with sedums and sempervivums:
http://www.localaccess.com/blackcatnursery/
NOTE: Our local squirrels consider sedums/sempervivums their winter salad. I have to spray them with hot pepper wax to keep them from being eaten any time of year. In the winter, I cover pots with a piece of window screen. Therefore, think carefully if you want a sedum roof if squirrels are common in your area.
Sedum acre is another excellent ground cover because of its fine leaves and bright yellow flowers:
http://www.bioimages.org.uk/HTML/P3/P34625.HTM
I could keep going on and on suggesting good ground cover sedums. A quick search online will lead you to many many many more.