I installed rubber roofing on my garage and kitchen extention last year, and am very pleased.
Go to the suppliers directory and check out the rubber roofing suppliers in there.. I used the 1 metre wide rolls and found them easy to lay. Just be careful as when the rubber sticks thats exactly what it does. I found that if I got it all lined up and then removed the backing slowely then all went well. It's great for around pipes as well.
I've run the rubber up the walls and to the edge of the block work and put coping stones over it, so the roof is basicly tanked.
In heavey rain I've had several inches of water on it and have had no leaks.
I found the chap from the company very helpful when I rang for advice.
May not be as cheap as felt to start with, but should be in the long run.
I wouldn't go back to felt now I've used this.
Hope this helps.