I'll be posting several pics soon of green roof tray systems tat have failed within 6 months. The failure occurred because the plants quickly became ROOT BOUND. You have seen plants stressed due to root binding before - the roots overlap again and again and soon strangle themselves.
Modular tray vegetated and green roof systems are quite popular in many areas now. Long term they do not provide adequate room for plant growth.
Mats, on the other hand, provide an unlimited horizontal plane for root growth and as the plants expand horizontally, roots can follow unimpeded. Mat systems will provide years of vegetated roofing support - refer to the Ford Motor Company Dearborn, Michigan 10 acre vegetated roof mat system.
See also www.metroverde.com for Florida Green Roof mat systems.