Emory Knoll Farms of Street, Maryland explains on their website the difference between an extensive green roof versus an intensive one.  The extensive uses plants with foliage of two to six inches, low-lying plants designed to provide maximum groundcover.  An intensive green roof uses foliage of one to fifteen feet to provide more of a natural landscape.  Emory Knoll Farms specializes in extensive green roofs.  To find out more go to their website:

www.greenroofplants.com