My understanding of the term "roof iris" is that this iris was sometimes allowed to grow on thatched roofs to help hold hte thatch, but it seems more likely that it would break it down. Plants on sod roofs, however, are useful and beautiful, as often seen in Alaska. Regarding the hardiness of I. tectorum, it seems quite cold-tolerant and is grown in many parts of the USA, including northern states. It does well here until the slugs get hold of it--none of the crested irises can survive them, even I. tenuis, which is native not ten miles away (and higher up in the mountains, above the slugs). Jane McGary Northwestern Oregon, USA