A great article on green roofing technical issues by Kelly Luckett appears on a recent Green Roof Website and I recommend the read - lots of good info.
You can find my comments to irrigation issues as follows: (Note - I am pretty much against irrigation on a roof - too much tort liability exposure in my opinion remember Byrne v. Boadle from 1L?)
I read your article on irrigation with interest.
I'd love to talk with you about how you can design a green roof to survive without irrigation.
I understand you may consider it hard to do, but it is very possible.
As a plant biologist for many years and a Florida nursery owner with my wife we too have held a great interest in green roofs, and have trials that have been in the field for years.
Now Florida is different from anywhere in the nation. We have the four H's - heat, humidity, hard freezes and hurricanes. And also a bunch of attorneys who got rich off of mold litigation and are looking at any irrigated roof as a lawsuit, arguing the mold in the building did come from the irrigated roof...
Lawyers aside, we wanted to develop a roofing system that was cheap and lightweight and would meet product approval for Florida Building Code - stricter than IBC - and was evergreen, could be used on sloped roofs, treated stormwater and provided wildlife habitat.
We tried trays and tray systems and mat systems like XeroFlor - http://www.xeroflora.com/ and everything in-between
We consider our 10 years or so of seeing trials not work, our biggest successes.
Tray based systems will not survive long term in Florida. This is our opinion. We do not represent XeroFlor but we have found mat systems are best for Florida's unique climate.
Florida is quite possibly on the verge of removing ballast, be it LECA or Expanded Clay or gravel from approved roofing materials lists because of hurricane damage possibilities. Imagine a roof full of gravel or expanded clay and a hurricane - now imagine the pellets flinging towards fenestration like a shotgun blast in a 150 mph gust.
Florida typically requires all roofing material to be permanently attached to the roof (DCA Roofing TAC guidance)and typically considers a green roof to be part of the building envelop.
So back to the irrigation...You can do a green roof in Florida without added irrigation - I don't know about north of Atlanta - but here in Florida it is certainly possible.
We have roofs in place that have not had a drop of irrigation from day one. And they are doing very well.
Sure, you see signs of stress during 2 month droughts.
But they still look pretty good...
And you don't have to fool with a mechanical system that can fail - one that supplies pressurized water...
And they do fine without fertilizer, but they like fertilizer too...
We heard the same advice years ago. You have to fertilize and irrigate. But our roofs speak for themselves. No irrigation. No fertilizer. No maintenance.
I spent years watching plants grow in Florida roof gutters, and in the cracks of hot asphalt pavement in over 100 degree heat, with desiccating winds, and watched plants grow in cracks of retaining walls for overpasses, and did they ever continue to grow!
One of my favorite green roofs is located in the roof gutter of the gas station next to our local Blockbuster Movie shop. No one irrigates it. No one fertilizers it. It is there year in and year out.
Maybe green roofs can be designed, installed and survive without irrigation. We know they can here in Florida...even with our harsh sun, heat, winds and storms...
I smile every time I read of someone saying it can't be done...