Green Roof Irrigation System - Can Nature Support a Green Roof?

Green roof irrigation can be accomplished via recycled rainwater pumped up from a cistern, potable water supply, a combination of recycled rainwater and potable water or from mother nature alone.

Importantly, a green roof should look good and not dry.  A dry roof could be a potential fire hazard.  Remember how those dry Christmas trees would burn.  Of course, some - like Ducks Unlimited - used to burn their green roofs annually - but most of us would not want to burn a green roof.

Other reasons for keeping the plants watered on a roof include;
1. More photosynthetic activity - hence more CO2 uptake and more O2 production,
2. Greater wildlife habitat,
3. Increased nutrient uptake
4. and others.

However there are also valid reasons for not irrigating a roof.

There is a water crisis here in Florida and across the southeastern US.  Remember the water wars of Alabama, Georgia and Florida a couple years ago?

50% of all potable water usage is typically consumed via landscape irrigation use!  Over withdrawal of aquifer reserves causes sinkholes to develop, wells to go dry and contamination of existing supplies to increase.  Native plantings are encouraged and water reuse is a municipal practice on the increase.

Additionally, roof leakage is a serious concern.  Creating roofs that can withstand a pond-like body of water is expensive and may cause mold problems.

Mechanical systems, including irrigation systems are always subject to eventual failure.  If the irrigation system goes down and the plants are used to significant watering then they may quickly die.

If the system is hooked up to potable water then a flood may occur.

Maybe there are other alternatives.  In our opinion, with the rush to implement new stormwater rules and nutrient removal criteria in Florida, the FDEP and certain testing organizations have jumped to the conclusion that the only way a green roof can survive periods of little to no rain is to install irrigation.

A quick review of Florida literature results in few if any research articles considering alternative irrigation methods.

Granted, FDEP's new applicant handbook suggests using rainwater cisterns to recycle rainwater as an irrigation supply for vegetated roofs, however this is but a selling point for their lack of research on non-irrigated systems and also is seriously flawed.

Suggesting rainwater cistern and rainwater recycling is low impact development is wrong.  Here in Florida half the year approximately sees only minimal rainfall amounts.  Today we are well behind - ten inches or so in our average annual rainfall amounts.  That means all those vegetated roof plants so used to plenty of rainwater irrigation from the cistern are now going to have to be irrigated with potable water or reuse water because they are not acclimated to the dry periods.  This philosophy is 'Anti-Florida Friendly Landscaping' and is a scam.

Certainly there is room for having stormwater volume and retention on the roof and in cisterns, but this approach cannot be the only sanctioned approach for green roofs in Florida.  And because FDEP and various educational institutions are pushing the 'Stormwater Pond on a Roof' approach the Florida Green Roof Industry has to adsorb and deal with the results of those misguided efforts.

Back to green roof irrigation.

Certain nations, such as Australia, New Zealand and others are stepping up and conducting serious research on alternative irrigation systems such as utilization of dew and air humidity.

We should too.

Here in Jacksonville we haven't had rain in three weeks.  Our yards and gardens are dry as a bone.  I'm sure many of the irrigated green roofs around the state are pumping out potable water (all the stored rainwater is used up by now) to keep the green roof plants irrigated.

Check out the following early morning photos.  remember - no rain here in the past three weeks however early morning water is everywhere.

Green Roof irrigation alternative - collect morning dew

Green Roof irrigation alternative - collect morning dew

Green Roof irrigation alternative - collect morning dew

Check out the article on dew collection found at http://www.inhabitat.com/2008/06/26/harvest-water-from-the-air-with-fog-dew-collectors/  

Possibly the research topic of an inspiring student or professor, the potential is great and the market place calls out for such inventions.

Green roof design already has taken advantage of the potential of harvesting dew for irrigation, MetroVerde utilizes compact dew catchers around many green roof plants in the field, harvesting significant early morning water for the plants.

Moreover, early morning water is captured on a more efficient basis by green roof plants because high daytime temperatures and evaporation process have not begun in the A.M hours.

So remember, next time someone says "Green Roofs in Florida Must be Irrigated", pause and ask yourself - has this person really done their homework or are they trying to push an agenda past you without your knowledge.


As always, feel free to contact us via email.

Happy Green Roofing,

Kevin!