Green Roofs, Many Times the Last Defense for Water Quality!

Green and living roofs are so very important to our water quality, sometimes being the last line of defense for removing pollutants before runoff enters our ecologically sensitive and important waterways.

Green roofs slow down stormwater, cleaning and sequestering pollutants 

Even small green roofs can provide a significant benefit to reducing peak runoff amounts and reducing loadings on creeks, rivers and ponds by removing nutrients such as nitrogen and phosphorous.

Blue Hole, Ichetucknee Springs

Once stormwater hits a roof and flows to the street below it may only be a matter of minutes before the water and any contaminants picked up as the volume flows across streets, drives and roads enters Florida's drinking water supply. Green roofs also moderate and attenuate the volume of stormwater leaving a building footprint.

Floating wetlands, Gainesville, Florida - greenroof on a pond

Storm drain allows pollutants to enter waterways

Florida has a unique geology across many parts of the state called Karst.  Karst formations are typified by limerock with caves, tunnels and holes throughout the  formation.  Once stormwater runoff enters the limerock above drinking aquifers the flow to the pools of underground water can be very quick.

Many times storm drains are a direct connect to water supply aquifers

Green roofs, living walls, floating wetlands and other best management practices can help keep water clean by slowing the runoff and removing contaminants from the water.

Wekiwa Springs, Florida - higher in nitrogen and algae

Wekiwa Springs, located just north of Orlando, Florida and shown above is surrounded by houses, streets, roads and commercial development.  Though many good best management practices are in place to contain nutrients and runoff, the springs still suffer from high nutrient contents such as nitrogen and phosphorous.

Salt Springs, Ocala National Forest, not as impacted by development

Salt Springs in the Ocala National Forest on the other hand, though threatened by development, does not have all the septic tanks, stormwater runoff and as you can see, the water is much clearer, contains less nitrogen and other nutrients and is so much more healthy.

Installing a green roof on your commercial or residential building is just one small contribution you can make in the Urban Core to help protect clean water supplies and ensure a healthy Florida for future generations.

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!

Green Roof Design and Stormwater in Florida

We are starting a series of blogs on green roofs and stormwater in Florida - especially since Florida's (FDEP's) new Applicant Handbook - the design manual for site Stormwater Permitting in Florida lists green roofs as a treatment option.

Click on the above link to download the manual.  The green roof section is located beginning on page 83 or so.

We are proud the State of Florida recognizes green roofs as providing stormwater treatment.

We are concerned with some of the material in the handbook though.

The handbook approaches green roof design from a stormwater storage function first, though.

As green roof professionals we believe the appropriate design for a green roof should be from an integrated approach - habitat, carbon sequestration, beauty, insulation and stormwater - not just primarily stormwater.

We are concerned the handbook allows primarily volume credit for green roofs.  Green roof design is approached from a 'stormwater retention pond' on top of a building approach.  The more volume you store between the roof and the associated cisterns, the less you have to treat for discharge.

This approach may be appropriate for stormwater treatment but in our opinion, stormwater treatment should be a benefit of good green roof design - not green roofs should serve stormwater treatment.

Not trival, our concerns are valid because under the stormwater handbook approach one ends up designing green roofs from a 'demand side' perspective - or peak load perspective.  Stormwater treatment systems are designed based on annual rainfall loading numbers, graphs and data as presented in the applicant's handbook.

A deep, intensive green roof may be great for helping handle stormwater on a site.  However a deep, intensive green roof may not be a sustainable system.  Lets discuss why.

Florida has an overall significant rainfall average amount of between 48" to 64" per year.  To handle that peak load, stormwater systems have to be rather large.  Green Roofs designed to attenuate or hold portions of that volume are usually substantial and, deep - 4" or deeper across the roof.  Cisterns are considered a vital part of a green roof system for additional volume storage.  The more volume stormwater your green roof can store, the less you must account for in other portions of your on-site stormwater system.


An intensive or deep extensive green roof system has more soil along with the additional storage volume.  More soil means deeper roots.  Check out the attached photo.

Green Roof Plant Root Architecture

The root system here is typical of a green roof plant with 6" of soil.  Roots grow down then out.

On deep stormwater based green roofs, the plants used will develop deeper root systems.  We've discussed this principle before - it is called acclimation - plants adapt to the site.  So with deeper soils found on stormwater roofs you will have deeper plant root systems.

Now, the dry months of the year run from mid-October to mid-March.  During this portion of the year the Florida average rainfall amount will be less than the required landscape plant evapotranspiration requirements.  In other words, during the wet summer months it rains enough in Florida to provide irrigation generally for most landscapes, including green roofs yet during the winter months irrigation is usually necessary because the rainfall amounts are significantly less.

Importantly, here in Florida though we have a large annual rainfall amount - over 50" per year, most rainfall events are less than 1" - and many less than 1/2 " in total volume.  This is representative of the typical afternoon summer quickie rain shower.

Going back to the photo above, many times after a rain event the first 1" of soil may be wet after an afternoon rain but the deeper soil can be very dry.  On deeper green roofs designed for stormwater systems this can be a problem for the plants.  Additional irrigation is usually required to reach the deeper plant roots.

So by building a deep green roof capable of handling heavy rainfall events as part of a site stormwater system we are designing in a landscape feature that actually requires irrigation.

The green roof helps reduce the overall annual rainfall discharge but requires irrigation to keep the plants alive year around.

This is what happens when you design a green roof to function primarily as a stormwater system feature.

We will talk about required fertilization in the next blog - and ask - why are we adding fertilizers to stormwater green roofs?

For now - let's agree green roofs can work wonderfully as a stormwater system - however - Florida green roof design criteria should not be based on green roofs designed for stormwater systems.

Florida green roof design criteria must also take into account the following - cost-effectiveness, habitat, water-wise design, beauty and other considerations.

A stormwater designed green roof will be very heavy when saturated with water and the cost associated with critical roof support may be way too high for many projects.

A lighter weight, thin extensive green roof system may not hold as much stormwater but will be less expensive.  A lightweight green roof may be used on many residential projects that could not afford a heavy, structural stormwater based green roof.

So the point here is - great news our state stormwater handbook specifies green roof design for credit in designing site stormwater systems.  Yet we must also realize - not all green roofs function primarily as stormwater systems and so design criteria cannot be restricted to stormwater purposes.

In future blogs we will explore habitat creation, food production, landscape beauty and other important criteria affecting green roof design.

Happy Green Roofing,  Kevin