Wednesday, September 15, 2010

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

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