Wednesday, December 15, 2010

Green Roof Design - New Years Resolution - Back to Green Roof Basics!

The copper and net dew catchers are coming down from green roofs today and we making a Green Roof New Years Resolution and concerted effort to 'Get Back To Basics" on Green Roofs.

Technology can be alluring and distracting.

Sometimes alluring technology can unintentionally shift core focuses.

 Development of metal and plastic dew and fog catching apparatus based on biomimicry is just 'that' - biomimicry.  The biomimicry approach of copying nature's solutions can be successfully substituted with nature's back-to-basics on Green Roofs  - simply use plants with dew and fog catching characteristics rather than mechanical  contraptions.

In fact advantages of alluring technology can be so great we sometimes tend to gloss over the downside, or disadvantages or serious problems.


(Advantage)  Copper dew catchers shine brightly in the afternoon sunlight and the plastic netting does a great job of harvesting air moisture.  Aesthetically pleasing.

(Disadvantage)  The metal and plastic dew catchers and fog harvesters may become launched projectiles, hurled dangerously through the air in hurricane force winds.  Something not normally considered on a day to day basis.

(Solution) Use plants nature has perfected to capture dew and fog - there are many.  Growing plants is what green roofs are all about anyway.

Solutions like the above are often over-shadowed by the alluring glow of technology.

Another technology with an alluring name is "Pollution Control Media".  Pollution Control Media is now required to be embedded in Florida green roofs under the drafted State of Florida Stormwater Quality Applicant's Handbook to receive stormwater credit.

The handbook begins with a short-sighted approach to green roofs by allowing for only stormwater volume credit for green roofs.   No allowance for water quality improvements is given.   Unfortunately, this may be due to the misconception that nitrogen and phosphorous fertilizers, pesticides and herbicides are necessary for successful green roofs.

Even the American Rainwater Catchment Systems Association (ARCSA) has questioned the approach and intent of the handbook's unusual requirements for green roofs and cisterns design.

Apparently, the Pollution Control Media is supposed to capture the nitrogen and phosphorous from the fertilizers, pesticides and herbicides applied to and sprayed on the green roof - from entering stormwater runoff.  In fact, according to the handbook, all green roof runoff should be captured by the cistern and then recycled back to the roof for irrigation on dryer days.  The green roof plants would then evaporate off the water through normal transpiration, leaving the nitrogen, phosphorous, herbicides and pesticides to accumulate in the closed loop system over time.  We wonder at what point the accumulating slurry of fertilizers, pesticides and herbicides would become a hazardous green roof waste.

A topic for another article - green roofs can and do actually clean stormwater.  Green Roofs do not have to be considered just another stormwater volume retention system. In fact, by allowing stormwater credit only for rooftop systems that have the structural capability of supporting deeper amounts of stormwater, incentive disappears for other types of green roofs.

But what is Pollution Control Media?

Pollution Control Media is primarily a blend of sand, crushed limestone, sawdust and mostly crushed or ground up rubber automotive and construction vehicle tires.

Pollution Control Media has shown promising results in removing nitrogen and phosphorous from spetic tank systems and other waste cleaning applications (Pollution Control Media was originally proposed to be installed under certain types of newly constructed stormwater ponds in an early handbook draft but was removed).

Therein lies the alluring technology scenario referred to earlier.  If fertilizers and herbicides and pesticides are to be used on green roofs (there are other more eco-friendly approaches) then why not add Pollution Control Media to keep the fertilizers and organo-herbicides and pesticides on the roof and out of the stormwater.

Studies even referenced a few bio-assay toxicity tests conducted around 2003 showing acceptable mortality levels.

A short-term acute bio-assay is one measure of safety but long term chronic exposure is an all together different issue, one acknowledged not to be answered about tire crumb in the supporting studies.

In fact EPA has changed their stance from recommendation of recycled tire products to one of - further study is now required - and listed the following substances as existing on ground recycled tires:

  • acetone
  • aniline
  • arsenic
  • barium
  • benzene
  • benzothiazole
  • cadmium
  • chloroethane
  • chromium
  • cobalt
  • copper
  • halogenated flame retardants
  • isoprene
  • latex
  • lead
  • manganese
  • mercury
  • methyl ethyl ketone
  • methyl isobutyl ketone
  • naphthalene
  • nickel
  • nylon
  • phenol
  • pigments
  • polycyclic aromatic hydrocarbons
  • polyester
  • rayon
  • styrene‑butadiene
  • toluene
  • trichloroethylene
Click here for more details.

For an interesting EPA memo about the questions on recycled tire products click here.

So by the State of Florida requiring an alluring technology to be a part of Florida green roofs for stormwater credit, we may be missing the more important issue of the potential wide-spread and long-term leaching of the above substances while nitrogen and phosphorous are kept in check.

While there has already been numerous installations in Florida of the Pollution Control Media, a pause in its use - until EPA completes their studies - may be a good idea.

Certainly the requirement for Pollution Control Media should be removed from the State of Florida Stormwater Quality Applicant's Handbook's requirement for green roofs to receive stormwater credit.

Efforts to recycle used tires are noble.  Applying ground up tires to green roofs is not an appropriate recommendation the State of Florida should be making until all questions about the long-term toxicity of zinc pollution, aromatic hydrocarbon pollution and the above substances are answered.

In fact - some studies are showing that though ground up tires may be safe for children playground exposure, recycled tire products do exhibit levels of bio-assay toxicity to wildlife in stormwater run-off.

So let's all get back to basics on green roof design.  Let's keep a wary eye on all the shiny dew catchers that may become projectiles during a hurricane and let's certainly not require use of Pollution Control Media that may contain substances no one wants on a green roof or in stormwater.

As always, email your comments and questions.

Happy Green Roofing!


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