Friday, April 11, 2014

Green Roof Soil Media Questions for an Ocean Front Green Roof

I have been asking myself a question over and over again lately.  How do I keep green roof soil media from blowing off a three story, ocean front residential green roof?

The Atlantic Ocean shore - great place to learn of #Greenroof plants and to build a green roof too!
I think I have the answer but I still am asking myself the question over and over.  Not because I have doubts necessarily, but because I want to examine the issue on a regular basis, over an extended period of time.

In the past I've had new insights arise when I regularly revisit a challenge.  Challenges, be they green roof related or otherwise, are usually solved if one puts enough thought into resolving the issue at hand.

This particular green roof project will be a challenge.  Weight will be a factor as it always is.  Light will be intense and strong, all day long with no shade available.  Salt spray will be constantly coating the plants.  Hurricanes are very likely as the house sits right in the middle of hurricane alley.

But I am as confident as any experienced green roof designer can be that all these variables will  be addressed in a manner that will minimize risk to the structure, plants and surrounding area.

The daily wind factor keeps coming back into my mind though.

Any green roof will have the potential to experience much higher wind loadings.  A green roof on the ocean front usually has much higher velocity daily winds whipping across the soil and plants.  Keeping the plants alive and free from desiccation is one issue and keeping the soil media from sand-blasting a neighbor's house is another design goal (as well as simply keeping the soil media on the roof).

Ocean lots are a challenge to the #GreenRoof designer for many reasons
Sand has it's advantages and disadvantages.  Coastal dune plants, those species that will survive on a roof such as this love beach sand.  But sand has a tendency to be blown around easily and is quite heavy.  The sand here is primarily a lovely brown hue and composed mainly of crushed coquina shell, full of calcium and other trace minerals.  It drains well and holds an adequate amount of moisture.

But I am not thinking sharp beach sand would be good on the roof.  With average daily wind velocities from 5-6 meters per second up to 10 MPS and higher, I am concerned beach sand would be blown away in a matter of days, if not hours.

Walking the beach not only relaxes but teaches much.  I see things on my frequent seashore strolls that remind me of how Mother Nature behaves.  She behaves as she wants too, with little to no regard for us humans and our designs.

Mother Nature and wind has a mind of their own with regards to sand deposition #greenroofs
I view seawalls constructed with many thousands of dollars intended to hold sand in one place or keep sand out of another place and despite our best efforts these structures ultimately always loose the battle. Mother Nature puts sand where her wind blows and in other places too.

Perhaps a larger diameter, lightweight inorganic substance like expanded clay may be better.  We will analyze this in future posts.  A mature, developed green roof plant root system will go a long ways towards holding soil media in place, but can take a couple years or more to for the roots to develop and I don't want the clay to come off the roof and act like shotgun pellets in a tropical storm, damaging adjacent fenestration as ICC notes warn against.

Ultimately, root architecture may be the answer.  Comprehensive root coverage can hold soil media, plants and the green roof system in place even under storm conditions.

Agar-based tackifiers and netting have been used to prevent wind scouring of green roof soil media.  On the ocean front site I am concerned the tackifier would quickly degrade under the intense solar heat and constant salty mist.  Last thing I want to see too, is wind netting loosely flopping about after becoming slightly dislodged, perhaps beating the plants down in the breezes.

Sustainability too and green building programs call for use of local materials and are other considerations to take into account.  Sometimes local material compounding is practical or even the only way to acquire soil media.  Other times local materials may not be suitable for green roof soil applications.

Ocean front #Greenroofs face salt, wind, sun, storms and other harsh impacts
We don't want the soil media to be too dark because of high solar gain.  I spoke with someone yesterday who told me about a free roof that would not grow plants because the expanded shale was so hot he could feel the intense heat through his shoes.

We want the soil media to have a proper, plant friendly pH to encourage good plant growth too, and it needs to be primarily an inorganic mix also.

There are also many standards and reference materials to consult.

European FLL have been the most looked to standards for specification and design.

There is a simple but good short description of the green roof soil media question published by Design Cost Data here.

One of the most comprehensive and helpful discussions of green roof soil media is located on the industry website, .  The article discusses green roof soil media ASTM standards and provides links to other valuable soil media references, especially to an article by Chuck Friedrich entitled 'Don't Call it Dirt!'.

I've got a lot to consider concerning this ocean front structure's windy green roof.

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