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, Greenroofs.com .  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.

Green Roof plant root architecture and the importance of shallow soil media in hot and dry climates and a really cool root architecture video (SimRoot).

Finding the right depth for green roof soil media is so critical in hot and dry climates for when the rains come the roots must be in place where they can take full advantage of any and all rainfall.

This design maxim is why we use shallow soil media depths of 100mm or less, usually ranging in the 50mm thicknesses.

Green Roof plant roots will spread horizontally once they reach an impenetrable root barrier or the bottom of the green roof system.  If your green roof system is deep, then many of your green roof plant roots will travel downward, vertically.  If your green roof system is shallow, then the roots will spread out horizontally.

Most plants in nature have a horizontal root architecture.  I see large oak trees sometimes blown over and their entire root mass is no more than 60cm or so deep (1-2').

Here in Jacksonville, Florida we have been experiencing a severe drought this year and for several years past.  Our weather service has classified the local vicinity as being under a severe drought.  I understand from speaking to others across the south, some areas, such as Houston has not received any significant rain in four months.

Yet for the last two days we've had strong intermittent thunderstorms and rain.

The flower pots were filled with water and buckets overflowed.  However this morning when I was working in the garden I noticed the soil below 20mm or so (an inch approximately) was completely dry!  I had expected this, having seen rain time and time again when it comes, barely penetrate the ground surface layer.

Shallow, extensive green roof systems encourage horizontal root architecture.  When the rains do come here, I need the green roof plants installed on our nature irrigated green roofs to be ready to adsorb as much of the rainfall as possible, wasting little if any.  They cannot exist deep down, hidden in the dry.

With a deeper green roof soil media, even if the media is well drained, I still see almost complete dryness just below the surface after a typical thundershower here.

Shallow green roof soil media can promote horizontal root architecture, and in turn capture more rainfall for use in the plant's photosynthetic processes.

Additionally, shallow root architecture can facilitate enhanced nitrogen and nutrient uptake.  Nutrient uptake is  important for several reasons, including of course plant nutritional requirements but also the removal of available nutrients from stormwater.  Shallow root architecture cleans stormwater and allows for optimal plant growth.

The root architecture model, SimRoot developed by Penn State is shown in the below video clip to illustrate how nitrogen is taken up in the upper margins of the plant root architecture system.  Note how the root hairs take up the most nitrogen and water.

I've heard many arguments for deeper soil media systems.  Yet they are heavy, expensive and require so much more water wasted.  Rather than build heavy stormwater ponds on top of buildings, put them under parking lots for a much more reasonable cost.

There are many benefits to shallow green roof systems.  Water efficiency and nutrient uptake are just two.

Need a Snazzy Natural Green Roof Water Retention material? Bonzai Me!

Hint:  Read all the way to the end for a natural replacement to SAPs.

Polymer water absorption crystals have been a popular way to help keep plants watered in containers, hanging baskets and even green roofs.  There are many websites selling this product, such as Watersorb.  Even other vendors can be found by Googling the phrase "polymer water adsorption crystals".

Yikes!  These crystals can be expensive.  $15.00 per US pound or $26 per three US pounds.

How could one afford to use these on a green roof at this price?

According to one of the most convenient information sites, Wikipedia, these crystals are commonly referred to as super-slupers.

The Super-absorbent polymers, also know as SAP's are created by cross-linking and chemically interacting polyacrylamide copolymer, ethylene maleic anhydride copolymer, cross-linked carboxymethylcellulose, polyvinyl alcohol copolymers, cross-linked polyethylene oxide, and starch grafted copolymer of polyacrylonitrile to name a few, according to the Wiki article.


OK - now for our suggestion.  This is only a suggestion.  The process is working quite well for us here in the very dry (we are considered as being in a Severe Drought zone).  However, field trials should be conducted in other areas before using a material similar to Attapulgite.   Attapulgite is marketed by Agsorb from Ochlocknee, GA and according to their website is a Attapulgite mineral in the non-swelling bentonite class. It is commonly called Fuller's earth. It's low bulk density and high absorbtivity allows higher liquid holding capacity. Our mineral processing maximizes the granule's micropore space. Agsorb heat treatments impart a hard inert granule with a high resistance to attrition.


Remember - you must work out your own blends!  Clay like bentonite or attapulgite can present different issues if used without proper design and research.


We suppose some of the problems could be compatibility with roofing materials, clogging of drains and root rot when used in incorrect blend mixtures.


However in hot and dry areas where every drop (I am very serious about the 'every drop' statement) of water is precious on a green roof, these clays may really help with plant survival.


Sure, clay is not a substitute for proper plant design.  Build you wind break and perimeter with CAM plants interwoven with appropriate Poaceae and C4 type photosynthesizing plant species.


But when you are at wits end (like we stay here in Jacksonville) about where the next drop of moisture will come from and how to preserve the liquid life, then this material just may offer help.


Watch your pH, watch your drainage, weights and water retention characteristics.  Make sure you always study the MSDS issues.


Success will be found in the appropriate blend.  Try starting with a 1-2% by weight blend, then adjust according to the site's wind and light exposure as well as temperatures.


We have had super great luck with these types of clays.  As an added benefit many contain an amazing array of trace nutrients our plants require.


For a very interesting read on soil science, and the area from which we first started thinking about using clays see this very interesting article about Bonzai Soils.

And there embedded in linked article was ou answer to the high cost of SAPs or water retention crystals - and the alternative was a natural product, readily available (though mined :( - however it would behoove us to study the carbon footprint of SAPs v clays, including environmental impacts).

Oil Dry!  Kitty litter!

We found a great deal at Sams Club here in the states - 40 pound bags of oil dry for several dollars.

Hey!  Our green roofs are suffering in the hot and dry!

Good luck with your experimentation.  You may find the effort was well worth your time.  And your green roof may be able to stay greener longer!

Green Roof Drainage Issues

Drainage is an essential design component on any green roof.

Not enough drainage and the green roof can quickly fail via leaks, dead plants and even a collapsed ceiling.

Too much drainage and  the green roof plants may suffer from not enough irrigation.

Inspecting the roof before installation of the green roof system is very important.  Be sure to review the plans and find the elevation breaks, low spots and ridges.  Take the plans and try to determine how the rainwater and irrigation water, if irrigation water is used, will flow.

Inspect the roof for drainage breaks before installation of Green Roof System

Once the underlying roofing system is installed, inspect the entire roof.  Watch for issues with the flow of water, look for depressions formed into the surface during construction where water may pool.  Ensure seams appear to be properly attached.  Always insist on a flood test and conduct a metal sweep before commencing any work.

When you feel comfortable with understanding the direction of rainwater flow and other issues and requirements for the green roof and install the base system, be it trays, monolithic built-in-place or other system, the soil media will be one of the first components to be installed.

Soil media is usually premixed and possesses the same water or rainfall permeability no matter the location placed on the roof.  Filling the green roof system with soil media possessing the same drainage and water flow characteristics may be adequate, unless the roof pools water in places it shouldn't, or drains water away from areas where water needs to stay and provide irrigation to the green roof plants to be installed.

If your roof construction requires a change in water flow patterns across the roof surface you can easily accomplish the re-direction of water by substituting materials possessing different permeability and water flow qualities for the soil media in the affected area, or by placing a layer of material with increased or decreased water flow characteristics under the soil media.

This obviously requires solid knowledge of  the materials you are working with, their permeability, saturation rates and other physical and chemical qualities that may affect the roof drainage pattern.

Sand for instance can be employed to either slow down or speed up drainage depending upon the type of surrounding green roof soil media.

Peat and coir are other materials with varying drainage characteristics that may be successfully used to after drainage patterns to your requirements.

Adding peat to affect drainage flow

Adding peat under fast draining green roof soil media allows for additional rainwater adsorption and storage.  Peat moves water through wicking action, and can direct much needed irrigation towards the plants needing the water the most.

Natural and geo-synthetic fabrics can also be used to accomplish the same water direction exercise, or re-direction as is the case.

Though all of the above water distribution efforts should in reality be accomplished through proper design,  we all know that ultimately we must make adjustments after the fact sometimes to accomodate anomalies in the as-built system.

Understanding how to 'tweak' rainfall flow through the green roof soil media is important.

Doing so will allow your green roof to mature into a successful planting.

Mature Green Roof Planting, Florida (MetroVerde)

Green Roof Soil Media Note

As standards are being developed by ASTM and other agencies, both private and governmental, here is a reminder to remember High Velocity Wind Areas.

Florida Building Code does not allow loose laid ballast, aggregate, gravel or pavers on rooftops due to hurricanes.

Loose laid gravel or aggregate may become airborne during cyclone winds and damage adjacent fenestration and windows, acting like large shotgun pellets.

Large beds of expanded shale, crushed tile or brick on a roof here in Florida could inflict serious or deadly damage to human life and property.

Parapet walls may be an answer to wind impacts on rooftop aggregate.

Yet the important point here is we cannot rush to adopt an incomplete standard.

A standard that is irrelevant to a geographic area is not a standard at all.

Miami-Dade County has developed agressive testing requirements for high velocity roof applications.  The State of Florida is also working on Building Code requirements as are many of the Florida Universities.

Lets proceed with standards, yet make sure they are applicable.