Green Roof Soil - What Kind of Soil Should I use in my Green Roof?

Choosing the right type of soil for your green roof project is very important.

Recently we talked about using a sterile soil mixture.  Non-sterile soils can bring in unwanted fungi and diseases and can damage or kill your installed green roof plants.


Today I want to discuss another green roof soil issue - water retention characteristics.  Again, realize we are coming from a non-irrigated, extensive Southeastern US perspective, and what works here in the long, hot summers and relatively cold but short winters may not work for your locale.  We believe in information sharing and encourage others to do the same so Green Roofs can proliferate successfully around the world!

Our practical experience has shown green roof plants do not like wet feet.  Dampness encourages disease and fungal growth.  Recently, an rather large Orlando green roof project was redone by the installer due to one of the components being installed upside down to where water was retained rather than drained away.

So your green roof soil should drain water away from the plant rather than hold moisture around the roots.

Many green roof plants acclimate to nature's background rainfall patterns and will quickly drink and store moisture from dew, humidity and rainfall.

However they usually do not like having their roots soaking in water.

I was reading about several efforts in Australia - from Green Roof's Australia newsletter - about how research is being conducted on green roofs and water - lot's of good information on Wilson Kath's blog.

Additionally, ASTM has several standards for green roof soils and drainage, including ASTM E2399 and others.

It goes without saying that your green roof system must be designed to accommodate the drainage properties of your green roof soil.  Small containers without drainage will result in pooling and plant health issues.  Large trays or open floors/mats should help with the facilitation of draining water.

A quick google search of "green roof soils" pulls up many good engineered soil vendors.  Moreover, many universities, such as University of Central Florida and others are researching green roof soils and obtaining patents for such.

Ultimately, the type of green roof soil will depend upon many factors - sun exposure, wind exposure, depth of green roof system, desired plants, climate and more.

The important maxim to remember is - generally you want your green roof soil to drain well.

Good luck and happy green roofing!  As always - you may email me with your questions.

Kevin

Green Roof Planting Media - Second and Third Generation Products

After literally thousands of soil mixture experiments we are making strong headway in developing the green roof soil medium that has the qualities we are looking for. As we look back over the years we can readily see how far we have come.

We've addressed the drainage and the flammability and the dust and the compactability and the water retention and the nutrients/organics issues.

Erosion was a huge issue that we wanted to resolve. Our focus was on lightweight, thin (low profile - primarily because of Florida's hurricane propensity), inexpensive vegetated roofs that would not only work on flat roofs AND we wanted our roofs to work on slopes - even severe slopes!

Today I installed a 20SF vegetated panel on a vertical wall at the house, less than ten minutes after the engineered soil and plants had been applied. Wow. We have come a long ways.

But we have a long ways to go.

Now that flexible vertical vegetated panels - less than 1" thick are a reality - now we are looking at technology that will allow us to apply vegetated panels directly overhead on ceilings. So here is the next challenge.

The era of filling planters or trays with dirt, loose soil mixtures, LECA or expanded clay and placing them on the roof is disappearing. Like the dinosaurs these applications will hang around for a while, with good usefullnes with large heftily
-engineered structures. LECA and Expanded Clay may soon be against building code in Florida for use on roofs - imagine a truck bed load of LECA pellets fling through the air in 150 MPH winds - something like a shotgun blast.

Toxicity free engineered planting media will be a requirement. LC 50 testing with minnows will become standard. Green building requirements will affect planting media design too.

The challenges are here - so are the opportunities...

Happy Green Roofing! Kevin

Green Roofs use of Expanded Clay and Lightweight Expanded Clay Aggregate

The State of Florida has recently requested studies analyzing the use of gravel and aggregate on roofs and the potential of gravel ballast to break windows and other fenestration components of a building during hurricanes.

Preliminary research by the University of Florida shows that gravel ballast may be the cause of unnecessary glass failure and subsequent water damage.

Hurricane Alicia in 1983 devastated the Houston area, and much of the urban fenestration damage was attributed to gravel ballast ripped off the roofs by the hurricane force winds and shot across the city like pellets.

As Florida's green roofing industry develops, designers should be aware of the potential for tort damage liability once their specified product damages anothers property.

We should be careful about placing any gravel or pellet like material in a green roof until all the appropriate studies have been conducted to determine the level of safety for such product use.