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.