Green Roof Primer
From the California Academy of Sciences Osher Living Roof of 197,000 square feet to the planters on top of the parking garage, green roofs are a common sight these days. By definition, a green roof is any planting located over a structure, so this is by no means a new concept. What is new, however, is the light-weight planting media, modern membranes, subsurface drip irrigation systems, and plant palettes. Brooklyn Grange in NYC farms over two acres of rooftops to provide over 500,000 pounds of vegetables to restaurants, CSAs and the general public. There are honey bees thriving on rooftops in every major city in the US, ensuring that our food crops are pollinated.
Green roofs provide many advantages over a conventional roof system: storm water management, increased energy efficiency, mitigation of the urban heat island effect, improved air quality and biological diversity. In our increasingly dense cities, we are offered accessible amenity space, recreation space, horticultural therapy and urban agriculture.
There are three basic types of green roofs: extensive, with 6″ or less planting medium; semi-intensive, 6″ – 12″ depth, with a more diverse plant palette; and intensive with greater than 12″ depth and the greatest diversity in plant material, including trees.
Extensive green roofs present some unique challenges. Lightweight planting media is a sterile mix and often does not hold water as well as deeper strata, causing plant stress in full sun. Some sections of roof may not see sun due to shadowing by the building. Plant material may need to be changed in these areas to suit the exposure. In some cases, peat-rich soil mixes will hold too much water, causing plant rot.
In all types of green roofs, the planting media is most often a lightweight mixture of soiless components which lacks the soil biology necessary for plant roots to take up nutrients. We have also found the lateral distribution of drip irrigation does not match our expectations in this type of soil. This is where our application of 
compost tea, with mycorrhizae, will make a world of difference. The mycorrhizae will provide the chemistry needed for improved soil structure and begin the development of the necessary biological community the plants will need to thrive. This improved structure will also increase water holding capacity and regulate water distribution within the media.
Consult with an Accredited Green Roof Professional by Green Roofs for Healthy Cities to help your green roof thrive.
Lauren Galanes, Green Roof Professional
