10.27.11Kelly Duke

Green Roofs and Water Management

An example of a modular extensive green roof at the University of La VerneAn example of a modular extensive green roof at the University of La VerneFenway Trilogy in Boston is an Example of an Intensive Roof Garden - Photo © Jay Graham, Graham PhotographyFenway Trilogy in Boston is an Example of an Intensive Roof Garden - Photo © Jay Graham, Graham Photography

Green roofs can play a significant role in water management by moderating the flow rate and quality of storm water.  For anyone considering a green roof here are some basic terms and considerations to help you determine which greenroof system will work best for you.

The first thing to know is that there are two basic types of green roofs.  These are:

Extensive green roofs are typically constructed with six inches (6”) or less of a lightweight mineral planting medium and utilize a largely homogeneous mixture of plants.  Such plantings generally require a simple maintenance routine with virtually no plants requiring individual care (plants such as various Sedum species are commonly used).  This type of green roof is typically installed for its role in “urban heat island” mitigation, its storm water management function, and its summer insulation capabilities.  Such a roof is a visual asset only and is not accessible as an activity space to anyone except maintenance personnel.  Aesthetically, this type of roof is generally viewed only from higher floors on the same or adjacent, taller buildings.

Intensive green roofs are generally installed with a deeper planting medium and are often feature a broader mix of plant types and sizes and requiring different and individual care and maintenance.  This type of installation is more of a “roof garden” accessible to people and serving both an aesthetic and functional role as a space for programmed activities.

For the purpose of this series of posts we will be considering only Extensive green roofs.  Within that category there are two basic methods of construction.  These are:

Monolithic: A “Monolithic” type of green roof that is an integral component of the roofing system.  With this type of installation the roofing system forms the substrate for the planting medium and the plants are installed from plugs, cuttings, or small pots.  As a result one must allow for the necessary post-installation maintenance needed to allow the plants to establish themselves and fill in.  Note that there is a way to achieve an “instant green” appearance using pre-grown custom mixes of appropriate green roof plants in roll-up mats that can be established in advance and installed similar to a sod lawn over the monolithic green roof planting medium.

Modular: This type of system utilizes pre-fabricated trays or either aluminum or plastic.  One of the nice things about this type of system is that the modular trays can be filled, planted, and pre-grown off site prior to delivery and the trays can be hand carried to the roof if necessary.  The limitations to this type of system are as follows:


Ÿ      The square or rectangular configuration of the module may be too aesthetically rigid and limiting if the roof layout is curvilinear.

Ÿ      From a horticultural standpoint, the individual modules can confine plant roots and limit the plants’ ability to access the soil moisture to only that water held in their individual tray.

Ÿ      In areas subject to high winds the modules must be adequately secured to prevent them from being lifted from the roof in a severe wind storm.


Note that there is a modular system which uses trays molded from coco fiber.  These are fairly sturdy and rigid at the time of installation but are bio-degradable over time.  This system (BioTray) gives you the installation convenience of a modular system that morphs over time into a monolithic system.

Most recently we have been advised of a modular system that uses a foam growth medium.  This system allows for higher volumes of water retention but with sufficient trapped air that even waterlogged plants do not appear to drown.  This tray system appears to have great promise due to its light weight.

The brand of manufacture will depend upon which is the most cost-effective for your specific situation.  A great source of information is the organization Green Roofs for Healthy Cities.

While green Extensive green roofs are intended to be partially or fully self-watering using collected storm water, they can require irrigation for establishment and during periods of drought.  Due to the high permeability of the planting medium it is generally necessary to employ some form of overhead irrigation.

Which approach you take will depend upon whether your building is on the drawing boards or built.  If already built, it may then depend upon the age and condition of the existing roof.  You should take into consideration your planned use of the roof, the load limits of the structure, regional rainfall, and, of course, your budget.

These questions can be resolved through the building Architect and Structural Engineer or a thorough site inspection by a competent roofing specialist.  Once you establish the above, you can move on to the next step in developing your greenroof program.

More to follow…  K. Duke

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Kelly Duke

Not many people can say that they have dedicated their life to the landscape industry. Kelly Duke can. His diverse background ranging from maintenance to estimating, to design, along with a passionate commitment to his trade has given Kelly a lifecycle perspective to landscaping. As the leader of the ValleyCrest’s Pre-Construction Services team, he analyzes early conceptual designs to determine whether or not and how they can be built within budget while meeting long-term design and maintenance goals. Many of the projects that come across Kelly’s desk require he examine the cost and savings of baseline water use in comparison to high efficiency alternatives.



  1. Friday, 9:59 Alan Harris

    I was going to ask you if anyone has tried to green roof with vegetable gardens, but before I could post my question I went to lunch and read this fromTwitter: Thanks to @StarkBrosCares and @SerenadeGarden for sharing. Pretty cool concept. While this example was for commercial hydroponics I could see raised garden plots on top of a Condo hi-rise being a great benefit. Any thoughts?

  2. [...] most recent post discussed green roofs and their role in storm water management.  I ended that post with a promise to elaborate more on [...]

  3. [...] have posted on this site some basic information about green roofs (Green Roofs and Water Management and Green Roofs and Urban Enhancement).  Fellow Blogger Alan Harris has posted informative pieces [...]

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