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Although they’re occasionally the focus of media attention in North America, green roofs and their potential benefits are still relatively unknown quantities unless you happen to be a landscape architect or architect.  In this commentary, Steven Peck and Damon van der Linde of Green Roofs for Healthy Cities aim to expand that base to include all watershapers – professionals, they say, who can play key roles in spreading the influence of these systems.


By Steven Peck & Damon van der Linde

Just think of it:  There’s a design approach that improves energy efficiency, reduces the urban “heat island” effect and provides much-needed green space in places where opportunities for natural stress relief are rare and much welcomed, yet it’s not something that has caught hold in North America the way it has in Europe and elsewhere.

I may be biased in my role as an advocate for the systems in question, but given all that they can do, I have to wonder why green roofs aren’t being installed with far greater frequency.

True, they are garnering more and more attention and are definitely up-and-coming in the construction marketplace, with 16.1 percent growth in 2009 and reports of similar expansion in 2010.  But given the fact that this technology may be the most beneficial of all “green” approaches, it’s a wonder it hasn’t seen even more dramatic growth.

In observing the situation, I’ve reached a simple conclusion:  While the green-roof concept has won the hearts and minds of landscape architects and architects, what’s missing is the practical component of the picture – that is, the involvement of professionals who know what’s involved in making waterproof systems work.

The missing piece, in other words, has to do with getting watershapers involved.


The use of plant material on rooftops dates to the beginnings of civilization:  Consider the Hanging Gardens of Babylon, for example, which draped over the sides of a huge ziggurat.  For all that, however, green roofs come across as a real oddity in the here and now.

To shed some light on the technology, let’s run quickly through some definitions and suggest a working vocabulary that will help watershapers get started.

A green roof is a contained space set atop a man-made structure as an extension of what would otherwise be a blank, existing roof.  The greening of these spaces typically involves the installation of quality waterproofing and root-containment systems along with drainage systems, filter cloths, lightweight planting medium and the plants themselves.  

1Within and around that planted landscape, these spaces can take on any of the characteristics of a park-like terrestrial space, complete with all manner of walkways, waterfeatures, lighting, garden art, shade structures and more.

This green space can be at, below or above grade – that is, atop subterranean parking facilities at ground level or perched high atop a skyscraper – but in all cases the plants are not put in the ground as they are conventionally but instead are placed in a medium contained on top of a structure.  In some cases, these systems are modular, with a drainage layer, filter cloth, growth medium and plants prepared offsite as movable, interlocking grids; in others, systems components are installed separately, as in any conventional landscape.

Green roofs are also categorized as being either intensive or extensive, basically depending upon the depth of growth medium.  If that material has a depth of six inches or less, it’s seen as an extensive green roof and is prized for its lower weight, cost and maintenance factors – although it sustains a less diverse mix of plants.  By contrast, intensive green roofs have more than six inches of growth medium (and therefore greater weight, cost and maintenance) while enjoying greater plant diversity.

Maintenance of these systems is crucial.  Once the plants have become established following installation, the relatively simpler extensive green roofs generally require two to three inspections per year for weeding and to ensure that the drains are clear.  For their part, intensive green roofs may require weekly maintenance, irrigation scheduling, regular pruning and as-needed replanting.

In both cases, irrigation systems require periodic maintenance and may have to be winterized and emptied with the arrival of cold temperatures.  In addition, the waterproofing assembly will require periodic inspection and maintenance.


Once installed, all types of green roofs have certain things in common, but mostly they are defined by their differences.  In fact, green roofs vary wildly, and each one is probably worthy of a separate article or case study.

Where everything comes together, however, is the point at which you begin discussing their benefits, which accrue no matter the setting or whether you consider it from the perspective of those who own the structure or who live and work around it.  Some of these benefits are huge, obvious and profound, while others are both surprising and subtle.   

Wildlife Resources

It’s not something that immediately comes to mind, but another benefit of green roofs has to do with the fact that they provide habitats useful to wildlife on a number of levels.  

Atop buildings, for example, they host birds and various forms of insect life, while on grade they serve a diversity nearly as great as any terrestrial parkland.  And if you project it out to a time when green roofs are common and widespread, it’s not much of a reach to see a prominent role for them in providing wildlife with substantial living space – not as a replacement for natural areas, but certainly as welcome relief in urban spaces.  

In that context, green roofs might become part of larger systems that complement green corridors and wildlife passages through urban sprawl.  If established with trees, shrubs and grasses, they’ll serve the needs of insects, birds, bees and butterflies, functioning as settled islands or serving as stepping stones for transient populations in need of food, water, shelter and cover.

-- D.v.d.L.

For their part, building owners and occupants will see reduced heating and cooling costs, reduced impermeable surface areas (an increasingly important part of the design process) and the intangible value of open garden space.  In addition, by expanding the space available for use, they increase the value of a property to all who have access to the roof.

On the flip side, the general public in the area around these buildings (not to mention the local government) benefits from reductions in stormwater run-off and the heat-island effect as well as improved air quality and provision of wildlife habitat.

To date, most of the consideration has leaned in favor of the public benefits.  Already, in fact, municipalities are advancing the cause by supporting green roofs in their building codes.  By contrast, interest in the benefits on the private side has been slower to develop, probably because of the higher investment required to make these systems work.

As a consequence, most of the driving force behind green-roof development to date has come from the public side of the equation.  Helpfully, lots of municipalities are making investment in green roof systems less burdensome by forming public/private partnerships that incentivize green-roof development and shed greater light on the advantages of these systems within the private sphere.

At this point, in fact, it’s all about exposure, education and performance:  As these systems become more common and as their characteristic benefits become more widely known and they rack up longevity as public resources, they’ll gain the sort of acceptance that will lead to rapid expansion of the concept and further refinement of the technology.

What we know as advocates of these systems is that their use must become more widespread if we are all to derive full benefit from their ability to mitigate the urban heat-island effect and reduce stormwater run-off.  Those are the biggest and most impactful benefits green roofs have to offer, and these systems need to exist on a much more widespread basis if municipalities and the people who live in them are to enjoy the advantages.

On this level, advocating green roofs has in some respects become a political exercise in addition to being a practical, educational pursuit.


Beyond all that, however, what we face is a need to have skilled practitioners working with these systems as designers, engineers and installers – and that’s where watershapers in particular enter the picture because of their familiarity with methods for containing and controlling water and guiding it where it needs to go.

2Functionally, a green roof is not unlike a pool or pond in that it is a water-retaining system that must be waterproof to prevent damage to the structure it surmounts.  The additional wrinkle here is that the water needs ways to flow through the system, where it is captured by drains and carried away.

In effect, what the overall system does is slow water down rather than retain it the way a pool or pond does.   

This retentive quality is important because many older cities have combined sanitary and storm sewers whose capacity can be overwhelmed by heavy precipitation.  In capturing and (temporarily) storing stormwater, green roofs reduce immediate runoff volumes and limit the occurrence of sewer-overflow events.  This in turn diminishes the quantity of untreated wastewater that enters bodies of fresh water in the aftermath of substantial storms.  Moreover, by reducing peak flows during storm events, green roofs reduce local incidences of flooding.

Studies have shown that green roofs perform remarkably in these contexts:  Indeed, they retain between 65 and 85 percent of stormwater runoff compared to conventional roofs (with the range depending on whether the green roof is extensive or intensive).  Excess water can also be collected on the roof or at grade in cisterns and used later for irrigation, thus reducing the green roof’s use of potable water.

Because the flow is slowed, smaller roof drains can be considered on some green roofs, thereby lowering the cost of installation and further controlling the volume of water ultimately released into the sewage system.  In addition, the plants and growth medium act as filters, cutting pollutant concentrations and thereby improving the quality of water that flows into sewers.

All of these concepts should be familiar to watershapers on some level:  You have expertise in designing and building waterproof structures; you’re familiar with hydraulics and drainage issues; and you know the materials involved in the construction of these spaces.  It’s a kindred endeavor, in other words, and it’s one in which your practical skills and know-how have great value.


As mentioned above, we at Green Roofs for Healthy Cities (based in Toronto) are advocates for these systems and are doing all we can to drive the technology’s success.  At this point, we see our best potential in forging alliances between the public and private sectors.

A Call for Professionals

As the green roof industry in North America continues to expand, the need for trained professionals who are familiar with green roof design, implementation and technology has never been greater.

That is why in January 2007, participants in the green roof industry gathered together to develop a set of occupational standards for Green Roof Professional (GRP) accreditation.

To become a GRP, the individual must pass tests in a wide range of subjects covering design, installation, waterproofing, drainage, plants and growth media.  The program is designed to establish accredited professionals as key members of any green roof team, fostering a multidisciplinary approach that gives them a vocabulary that cuts across all of the professions that get involved in these projects, from suppliers and manufacturers to designers, engineers and subcontractors.

For information, contact Green Roofs for Healthy Cities at (416) 971-4494 or visit www.greenroofs.org.

-- D.v.d.L.

This is one of the reasons we spend so much time in studying the role of green roofs in the LEED rating system and are pleased to note that the addition of a green roof can contribute as many as 15 credits, depending on the design and the green roof’s level of integration with other building systems.

Whether used on their own or combined with other green approaches (such as solar power or constructed wetlands or recycled building materials), green roofs can contribute LEED points in several key categories, including reduced site disturbance, protecting or restoring open space, reducing urban heat islands, stormwater management, water-efficient landscaping, innovative wastewater technologies and innovation in design.

What this adds up to is tax benefits enjoyed by green-roof owners that help to offset the higher initial cost that comes with installation of these systems – just the kind of incentive that will spur industry growth as LEED qualification gains in significance.

In Europe, private/public sector alliances are much further along than they are in North America, probably because governments there have become actively involved with legislative and direct financial support.  That boost gave the European movement a kick-start it hasn’t enjoyed here, but we’re catching up as best we can.

Indeed, a number of jurisdictions (Chicago, New York City, Washington, D.C., and Portland, Ore.) have established policies in support of green roofs on both public and private buildings, while Toronto recently mandated a requirement for green roofs on all new buildings and commissioned a rooftop park for its city hall.

Working in another direction, some jurisdictions have begun to implement stormwater-related utility fees that help raise capital to improve stormwater-management systems.  The fees, which keep going up, are typically assessed to property owners based on the impermeable surface area of their property – a real incentive to taking steps such as installing green roofs to reduce those surfaces.


The one point we haven’t considered so far is the fact that green roofs can be beautiful spaces.  While it’s great that they save energy, reduce storm runoff and benefit wildlife, the fact that they can also greatly beautify unsightly rooftops provides a host of psychological benefits to people who live and work in their vicinities.

For all their material benefits, there may in fact be none more important than the ability of green roof designers and builders to create serene, beautiful spaces where people can relax.  There’s a wonderfully unexpected joy at finding a naturalistic environment atop a building – and surrounding views take on all new meanings when enjoyed from a green oasis.

As we see it, the greener our building tops become, the better life will be everywhere.  To make this happen, the watershaping industry needs to get involved!


Steven Peck is the founder and president of Green Roofs for Healthy Cities, a Toronto-based association that serves North America’s green roof and wall industry. The organization’s mission is to increase awareness of the economic, social and environmental benefits of green roofs, green walls and other forms of living architecture through education, advocacy, professional development and celebrations of excellence. He can be reached via email at This email address is being protected from spambots. You need JavaScript enabled to view it. . Damon van der Linde is the association’s marketing and research coordinator.

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