The concept of a green roof is pretty straightforward – a thin layer of vegetation used as a rooftop covering instead of traditional, petroleum-based exterior weatherproofing materials.
There is a growing trend towards green roofs, which stems from (pun fully intended) efforts to incorporate the vegetation that surrounds a building site into the architectural design. But the idea goes beyond aesthetics. Green roofs are a sustainable solution, not only because of the reduced need for off-the-shelf, fossil fuel-based products (such as shingles or rolled roofing), but also because they have a number of other ecological advantages.
For example, in the deep heat of summer, a 7cm (2.75 inch) layer of soil, rock and plants can dramatically reduce the amount of heat that transfers into a building. So one immediate benefit is reduced energy consumption, since air conditioning systems would not have to work as hard as with asphalt-based roofs.
There are a host of other great benefits that come from going green:
- Water retention: Rainwater is retained longer and released more slowly, reducing the possibility of harmful erosion on the surrounding landscape.
- Water filtration: Rainwater is cleaned by the roots of plants and the microorganisms that exist under the soil.
- Natural cooling effect: As roof plants transpire, the water they release often helps to cool the roof area dramatically.
- Increased lifespan: Green roofs block ultraviolet radiation and other environmental effects that typically reduce the life of a roof.
This last point is especially true for a green retrofitted to a traditional roof.
A quick word of advice: when retrofitting a green roof system on a traditional roof covered by shingles, use a heavy duty a filter fabric so the roots do not penetrate the old shingles and create issues. In new construction, rubber membranes or other materials can take the place of shingles, which reduces construction labour costs.
Considerations for going green
A primary consideration when deciding on a green roof is climate. The right roof plants must be chosen to grow in a given environmental context. Think about where you live: Is it hot? Is it dry? Is it always sunny? Is the roof subjected to constant wind? Draw up a list of the basic environmental conditions the roof plants are likely to face.
If you don’t have an external water source available for the roof, the plants should have some mechanism to survive dry conditions. The most common dry-tolerant plants are sedums or stonecrops, which are traditionally very shallow-rooted, with thick, fleshy leaves that can hold significant quantities of water to last through periods of extended drought.
Roofers are often scared of the idea of vegetation on roofs because roots can find their way through underlying building materials and cause leaks. However, many plants are suited to roofing without compromising the weathertight seal; it depends mainly on what the roof is to look like and what the filtering requirements are, if any. Native plants, other herbaceous plants and many exotic plants from around the world can create a roof that has great benefits to the surrounding community.
When it comes to green roofs, there are two ways to go: intensive roofs are thicker and can support a wider variety of plants, but they weigh more and require more maintenance. Extensive roofs are lighter as they use only a thin layer of lightweight materials (pumice, volcanic stone, and expanded shales) mixed with sands and long-term nutrients to support the growth of plants. This additional weight can usually be tolerated by standard roof designs.
Note however, that common perennials or traditional grasses require increased soil depths, which not only means increased weight, but the possibility that additional structural support may be necessary.
Precautions to consider
A waterproof membrane is required under the growing medium, which is likely to last 30 or more years. Often, a used pool liner works as a D-I-Y solution, providing it does not have a leak.
Green roofing systems are typically designed for low slope roofs. Steeper pitches usually require special modifications to ensure flow of water over the roof does not contribute to washout, or does not leave a patchwork of extreme wet or dry areas.
As a general precaution, it’s a good idea to consult a qualified building engineer before getting started to ensure a project meets building codes and is structurally safe.
The Internet has tremendous resources for research. Here are a few to get started:
- Centre for Architectural Ecology
- Penn State Center for Green Roof Research
- Harvard University – 29 Garden Street
- Green Roof How To: Installing Your Own
- Small Green Roofs, Low-Tech Options for Greener Living
Green Roofs in Action: There’s not much entrepreneurs Henriette and Dieter Maier can’t tackle. When Dieter built a small extension onto their West LaHave, Nova Scotia home for Henriette’s fibre art operation, he opted to try a green roof, inspired by a trip the couple took to Norway (where, they say, green roofs are abundant).
“We love the look of it, we love the sustainability of it, and for me – personally – as a fibre artist, it’s a natural thing to do,” says Henriette. The couple says the roof, which has been seeded with a mix of grasses and wildflowers, is extremely easy to maintain and very energy efficient.
Dieter’s latest building project, a two-bay garage, was only recently completed. What kind of roof does it sport? Green, of course.
The Maiers recommend “doing your homework beforehand regarding construction, permits, soil preparation and the selection of seeds.” Other handy advice: talk to someone who has a green roof, get tips, read up on it, then go for it and enjoy! [Henriette and Dieter are very approachable and would welcome those who are interested (or curious) to stop by and have a look — Ed]
Jeff Morton has worked in environmental horticulture for 30 years, as an educator, nurseryman, landscaper and consultant. He enjoys landscape design challenges and has installed green roofs and remediation landscapes in Nova Scotia and Ontario. Jeff lives and gardens in Truro and on Caribou Island.