Native grasses rustling in the wind, birds and butterflies finding food and shelter – just another summer day at Toronto’s City Hall – that is, if you happen to be up on the Podium Roof, which features over 3200 square feet of green roof demonstration gardens. Whether on public buildings, commercial sites or even atop your neighbour’s garage, green roofs are an environmentally sound design concept, rapidly gaining support in Canada.

“Just think of all the waste space waiting to be used,” says Toronto architect, Monica E. Kuhn, who has designed both commercial and residential green roofs.

Kuhn points out that green roofs should not be confused with roof gardens, which usually involve planting in containers. A green roof is actually another layer of the roofing system itself. However, an accessible green roof (as opposed to one designed only for viewing) may combine elements of both, with the addition of planters and pathways, for instance.

While currently regarded as an innovation in North American design, the idea of green roofs is not a new one. In ancient Rome, Pliny wrote about imported trees for green roofs. In Canada, Western pioneers commonly built their first homes with sod-covered walls and wooden roofs topped with prairie grasses. Since the 1970s, green roofs have been increasingly incorporated in commercial building design in many European countries and have gained active government support, particularly in Germany, where green roof technology and installations have become accepted elements of the construction industry.

Monica Kuhn, who sees green roofs as perfectly viable for private residences, explains the basic components.

“For most homes, the roof structure is probably wood with the framing and the sheeting on top of the joists and rafters,” she says, “and then you have the roof membrane which can be tar and gravel, bitumen or asphalt shingles.”

You can work with your existing roofing, but you must be sure to protect it in some way and to separate it from the wet soil. The drainage layer which is part of every green roof design fulfills this function. “This can be just air or gravel or even Styrofoam,” says Kuhn. “Its purpose is to separate the soil from the wood and give a pathway for water to run off.”

The fourth layer is filter or landscaping cloth to contain the soil and roots. Next are the growing medium, and finally the plants.

If you are planning an entirely new roof, you might consider installing one designed specifically for green roofs. Some include a very thin metal sheet between two layers of roofing to prevent root penetration.
Green roof research and demonstration projects are on-going throughout North America, reflecting the current interest in their environmental, economic and aesthetic benefits. The demonstration gardens at City Hall are an endeavour supported by a partnership between the City of Toronto, the Toronto Atmospheric Fund, the Federal Government and Green Roofs for a Healthy City, a consortium of related businesses.

“Just as we invest in roads and bridges, we need to invest in our roof spaces as a part of the city’s infrastructure,” says Stephen Peck, Executive Director of Green Roofs for a Healthy City, whose members include Elevated Landscape Technologies, Flynn Canada Ltd., Garland Canada, IRC Building Sciences Group Inc., M & S Landscape Architects, Sheridan Nurseries and Soprema Canada.

“In order to accommodate urban population growth, we can’t just keep developing outward,” says Peck. “We need to bring people into the city and offer an improved quality of life by bringing nature back into the city.”

While the aesthetic advantages of green roofs are clear, they offer very practical benefits as well. Plants act as insulation, with a resulting savings in heating and cooling costs. They also act to moderate the Urban Heat Island Effect, which is the difference in temperature between the city and its surrounding countryside, caused mainly by the expanse of flat surfaces, such as roofs, reflecting solar radiation as heat.

Air quality can also be improved as airborne particulates are filtered by the plant life as the air moves across a green roof. As well, the soil, plants and the drainage layer of a green roof can act as sound insulation, absorbing traffic and machinery noise.

And, according to Stephen Peck, their value also lies in “retaining stormwater, providing biodiversity opportunities and even the potential for food production.”

While projects, such as the City Hall demonstration gardens, are designed to produce Canadian research data on green roof technology, the concept is readily adaptable to individual homes.

Kaaren Pearce, Horticulturist with Elevated Landscape Technologies, worked on the installation of two green roofs on a Toronto home last year. One is over top of a double garage and the other over the family room. Both are designed primarily for viewing, rather than accessibility.

“The plant material was chosen for minimal maintenance,” she says. “The only big concern is the trees.”

Pearce notes that most of the trees in our area reproduce by windborne seeds, so that it’s important for green roof owners to be particularly watchful after they have seeded. “Look at when they start growing in your garden and when they get to about three inches, that’s when you want to check the roof.”

“In this case, the homeowner loves the plants so she spends a lot of time looking at them.”

While this particular installation involved two flat roofs, it is also possible to install green roofs on slopes.

“Homeowners should definitely have the grade of their roofs assessed by professionals,” says Pearce, who is planning to install a green roof on her own sloping front porch this season.

Experts agree that the most important factor in mounting a green roof is the loading capacity of the roof. The solution is to know the structure of the building so as to place the load on bearing walls or columns. Using a lighter weight growing medium than garden soil is also necessary as wet soil weighs approximately 100 lb. per cubic foot. Soils that are part vermiculite, for instance, can weigh in at 70 or 80 lb per cubic foot. The depth of the growing medium can also be reduced considerably from that of the average garden.

Monica Kuhn recommends consulting the drawings of your home to discover its loading capacity. “Your city hall may have them on record if there’s been work done or you are in a new home.”

You may need to consult an architect or engineer to interpret the drawings and if you cannot locate any, employ an engineer to inspect your home and assess the load capacity.

“If your green roof is accessible you will need to comply with the Ontario Building Code,” says Kuhn.

For instance, there must be a guard rail around the roof at a height of three feet six inches. Any planters or other permanent structures such as benches must not be placed against it where they might act as steps up.

The key to adding a green roof is to ensure that the existing roofing is protected and separated from the wet soil with a drainage layer. Skylights and vents must also be protected, usually with a gravel barrier, according to Kuhn. Roof edges customarily have a protective barrier as well.

As for what to grow on your green roof, Monica recommends experimenting.

“As in every garden, those on a roof will have microclimates depending on their orientation, any wind shading, or building shading. Typically it will be hotter and there will be more extreme conditions, especially wind.”

The demonstration gardens at City Hall include a bird and butterfly meadow with native plants such as eastern columbine and bottlebrush grass. The Kitchen Garden, designed to demonstrate the potential for urban agriculture (and supported by Toronto’s FoodShare food bank), features peppers, tomatoes, sage and chives.

Monica Kuhn also suggests hardy alpine or Mediterranean plants.

“Lavender, rosemary, portulaca, geraniums – all would work well,” she says. “Or try native species of wildflowers or grasses.”

Green Roofs – by Mary Lynn O’Shea

Mary Lynn O’Shea is a Master Gardener and member of the Garden Writers Association.

She has been a freelance writer for the past 20 years, specializing in gardening topics for the last five. Her articles have appeared in Canadian Gardening, The Toronto Star, Ontario Gardener, Landscape Trades and Today’s Parent.

Her personal garden favourites include passion flowers, roses of all kinds and good mulch.