In 1997, when Neil MacDougall decided to build a workshop (for him) and studio (for his artist wife Linelle) in their North Toronto garden, his imagination was sparked by a magazine article showing a small building with a cow grazing on its sod roof. “I did not think the neighbours would like a cow”, recalls Neil, “but the idea of a functional roof planted the seed which germinated into our rooftop garden.”

An art school acquaintance of Linelle’s recommended architect David Lieberman. “He had an art and woodworking background, critical for the design and layout of my husband’s workshop and the studio,” says Linelle. He also had an established track record in designing green roofs, beginning thirty years ago with a cottage in England.

Unlike a “rooftop garden” which usually describes a flat roof with some type of paving or decking and movable or built-in planters, a green roof lies directly on the roof. Europe has a long green roof tradition with cities like Mannheim, Germany mandating the replacement of ground level green space lost to new commercial buildings with an equal area of rooftop planting. It is only in the last decade or so that the North American design community has really embraced green roofs, thanks to technological advances in roof membranes, improvements in soilless mixes and a desire to create landscapes that give back to the environment more than they take.

In green roof design, the roof itself becomes a massive shallow planter with the soil and plant roots absorbing rainwater and pollutants that would otherwise run off into storm sewers. The green roof insulates the building below against temperature extremes, resulting in lower heating and air conditioning costs. It also extends the life of the roof membrane by radically reducing the roof’s summertime temperatures (a planted roof can be at least 21C or 36F degrees cooler on a hot day than a typical tar-and-gravel roof) and blocking damaging ultraviolet rays. A green roof invites wildlife to a leafy, above-ground habitat and, best of all, it transforms a bleak, utilitarian surface into a delightful vista for those fortunate enough to overlook it.

It was the view from Neil MacDougall’s home office that David Lieberman was intent on enhancing when he set to work. “The workshop was purpose-designed with a green roof in mind from day one,” he notes, adding that although it is possible to retrofit an existing building, the best results come from designing the green roof from the ground up. With an existing building, he says, “It’s very doable, but what has to be examined is not only the quality of the roof membrane from a technical standpoint, but the bearing capacity of the structure, the roof angle, drainage, all sorts of things.”

In other words, a green roof – particularly over an occupied building -- is not a do-it-yourself project, unless structural engineering is your hobby. And though there are now many “out-of-the-box” modular green roof systems available from companies like Soprema and Garland, it is vital to ensure your building’s architectural integrity before topping it with a garden that can weigh up to 30 pounds per square foot when wet.

For the MacDougall workshop, David created a vented air space between the building’s insulated ceiling and the green roof. The vents look like decorative portholes on the side of the building but they serve to keep the soil above frozen in winter so the plants remain dormant. The roof features a waterproof, single-ply EPDM membrane and is sloped slightly to assist drainage. To allow for a range of soil depths, a 12-inch high metal parapet surrounds it.

When the time came to do the planting, David called in Terry McGlade of Perennial Gardens Corporation. The pair has collaborated on several green roofs over the past ten years, including the Merchandise Building in downtown Toronto. It is Terry’s expertise with soil mixes, plants and climatic conditions that ultimately determines the success of each project. Rooftops on tall buildings in the downtown core, for example, are buffeted by wind and much colder than residential rooftops.

Terry’s team began by laying a perforated drainage tube over the roof, connecting it to an overflow scupper which drains onto the art studio roof. A 3-4 inch layer of Styrofoam pellets went in next, followed by filter cloth to keep soil and plant roots out of the drainage area. Then came 6-9 inches of compost-rich soilless mix. After wetting that thoroughly, it was time for the plants. “This garden was designed in response to sun and shade conditions,” says Terry, “with hostas under the trees to the east and succulents, grasses, alpine plants and prairie flowers in the sunnier west side..” In went sedums, cranesbills, strawberry plants, phlox, thyme, liatris and feather reed grass.

Despite being fertilized only once, almost all the plants have thrived in the six years since the roof was finished. (Purple coneflower did poorly, possibly because it prefers sandy soil.) “It has all the challenges and responsibilities of a regular garden.” says Linelle. “If it doesn’t rain, we water it as we do the other gardens around the house.” But the workshop green roof is such a success that she eventually plans to install a second one on the studio featuring only succulents, sedums and grasses that require no deadheading.

Terry McGlade puts it best: “When you think of different ways to garden, the sky’s the limit!”

Adapted from an article that appeared in Gardening Life magazine

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