Story and photos by Colin Viebrock

From my second-floor home office I can count 57 garages dotting the landscape, the vast majority roofed in black shingles or gravel. Laneways are not particularly attractive at the best of times, and homeowners who pay great attention to curb appeal will simply hide their unsightly garage with a trellis or grapevine.
When it came time to replace my decades-old, sagging structure, I decided to try something different: design and build a garage with a green roof. Green roofs have fascinated me for many years, and there are a number on large buildings in Toronto, where I live, but few on residential properties, and to the best of my knowledge, there were no green-roofed garages in Toronto (now there are three). The benefits of a green roof, even a modestly sized one, are many. They can reduce your heating and cooling costs, increase your roof’s lifespan, provide sound insulation, and add aesthetic appeal. On a broader environmental level, green roofs reduce and filter storm water run-off, filter pollutants out of the air and water, and actively cool the surrounding air, helping mitigate the “urban heat island” effect (where cities are up to five degrees hotter than the surrounding countryside). Green roofs may provide additional habitat for birds, butterflies and other wildlife, which is a wonderful addition to the view.
With some caveats, a green roof is also an easy way to expand your garden. Those of us living in urban centres often have to make do with small gardens. The green-roofed garage has almost doubled the amount of green space on my property—and increased the property value.
This became the ultimate do-it-yourself project, requiring a good mix of chutzpah and arrogance. Research led me to a supplier who, fortuitously, was offering a course in installation and I began my practical education there. I designed and built the garage, with the help of a contractor, over a one-month period last year, then installed the green roof in a week this spring.
A green roof starts with a waterproofing layer. You can install it over asphalt or tar and gravel, but for new construction, a rubber or plastic membrane is better because it also acts as a root repellent. On top of the waterproofing layer are drainage, water retention and filter layers. These layers also act as root barriers but, more importantly, they help manage the amount of water that is retained on the roof and ensure that the growing medium doesn’t clog the drainage layer and wash away.
Next up is the growing medium. This is usually a lightweight, custom mixture, composed mostly of expanded stone, volcanic rock, perlite, with only a 10- to 20-percent organic content. The goal is to find a balance between a soil that will sustain the plants, but that won’t weigh so much when wet as to require excessive support from the building’s structure.
The top layer is, of course, the plants. These are chosen to match the composition and depth of the growing medium. For shallow, lightweight roofs, a mixture of sedums and iceplant (Delosperma) varieties is typical. These shallow-rooting succulent plants do well in dry conditions (requiring less maintenance) and in poor soil. With deeper roofs, the plant selection can expand to include native grasses, wildflowers … even some herbs and vegetables. But on our garage we kept it simple. Even still, the 10 or 12 varieties of plants create a unique palette. Since they all have different leaf and flower colours, and flower at different times of the year, the view from the home office is constantly changing.
One of the most important considerations when designing a green roof is weight. Green roofs are heavy, and (unlike every other roofing material) they get heavier when wet. Even my two-inch deep roof weighs around 15 pounds per square foot when it rains—that’s around three tons for the whole garage. It’s comparable in weight to a tar and gravel roof, but even still … I can’t stress enough how important it is to get the advice of a structural engineer, whether you are building new or retrofitting an existing building.
The question I’m most often asked (after “You’ve got plants up there?”) is how much does it cost to build a green roof. The short answer is: it costs more. First, the structure needs to be beefier to support all that extra weight. Figure in $500 for a structural engineer to review your drawings (all dollar values approximate), and then the extra cost in materials (maybe an additional 10 to 20 percent). As for the actual waterproofing and greening of our garage, the cost of materials worked out to around $12 per square foot. If you were to add labour costs, you’d probably end up somewhere around $15 to $20 per square foot. A call to several reputable roofing companies yielded quotes of $10 to $15 per square foot for a high-end modified bitumen flat roof.
To defray the cost, if you happen to live in Toronto, you can get in on the second phase of the Green Roof Pilot Project. Under the first phase, of which I was a part, owners were eligible for a subsidy of $10 per square metre (about $0.93 per square foot) for installing a green roof. Phase two is increasing that subsidy to $50 per square metre ($4.65 per square foot). This brings the cost of a green roof right in line with a traditional system. (See www.toronto.ca/greenroofs for all the details.)
Of course, I now have a longer-lasting, environmentally friendlier, and prettier roof for my troubles. Which, as I write this from my home-office, is definitely worth every penny.

*Originally published on Ontario Gardener 2007 Fall issue