We are currently in the process of getting a green roof on our backyard shed, and the question I’ve been asked most often is why?

Indeed, why would we do this? My father shudders at the notion of putting soil and plants up on a roof. It could leak and compromise the roof, he warns darkly. Some of my older neighbours shake their heads doubtfully at the idea.

Even some of my friends look blank and want to know what it is and how it could possibly work when I mention our green roof.

But interest and knowledge is growing. In many neighbourhoods, green-roofed areas have started to appear, mostly on sheds or on flat overhangs over porches or backyard decks.

We’ve been dealing with a company called Xeroflor (xeroflor.ca), which installs green roofs using specialized materials, and has offices in Toronto, Vancouver and Montreal.

This company has been doing it here since about 2001. So far, it’s been mostly on commercial buildings and condos, with only a sprinkling on private residences. But with increasing interest in the residential sector, and an upcoming new Toronto bylaw mandating green roofs (January 2010) on many commercial, industrial and institutional buildings, interest and knowledge will continue growing.

A green roof is what it sounds like — a roof that is green with plants rather than black with roofing tar. The best scenario is a roof that is engineered with a green roof in mind. Existing roofs can also be used as a green roof, with some conditions related to reduced weight load.

The roof must be finished before the green roof is applied in order to waterproof it. Some larger roofs may be built with access, but for most residential applications, a green roof is visual, meaning that the view can be enjoyed from a window, but does not allow access. And it is usually on a flat roof — it’s not recommended on a sloped roof.

Different plants can be used, and will grow on a growing medium selected by the green roof company, but plants are all selected to handle the hot, dry conditions on most roofs. If you’ve got a shady space, you may want to consider a moss mat. The one being installed on our shed is a low-growing sedum (4 to 6 inches high), and does not normally require watering.

The cost of installing the Xeroflor system is about $15 per square foot, depending on size and accessibility. Doing it yourself is not recommended for green roofs because of its specific requirements.

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