A green, or “living,” roof is one that is partially or completely covered with vegetation. Forward-thinking cities are looking to green roofs and gardens to help reduce building temperatures, filter pollution, lessen pressure on sewer systems, and reduce the heat island effect.

Toronto is one of these forward-thinking cities. In January 2010, Toronto began offering industrial and commercial building owners up to $100,000 per year to make so-called “green roofs” of their rooftops.

Moreover, Toronto became the first city in North American to require green roofs for developments over a certain size. Starting in 2010, large, new urban developments had to devote 20-60 percent of available roof space to a green-roof construction.

A New ‘Last’ Frontier

Steven Peck of the Toronto-based non-profit Green Roofs for Healthy Cities (GRHC) calls city rooftops “the last urban frontier.” He says the “wasted space” on rooftops makes up 15 percent to 35 percent of the total urban land area. GRHC also concurs with the U.S. Environmental Protection Agency’s assertion that green roofs mitigate the urban heat island effect.

Just as the green roofs cool city neighborhoods, so too can green roofs save owners money on the cooling — and heating — of their buildings.  GRHC cites field experiments in Ottawa that found that an extensive, 6-inch green roof can reduce heat gains by 95 percent and heat losses by 26 percent, compared to a conventional roof.

Rooftop Greenhouses

In Canada and the Northern United States, open-air farms on green roofs are limited by the short growing season. So, in colder climates, rooftop greenhouses are emerging as logical vehicle to deliver a year-round harvest of fresh, local produce to city dwellers.

For building owners, rooftop greenhouses will likely offer some of the same benefits as green roofs, namely, efficient water use and collection, heat-island mitigation, and better insulation.

My company, Lufa Farms, uses a 31,000-square-foot rooftop greenhouse in Montreal to grow garden-fresh greens and vegetables atop a sturdy commercial building, near, fittingly, the Marche Central, the one-time farmers’ wholesale market that is now a retail shopping center. We’re also building a second rooftop greenhouse in Laval.

Since Lufa’s greenhouses are the first of their kind and size in the world, my colleagues and I have had to become evangelists and urban pioneers to overcome hurdles related to zoning, building codes and engineering, legal, leasehold, and taxation issues.

Overcoming Zoning Challenges

Finding the right buildings also pose a challenge, with crucial questions to be answered: Is the building right structurally? What are the height issues? Does it have the proper access and zoning?

Prior to building our first greenhouse, Montreal had no zoning for agricultural buildings or local building codes for agricultural construction. (The last of the city’s farms had disappeared some 30 years ago when Montreal urbanized and the old farmland was rezoned.)

Ultimately, the local zoning departments have been receptive and have revised the zoning and have accepted national building codes for the permitting.

It’s Working in Montreal, so…

Over the next five-to-seven years, we will be scouting in Toronto, Boston, New York, Philadelphia and Chicago for existing buildings on which to site more greenhouses. We are also looking to partner with developers interested in constructing new buildings with greenhouses integrated into the designs.

If you’re planning a large building, a greenhouse is even more feasible if you plan it from the start. The optimal (and less expensive) solution is to design the building to support hybridized use.

In addition to a rental revenue stream, an obvious economic benefit for city buildings are lower maintenance cost, since a roof beneath a greenhouse is protected from the weather.

Similar to a green roof, a roof greenhouse will typically reduces a building’s utility costs, too: the structure insulates in the winter so the floors underneath do not need as much heat. In the summer the greenhouse provides a degree of cooling through plant transpiration, so that the building will have a lower demand for air conditioning.  If the design operates as planned, the owner will be able to earn credits toward certification as a Leadership in Energy & Environmental Design building.

A couple of things to keep in mind if you are investigating a rooftop greenhouse for your building:

• The structure must support the weight of the greenhouse
• The roof must be flat
• The structure must meet local snow-load requirements
• The roof should have an area of 100,000-200,000 square feet of space for the greenhouse
• To pass through zoning, the building should ideally be located in a commercial, residential or industrial area

Got questions? I’m happy to talk to you anytime about what else we’ve learned at Lufa Farms.