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Building more green roofs could create the "Hanging Gardens of Halifax"

There are secrets to greening Halifax in the coastal barrens of Peggys Cove.

Scott MacIvor is conducting groundbreaking research there as part of his master's degree in applied science at Saint Mary's University: "I'm studying the plants."

MacIvor notes that coastal barrens plants live in similar conditions to those you might find on a Halifax rooftop. "There are high winds, they are exposed to sun and drought and they are regionally specific, accustomed to our climate," MacIvor says. He and his supervisor sought local plants that survive in harsh conditions in the hope of revolutionizing Halifax rooftops.

A green, or living, roof is a plant-based extension up from an existing roof. It requires extensive waterproofing and protection from root growth, a drainage system and relatively lightweight plants.

"I'm passionate about green roofs because I've always lived in urban environments," MacIvor says. "We have a lot of impervious surfaces, roads and buildings, which don't allow water through."

Concrete and asphalt create storm-water runoff, one of the reasons Halifax Harbour is polluted. These hard surfaces also trap the heat of the sun, creating the "urban heat island effect," whereby cities tend to be a few degrees hotter than surrounding greener pastures.

MacIvor lists the many benefits green roofs provide: "They counter the urban heat island effect and mitigate carbon dioxide and other air pollutants; increase biodiversity; provide refuge for valuable urban wildlife including carnivorous insects for pest control; allow for storm-water retention; provide cooling, insulation and energy savings and they protect the roof membrane so the roof lasts longer." Green roofs also increase sound insulation and fire resistance, improve general health and reduce stress.

The environmental and health benefits of green roofs have been documented as far back as the hanging gardens of ancient Syria and in Western European nations such as Germany and France they blossomed into a multimillion-dollar industry in the early '90s. We've been slower on the uptake in North America, where the benefits are poorly understood. According to MacIvor, we have the potential.

"I see increasing growth across North America," he says. A few years ago, a survey of green roofs in Halifax turned up more than 50 examples, atop institutional buildings and personal residences. MacIvor notes a few recent highlights, including a green roof at Citadel High and a second, bigger green roof being installed at SMU. He says that with an increasing understanding of locally appropriate plant technology, the number of green roofs in town could explode.

"Green roofs in North American conditions are not well researched," says MacIvor. "And you have to do it city by city; you can't copy what we know from Toronto in Halifax." The climates are too different. Most of the world's research on green-roof technology comes from Germany, which means that we may not be using the best available plants for our climate.

MacIvor's research, conducted out of SMU's Green Roof Testing Facility, aims to enhance our green-roof plant selection and bolster efforts to green Halifax's skyline. The data will be used to create "habitat templates," plans for real living rooftops.

If these hardy flora prove effective, Halifax may be the perfect milieu for a green- roof renaissance because of our low density and relatively flat cityscape. According to a recent paper from the American Institute of Biological Sciences, "Green roofs will have the greatest effect on energy consumption for buildings with relatively high roof-to-wall-area ratios." In other words, low-rises.

Green roofs offer the greatest energy savings for homeowners, who may eventually benefit from another advantage of coastal barrens plants: "They can grow in only two inches of soil," says MacIvor. This saves money and reduces weight, a factor in making green roofs workable on smaller structures.

Aside from the lack of local research, cost is the biggest reason green roofs have yet to flourish on your street. Costs vary widely depending on a roof's size, type and angle, but generally fall between $10 and $20 per square foot. It's a serious investment for something that could cause its progressive new owner a winter of suffering over die back---when the plants go dormant.

For that reason, many "green" builders prefer to stick with tried-and-true solar panels, but MacIvor resists the garden-versus-solar-panel debate. "Solar panels can be used in combination with green roofs," he says. "But green roofs offer benefits to the environment beyond thermal benefits, like absorbing storm water." Green roofs also save money long-term due to energy savings and increased longevity of roof membranes.

Overseeing the economic and the scientific is the political. In European states with large green-roof industries, inevitably you find legislated green-roof requirements written into building codes. So far, Halifax has talked good sustainability talk, but regulated little other than building heights and planning processes.

Do you have a green roof? Drop Chris Benjamin a line at chrisb@thecoast.ca.

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