As big Canadian cities grapple with climate change, green roofs are catching carbon, shielding buildings from sunlight and moderating temperatures. Equally important for Halifax, which averages more than 1.35 metres of rain per year, is how green roofs reduce flooding by absorbing storm water and filtering runoff into sewers.
According to Halifax’s Ecology Action Centre (EAC), by 2009, Halifax Regional Municipality had more than 50 green roofs. More recently, specialized landscapers and ecologists have recommended simpler roof layering and resilient native plants as recipes for successful green roofs.
Sue Sirrs, founder of Outside! Landscape Architects, says that many green roof companies apply numerous layers and, in the process, make rooftops “overdone,” heavy to support and expensive. Working with a crew from Denmark’s Faroe Islands on a green roof for a Newfoundland home, Sirrs learned about the Danes’ method of anchoring sod with fishnet and rocks.
“They had a good laugh at all the crazy things that we do here with all the layers of green roofs,” she said. “[Theirs are] very applicable to our climate here too, simple to maintain, low cost to install, and easy to fix if there’s a heavy wind and the sod is pulled off the roof.”
In terms of design, the gaps in the netting allow the inner sod layer to nourish the exposed outer layer. Sirrs’ team has now built similar sod green roofs in Halifax. She said residents can follow suit by using plant mats from a garden supply store. She also recommends native sod to sedums (easy-to-grow succulents), which are often used for green roofs, yet non-native to Nova Scotia.
Jeremy Lundholm, an ecology consultant and adjunct Saint Mary’s University professor, spent two decades testing approximately 20 native plants on Halifax green roofs. To find species fit for windy rooftop environments with minimal soil, he tested plants that survive in harsh coastal barrens.
Species Lundholm recommends include silverrod, three-toothed cinquefoil, poverty oatgrass and roseroot. He described these as “really bomb-proof” and providing good rooftop cover to prevent wind erosion of the soil.
“They strike a good balance in terms of being drought-tolerant,” Lundholm says. “When there is water, they’ll suck it up and grow a lot — more than the sedums.”
Soil as shallow as five centimetres can suffice, but to offset the impacts of seasonal drought, Lundholm advises 10 to 15 cm for commercial green roofs. Deeper soil also accommodates more shrubs, which add biomass and help maximize carbon capture.
“If you look at all the green roofs in Halifax, probably only five per cent have trees or substantial shrubs,” Lundholm noted.
Halifax Central Library hosts an especially popular green roof. Its main section, publicly accessible and containing 650 square metres of sedum, does require “considerable maintenance,” according to Megan Gainer, facilities director for Halifax Public Libraries.
Stormwater unabsorbed by the lightweight soil is filtered through roof drains into the library’s grey water cistern, which uses the non-potable water for the building’s toilets and directs leftover water into the city’s sewer. Amid a projected rise in flooding for Nova Scotia due to climate change, Lundholm wants more buildings fashioned with similar integrated rainfall retention and filtering systems — to reduce urban storm water. In its storm water management standards, HRM recommends green roofs.
The library’s green roof is also “much loved for its ability to introduce natural elements, and its calming presence within the downtown,” Gainer says, adding it’s a “rare feature in Halifax” that enhances the rooftop patio.
As Halifax’s skyline grows, Sirrs says having these green roofs visible and accessible to people living and working higher up is beneficial.
“I think we do undervalue beauty,” she says, “…in terms of seeing green spaces, when we have access to them, and when we can engage with them.”
Sirrs emphasized that green roofs always need at least minimal maintenance, by “somebody who cares about plants.” Habitual upkeep in the first two years is vital to establishing the vegetation.
On her own shed roof, Sirrs experimented with native plants for 15 years, only assisting twice — once to water them the first year, and then to clear off heavy snow in 2015. With minimal maintenance, she was eventually harvesting a large raspberry crop.
Halifax’s Regional Plan is currently being updated to allow green roofs and greenhouses to exceed maximum building heights, enabling more local food production.
“Hopefully we’ll see more green roof construction as people take climate change and its consequences more seriously,” Lundholm says.