Green roofs are celebrated for their environmental and financial benefits. Commonplace in Germany, they are slowly growing on Americans.
When her two children were young, Barb Bryant wanted to get them interested in gardening. They couldn’t have cared less, but the endeavor was an inspiration. In 2023, Bryant turned the rooftop of her Cambridgeport garage into a 1,200-square-foot green roof, complete with perennials, vegetable plants, and an apple tree.
“Years went by, and we kept daydreaming about this garden on the garage roof, and then just decided to make it happen,” said Bryant, a computational biologist who also runs a nonprofit called Navigation Games. Her domestic partner, Dave Yee, works at Novartis.
Bryant and Yee weren’t alone in their dream to turn the garage into a green roof, a vegetative layer grown on a rooftop. Commonly referred to as a “living roof,” green roofs are commonplace in Germany and celebrated for their environmental and financial benefits. But on this side of the pond, the practice has barely scratched the surface.
Bryant reached out to Recover Green Roofs, who designed and built rooftop farms at Fenway Park and Boston Medical Center. First, they had to make sure the garage was strong enough structurally. Straddling her property and the neighbor’s, the garage wasn’t entirely owned by Bryant, who negotiated with the neighbors to craft an agreement in which she would pay to rebuild the garage, which would be necessary to support the weight of the roof deck and garden. In exchange, they would allow her to garden on their side of the new roof deck for the next 10 years. Next, she had to get approval from the city of Cambridge and the zoning board, which wanted to make sure the roof deck wouldn’t become a party space. She ultimately received approval.
These days, a staircase from Bryant’s driveway leads up to the green roof, and there’s also a flyway that goes from the home’s second floor to the garage roof. There, you’ll find a corner of the garden filled with sedum, grasses, and a variety of other small perennials. There’s an apple tree, as well as a couple of peach and pear trees. The rest is filled with food plants and flowers, allowing the family to grow what they eat. A picnic table provides extra living space, essentially expanding the square footage of the home.
“We really enjoy being able to produce our own food,” said Bryant.
Drive down Route 1A in Revere, and you’ll spot a 6,000-square-foot living roof, reportedly the largest and steepest on a residential project in New England, at Gibson Point, a new 291-unit apartment community. Cascading diagonally to provide visibility from the ground up, it’s filled with 15 to 16 species of sedum and will fill with pink, yellow, and white blooms come springtime.
From an environmental perspective, the benefits are significant. At Gibson Point, the living roof’s soil serves as an insulator. It absorbs stormwater, and in higher-volume storms, tempers water that travels downward. When sunlight hits the diagonal slope, it decreases energy costs for the building and sends water vapor back into the sky.
“I think this plays well into the concept of regenerative design, which is the principle of leaving the site in a better place than where you started,” said Amy Korte, president at Arrowstreet, the architect behind Gibson Point. “You’re thinking beyond just designing for the people in the space, thinking about the climate and habitats that you can create.”
Green roof temperatures can be 30 to 40 degrees lower than those of conventional roofs and can reduce citywide ambient temperatures by up to 5 degrees, according to the US General Services Administration. Green roofs provide shade and stormwater management and reduce heat islands.
“A green roof is a nice antidote to help cool our cities without using electricity,” said Mark Winterer, owner and cofounder of Recover Green Roofs.
But of course, there’s one glaring question: How much does a green roof cost? Winterer said the cost runs “a huge gamut.”
Installation costs depend on “the size, the depth, the types of plants, and how high it is on a roof,” said Winterer, noting a smaller residential green roof would go for $55 to $75 per square foot, but prices go down the bigger it is.
“The size, depth, and how we access the roof will impact the price,” Recover said in an email to the Globe. “We installed a lightweight green roof on a garage in Lexington last year that we could back our trucks up to from their driveway, so it was super easy to access without heavy equipment. That covered half of a 1,200-square-foot garage roof and cost $35 a square foot. If that client covered their whole roof (they had a deck on the other side), it would have cost $25 per square foot.” This doesn’t include the cost of shoring up the roof to handle the weight of the garden and the snow load.
“We work with structural engineers on all our projects, designing our systems within the buildings’ available loading capacities,” Recover wrote. “Most roofs built within the last 70 years were built to only support the region’s snow load, and those clients will need to add reinforcements to support a heavier green roof system, similar to Barb. She actually tore down her old garage and built a new one with the necessary reinforcements. We also have a few ultra lightweight green roof options that might be within an existing building’s available loading capacity that we’ve used successfully on retrofit projects without having to add reinforcements.”
After installation, there can be financial benefits, but there are a lot of factors. Green roofs help cool a building, which reduces air conditioning costs at a time when Boston-area residents are seeing electric bills skyrocket. Researchers at Lawrence Berkeley National Laboratory who conducted a cost analysis found that while green roofs are more expensive than conventional ones, they provide significantly higher relative benefits per square foot over a 50-year lifecycle due to energy-cost savings, avoided emissions, and reduced stormwater fees, the Environmental Protection Agency stated.
To keep costs down, Recover often uses low-maintenance plants that require anywhere from two to eight visits per year. However, rooftop farms that produce vegetables can require two visits per week. What do those visits entail? They cover hand weeding, clearing drains, removing debris, soil testing, organic low-dose fertilizer as needed, organic pest control, organic agricultural farming and harvesting for the vegetables, pruning, removing dead plant debris, replanting any die-off, and irrigation upkeep. “Our least expensive contract is a small local Somerville client, and all we do is start the irrigation in the spring and turn it off in the winter, which is $250 per visit ($500 total). The homeowner does all the gardening and cleanup.“ Recover said via email. “The other residential clients range up to $1,000 per visit and anywhere between, depending on the roof size, the planting type, the time of year, how many people we need, and how we can get materials up and down to the roof.”
In New England, where the climate is anything but predictable, green roofs generally feature perennials that go dormant during the winter. However, some can feature evergreen plants that have winter interest.
Bryant, Yee, and their neighbors love to have a green space to look at instead of a black membrane roof. Occasionally, passersby will see her gardening, and she’ll invite them up for a look, sending them away with a flower or herb bouquet. Her favorite moments are when kids come by and see the journey of food being grown.
Bryant also feels as if the green roof expanded the square footage of her home, but on the whole, it just makes her smile.
“From a personal satisfaction point of view, it just makes me happy to look at it and to garden there,” Bryant said. “The gardener’s rush of things happening a little bit differently every day.”
By Megan Johnson from Boston Globe