A ‘green’ roof adds insulation and helps manage storm water runoff

When their roof needed to be replaced, D.C. architects Mark and Lucia Freeman installed a green roof. (Jennifer Chase for The Washington Post)

When most people offer guests a tour of their home, they give you a spin through the kitchen, living room and maybe their yard. At Mark and Lucia Freeman’s home, an elegant mid-century modern residence in D.C., you might also get an invitation to climb a ladder and check out the roof. It has become the talk of the neighborhood — and occasionally of social media — since the couple installed an array of native plants up there last year and added a bracing system to keep it all from sliding off.

The pair are architects who try to incorporate eco-friendly features whenever possible, so when they needed to replace their roof, Lucia advocated for a green roof instead of a traditional asphalt or metal surface.

Green roofs are far from a new architectural phenomenon; the Hanging Gardens of Babylon and the turf houses favored by Vikings both provide early examples, and modern European buildings have been incorporating green roofs for decades. But as climate change makes extreme weather more common, green roofs can check off a lot of eco-friendly boxes for homeowners or developers. Still, they are relatively few and far between; there are only a handful in the District.

Installation requires placing alternating layers of waterproofing and water-retention membranes, filtration sheets and drainage materials. That is topped with a few inches of substrate and plants, serving as a functional Dagwood sandwich that helps protect the standard roof underneath. Installers often use preselected assortments with low-growing, shallow-rooted plants tailored to the local climate.

In the Freemans’ case, those plants include fragrant chives and nubbly succulent beds; plump mushrooms have also cropped up since they installed the roof.

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Now that the vegetation on the Freemans’ roof is noticeable from the road, passing drivers frequently reverse their cars for a second look. When the family works in the yard, they regularly see pedestrians examining the feature. Some linger longer than others, asking detailed questions about what’s going on up there.

Most anyone who does stop by — whether for the Freemans’ regular crawfish boil, a family gathering or just on their way home from work — tends to pose the same question.

“Why would you want this on your roof?”

The primary answer: They had too much water where they didn’t want it.

Lucia remembers the “river of running water” that engulfed the alleys around the property. The nearby drain, stationed at the bottom of a hill, would overflow with several feet of water.

Other storm water management practices may have helped mitigate the alley flooding, and the couple does have permeable pavers in lieu of an asphalt driveway. But they needed something to mitigate all the water pouring off their roof during summer storms.

The water “would still essentially be packing up against the retaining wall, so for us it was both utilitarian as well as a step in the phase of the work to fix the property,” Mark says.

The Freemans also had water coming into their home. Although they knew the roof had leaks before they bought the house, new leaks began springing up after they moved in, and pools of water were forming on the roof around the back wall and chimney. The roof had to go, and they believed their best option for managing the runoff was a green roof, which is intended to hold a gallon of water for each of its 2,500 square feet.

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Since installing the roof, Lucia says, they’ve noticed a big difference in the drainage, particularly during heavy summer rains. Their experience aligns with federal research that suggests “green roofs are capable of removing 50% of the annual rainfall volume” that lands on them, according to an Environmental Protection Agency study.

Another bonus is that while a conventional roof is a biodiversity desert, its greener cousin offers a small oasis for migrating birds and pollinators.

“This spring, it was nuts, we had birds all over the place,” Mark says, including an abundance of blue jays, cardinals and orioles.

“We’ll be sitting here sometimes and we’ll see different birds in the trees: grouse, robins — definitely a lot of robins — and doves,” says Lucia. And the birds are feasting on the wave of insects that visit the new plants.

The Freemans found the new infrastructure also dulls the noise from summer storms to the point that “you can’t even really hear it” now, Mark says. The stiffness of common roof finishing materials can reverberate sounds, while the more flexible, denser green roofs dampen noise.

And homes with green roofs see an energy efficiency bump from the insulation provided by the vegetation and substrate — between 15 and 25 percent in summertime energy costs for a two- to three-story building, according to the American Society of Landscape Architects. The green infrastructure also can help cool urban neighborhoods struggling with the heat island effect, according to the EPA.

There are, of course, downsides, including higher installation costs. A green roof runs between $30 and $35 per square foot, compared with between $4.35 and $11 per square foot for a traditional roof, according to the home maintenance site Angi.

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That range assumes a property owner doesn’t need additional structural reinforcement to support the weight of the plants and soil. If a roof needs reinforcement, that can usually add $2 to $4 per square foot; with a pitched roof, the price generally rises an additional $1 to $3 per square foot because it requires a bracing system to hold plants and soil in place.

While green roofs do carry higher installation costs, they don’t need to be replaced as often as their conventional cousins. The General Services Administration reports they can last twice as long as traditional roofs.

The Freemans have found their roof remarkably low maintenance. A few times a year, Lucia climbs a ladder propped against the front of their carport and hoists herself up to do some minor chores. Her son would love to join her, but he’s not quite old or tall enough for that yet. In late September, she plucked a few foot-high tree saplings trying to grow out of the now-fertile space, cautiously hopscotching among the few patches of the roof that the plants haven’t colonized, and tossed the plants over the edge.

She noted that, even on a warm day, the roof was notably cooler than roofs covered in asphalt or metal would be.

“It was cool enough for me to leave my phone and my water out on the roof,” she recalls with a laugh.

Bridget Reed Morawski is a freelance writer in Washington, D.C.


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