Green roofs bring complications and joys

Green Roof
Kenny residents Mike Lee and Lori Johnson stand with their dog Casey, a 12-year-old Yorkie-poo, outside their home near Grass Lake. Photo by Chris Juhn

Minneapolis homeowners Mike Lee and Lori Johnson wanted to make a statement with their new-construction home in Kenny, a stone’s throw from Grass Lake. Specifically, they wanted a flat-roofed home that minimized runoff into the nearby lake while adding usable outdoor space to the property.

So, in the fall of 2017, they hired Roger Grothe, owner of Mendota Heights-based Aloha Landscaping, to add a 900-square-foot green roof to the main residence — about half the structure’s total roof area. The remainder is given over to a seating area with lake views over the greenery. The entire roof’s drainage system runs into a rain garden, leaving the driveway as the property’s only runoff source.

The project lived up to its potential. Two years on, Lee and Johnson enjoy a peaceful rooftop green space with great views, environmental benefits and noise-dampening properties.

“We consider the green roof to be a positive attribute of the house,” Johnson said.

But Lee and Johnson are iconoclasts, even in their upscale neighborhood. Residential green roofs remain relatively uncommon in Minneapolis, despite clear environmental and aesthetic benefits. Homeowners considering the project may need to look beyond their block or immediate neighborhood for comparisons. And the twin elephants in the room — project cost and feasibility — weigh heavily.

That’s not to say adding a green roof is impossible, or even that difficult, for homeowners with ample budgets and suitable onsite infrastructure. It’s just important to know what’s involved before work begins.

Green Roof
Their roof has great views, environmental benefits and noise-dampening properties. Photo by Chris Juhn

Worth the cost?

When Debbie Petschl, the owner of Twin City Landscape, installed a green roof over her garage in the Hale neighborhood of Minneapolis, the endeavor was basically a really expensive research project.

“We undertook the project mostly to see how much it cost, what sort of practical benefits it had and to see if maybe we could install in other places,” she said.

In the 15 years since, Petschl hasn’t installed another green roof, on her property or on any other — not because she isn’t satisfied with how things turned out at her Elliot Avenue home, but because designing and installing residential green roofs is not really worth her company’s time.

“You have to work in conjunction with a garage builder,” she said — or a homebuilder, if you’re installing a green roof on the property’s primary structure, as did Johnson and Lee. “That takes time. We’re a landscape company, and we’re super busy.”

Still, Petschl would “definitely recommend” green roofing to environmentally conscious homeowners able to absorb the project’s substantial cost.

Green roof installation costs range from $10 per square foot or less to around $25 per square foot, Grothe said. These figures include the roof itself, plus a drain mat, filter fabric, organic substrate and plants. They don’t include waterproofing — an essential, and potentially costly, element of any roof.

So for a typical 500-square-foot Minneapolis garage, homeowners should expect to pay roughly $5,000 to $12,500 on just the green roof. Since green roof retrofits are less common than new builds, the actual project — building a new garage from the ground up with a green roof — is likely to cost far more.

The actual cost of a finished green roof turns on several factors, according to Petschl and Grothe. Some costs are baked into any green roof project. For instance, a small crane or lift is always necessary to hoist materials up to the roof. Grothe said that alone accounts for “several dollars per square foot.”

Green Roof
Residential green roofs remain relatively uncommon in Minneapolis, despite clear environmental and aesthetic benefits. Photo by Chris Juhn

The most effective way to reduce green roofing costs is to babysit sprouted plants for a season or two before putting them on your roof, said Grothe. The alternative, ready-to-install tray systems like those sold by LiveRoof, roughly double project costs – $20 to $25 per finished square foot, compared with $10 to $15 for plants grown from sprout.

The roof’s thickness matters, too. Thinner roofs — 3 to 4 inches of growing media, the threshold for viability in Minnesota — cost less than 6- to 8-inch roofs, which are more common in commercial construction. But plant choice decreases as roof thickness rises. At 3 to 4 inches, your best bet is a handful of cold-hardy sedum varieties, Grothe said.

“People envision green roofs with all sorts of natives, but that’s not really practical at 3 or 4 inches,” said Grothe. Bluestem, prairie Junegrass and other sought-after natives survive at 6 inches, but many garages can’t support 6 inches of substrate anyway, Grothe added.

On the other side of the ledger, green roofs have longer replacement lifecycles and may reduce other common homeownership expenses, Grothe said. For starters, about 70% of the water that hits a typical green roof is absorbed — potentially reducing the need for drainage infrastructure elsewhere on the property. Due to the natural temperature and radiation shielding afforded by the plants and substrate, properly installed green roofs may extend roof lifespans by 30% or more.

Petschl has experienced this second benefit firsthand. Since installing the green roof on her garage, she’s had to re-shingle the roof on her main house, but the garage roof is chugging along “with no signs of aging,” she said.

Green roofs are also formidable insulators; Petschl’s garage is easily 10 degrees cooler in summer and 10 degrees warmer in winter. Such insulating power may not matter much in an unheated garage, but it makes a big difference in a primary residence.

Green Roof
Lee and Johnson’s 900-square-foot green roof anchors a rooftop amenity space with comfortable seating and a view that changes by the season. Photo by Chris Juhn

Beyond the garage

Because it’s very expensive to safely retrofit older homes’ roofs to support the added weight, garages are the most commonly green-roofed residential structures in Southwest Minneapolis.

New builds are better green roof candidates, Johnson and Lee learned. They’re both architects, so they felt confident designing a flat-roofed structure capable of supporting plants and soil. But they knew their limits, and that’s where they stopped.

“We can handle everything under the roof, but we left decisions about drainage media and plantings up to Roger,” said Lee.

Though their roof is still in its early days, they’re satisfied with their choice. The 900-square-foot green roof anchors a rooftop amenity space with comfortable seating and a view that changes by the season. “It looks really nice as the flowers and foliage change throughout the summer,” said Johnson. Later in the year, the rooftop is a great vantage point to observe autumn colors around the lake.

This sort of private amenity is increasingly popular with homeowners in Southwest Minneapolis, Grothe said. “They get a kind of bonus entertainment space plus the green space,” Grothe said. “A lot of homeowners are headed in that direction.” This summer, Grothe led a similar green roof project on a home near Lake Harriet, complete with a rooftop seating area overlooking the green roof and the neighborhood beyond.

Besides leafy solitude and aesthetic appeal, Lee and Johnson enjoy the roof’s noise-dampening properties. “It is quieter under that portion of the house,” Johnson said.

Like Petschl, they’ve been pleasantly surprised by the maintenance requirements. Through late July, Johnson weeded the roof just one time, and expects weedings to become even less common as the roof’s plants fill in. The only hiccup thus far came when a significant share of the plants died in a particularly vicious late-winter freeze-thaw cycle. Grothe replaced the casualties with even hardier strains, and things have gone well since.

Nothing manmade lasts forever, certainly not in Minnesota’s harsh, changing climate. While they last, though, green roofs do their part to lessen the destruction wrought by everything else we’ve built.

green roof
Homeowners should think carefully before adding greenery to an existing roof. Submitted photo

Green roof dos and don’ts

Debbie Petschl, the owner of Twin City Landscape; Mike Lee, an architect who’s installed a green roof on his Kenny home; and Roger Grothe, the owner of Aloha Landscaping, shared their advice for those looking to add a green roof to their houses or garages:

Should you do it?

Perhaps most importantly, homeowners should think very carefully before adding greenery to an existing roof. Lee recommends hiring a structural engineer before doing anything else. “You’re potentially putting a lot of weight on your house,” he said. “Better to see if the project is possible than to put a lot of money into it, then have the ceiling coming down after it’s done.” Retrofitting an existing roof to hold more weight may not be cost-effective, if it’s possible at all.

Even when a house can physically support a green roof, pitch presents another challenge. Most older Southwest Minneapolis homes have largely or wholly pitched roofs. Though green roofs are generally “off-flat,” said Petschl — subtly pitched to facilitate drainage — gravity and runoff make it difficult to keep growing media in place on even a modestly pitched traditional roof. Though costly, retrofitting a sloped roof to support relatively lightweight green infrastructure is not impossible, said Grothe. Designing a green roof to support the additional weight of an adult human is another story, he added.

Homeowners fortunate enough to have flat-roofed additions or sun porches shouldn’t assume that they’re built to hold modern green roofs. Older homes’ porches aren’t built to contemporary codes, said Lee. Professionally, he’s seen “a fair number of shakeout porches fall down” due to overcrowding.

The danger of DIY

In Lee’s opinion, at least, the physical task of installing a green roof is within the capabilities of a committed DIYer. It’s what comes between the engineer’s OK and the roof’s first nails that can trip up amateurs. Experts like Grothe are better positioned than the average home gardener to assess the specific growing media, plant species and roof infrastructure appropriate for a particular roof, based on sun exposure, roof type and other factors. In many cases, the ideal inventory list can’t be assembled from in-stock items at the nearest home improvement store.

“We certainly would not have chosen the correct growing media if we’d tried to make
our roof out of stuff available at the Home Depot,” said Lee.

Carbon footprint

Grothe advises eco-conscious clients to pay attention to their growing media’s carbon footprint. Some contractors use expanded shale, a relatively inexpensive product that works well on roofs. But the carbon inputs required to produce expanded shale nearly outweigh the roof’s climate benefits. Grothe prefers a lower-carbon pumice medium.

Risk of water damage

All roofs carry an expiration date. A decade and a half on, Petschl’s green roof is holding up well, but her experience may not be typical. Johnson is already watching hers for signs of moisture damage. Since her roof covers part of her home’s living area, not just an uninsulated garage, water intrusion is a serious concern.