One look out the back window was all Vivian deZwager needed. As her husband, Joe Maplethorpe, worked to build the garage he’d always dreamed about behind their South Tacoma home, a solid beige wall was beginning to take shape.
• Related photo gallery: A step by step look at creating a green roof

DeZwager had agreed to sacrifice much of her backyard gardening space so that Maplethorpe would have a place for his woodworking hobby. But as the edifice went up, she started having withdrawal pains.

Then inspiration struck: Why not turn the garage roof into a garden?

The garage is on a downward slope, so the roof would be clearly visible from the back of the house.

“I had heard about green roofs,” says deZwager, a Pierce County Master Gardener who works as a gardener for Tacoma Power.

Soon, she would learn much more.

DeZwager, Maplethorpe and deZwager’s mom, 70-year-old Janine deZwager, were prepared to do much of the grunt work involved in planting a garden in the sky.

But DeZwager knew enough to know she would need help designing and implementing the idea.

CALLING IN A PRO

She started with a Google search that brought her first to a green roof expert in Oregon, then to Patrick Carey, of Hadj Design in Snohomish County.

Carey says it’s crucial for a licensed engineer to sign off on the adjustments needed to keep the roof from collapsing.

The pitched garage roof first had to be engineered with extra bracing so it could support the weight of the soil and other materials that would top it.

“It had to have extra trusses all the way across to hold the extra weight,” says Maplethorpe.

If you have a 4-inch deep green roof, it should weigh about 25 pounds per square foot – a little heavier than a tile or slate roof, says Carey.

Installing a green roof isn’t necessarily a do-it-yourself project, even for serious horticulturists. Carey says he has heard of landscape architects installing green roofs, only to have them blow away in the first windstorm.

“Patrick designed the system, and he helped us install it,” says deZwager. “It was definitely a learning process.”

LAYER IT ON

DeZwager’s green roof was built in layers:

Waterproof membrane: Carey started deZwager’s project the way he starts all of the more than 50 green roofs he’s designed: by installing a waterproof membrane on top of plywood sheathing. All the seams of the thick vinyl sheeting were heat-welded to prevent leakage from the green roof, which retains water.

“All green roofs have one, whether they’re residential or commercial, a steep slope or flat,” says Carey. “Ones that don’t won’t remain roofs for long.”

Drainage mat: It consists of a polystyrene sheet molded with cups, like an egg-carton. The cups retain water for plants on the green roof to absorb.

Soil filter fabric: This porous fabric allows water and air to flow through, but retains soil so it doesn’t wash away. A layer of mesh that will eventually decompose was added to help control erosion until rooftop plants get established.

Edging: Aluminum curbing creates a shallow “wall” around the roof’s perimeter. “You’re building a shallow planter,” says Carey.

Growth medium: Getting the mixture just right for the type of plants the green roof will grow, and the microclimate in which those plants must be supported takes serious consideration.

Carey, who custom blends special soil mixes for clients, calls this step the “rocket science of green roofing.”

The soil in deZwager’s roof garden mix is “very mineral oriented,” he says. The mix contains a generous portion of pumice gravel. The volcanic rock is lightweight, due to the air pockets inside, and retains heat.

“The kinds of plants she uses are alpine drought tolerant,” Carey says. “These are plants that are used to growing on rocks.”

The mix also contains compost that is biostable – it’s finished decomposing.

Erosion control blanket: This mesh webbing helps keep soil in place until plants are well-established.

COSTS

DeZwager’s entire project cost about $12,000, including trusses for support, mulch, other layers and plants

“It could have been more, but we did much of the labor,” says deZwager.

But green roofs are designed to last two to three times longer than a standard asphalt roof, Carey says. So that makes the extra upfront cost a little easier to take.

He says homeowners should expect to spend between $17.50 and $22.50 per square foot on a green roof assembly.

PLANTING A ROOFTOP

Installing the green roof hardware on the Tacoma garage took several days last fall. Maplethorpe lifted huge bags of soil – 11 yards in all – up to the roof with a mechanized hoist.

After the soil was smoothed, it was time to plant.

DeZwager chose succulents and other drought-tolerant plants like hens-and-chicks, sedums and ice plants. The garden contains allium bulbs and brass buttons – more than 2,000 individual plants in all.

DeZwager bought some, but borrowed others from friends and neighbors and propagated them in pots.

But she wasn’t content to plant a mere garden up on the roof.

Instead, she decided to use the plants to draw a picture. She imagined a pastoral scene: a tree with generous branches, supporting a garden swing, the sun’s rays shining down.

Hens-and-chicks, along with terracotta mulch, form the tree branches; ice plants, mulched with pea gravel, make up the “lawn” beneath and behind the tree; the sun’s rays are made from yellow and blue sedum, while the body of the sun is another variety of ice plant.

When her plants grow to maturity – and when she finishes the second half of the roof this spring – the picture will be completed.

She’ll tend her low-maintenance garden by climbing a ladder and says she’ll water – on the rare occasions that it’s needed – with a hose or a sprinkler.

DeZwager acknowledges that it might have been easier to spray paint a mural on the roof. But her husband reminds her that she’s just a “tree-loving hippie.”

“I’m a gardener,” deZwager concludes. “I got hooked on the idea of a green roof. The design just sort of evolved.”

Debbie Cafazzo: 253-597-8635

ADDED BENEFITS

Green roofs are a small but growing segment of the roofing industry. Green-roof advocates offer a long list of advantages. They say a green roof:

 • Pleases the eye.

 • Pleases the Earth: A green roof emits oxygen and absorbs carbon dioxide, helping to reduce global warming

 • Stores stormwater: The soil and drainage mat both retain water when it rains. What water does run off the green roof has been biofiltered through the soil, meaning the water is cleaner. The water also runs through a green roof more slowly, which means it doesn’t contribute to sewer system overflow and flooding.

 • Absorbs sound: A green roof can reduce overhead noise – say, from a McChord C-17 rattling by.

 • Provides bird habitat.

 • Can reduce building cooling costs. resources

 • “Green Roof Plants: A Resource and Planting Guide,” by Edmund C. Snodgrass and Lucie L. Snodgrass

 • “Planting Green Roofs and Living Walls.” by Nigel Dunnett and Noel Kingsbury

 • www.hadj.net

 • www.greenroofs.org

 • www.greenroofs.com

 • www.madelinearts.com/roofspace