On the fourth floor of downtown Seattle's new Olive 8 hotel/condo the large green roof helps mitigate heat and soaks up storm water while providing respite for humans and habitat for the birds, bees and butterflies trying to find a haven in the harsh environment of downtown.

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PLANTED ROOFS, despite their casual country air, are going uptown. And where else but in the heart of the city is their green relief more welcome? We now have three seriously large green roofs in downtown Seattle: the one at the Justice Center, the all-sedum roof on the Bill & Melinda Gates Foundation garage near Seattle Center, and the equally sedum-heavy roof at Olive 8.

The view from the windows of the new skyscraping Olive 8 hotel/condo complex includes plenty of bleak downtown roofs. A very few have been fashioned into oases with potted trees and seating for employees or residents to take a break or eat lunch outdoors. But only when you look down on the fourth floor of Olive 8, as many of the hotel rooms do, is a working green roof on view.

It’d be so cool (literally) to see more green expanses as part of our skyline. Planted roofs not only mitigate city heat by absorbing sunlight, but also soak up storm water like a big sponge.

Located at the corner of Eighth Avenue and Olive Way (which you could probably guess from its name), this new condo-topped Hyatt with its glossy, modern facade drops nary a hint that it’s the greenest such building in Seattle. Local developer R.C. Hedreen Co. earned a taller building in return for going green, and is aiming for Silver LEED (Leadership in Energy and Environmental Design) certification. The 8,355-square-foot, drought-tolerant sedum roof is an important element in the quest for LEED status, and also a pilot project for a developer considering green roofs on future buildings.

I turn to Jonathan Morley and Rachael Meyer of the Berger Partnership, who designed the roof, to explain why they planted solely sedum at Olive 8. What happened to green roofs sprouting wildflowers and grasses?

“Sedum is the key to a tidy appearance,” says Meyer. Sedums, which evolved on rocky Mediterranean hillsides, survive just fine in a scant 4 inches of soil, while meadow-type roofs require deeper soil. More soil means more weeds.

Sedums are also the best bet to survive without any irrigation once they’re established, and saving water is important for LEED certification. Hose bibs were installed nearby to water during drought for the first couple of years; after that the sedums are on their own. Will they make it up there in all the sun, wind and drought? Morley admits the sedums will change color to more red tones, and maybe even go dormant at the end of a dry summer, but he’s convinced they’ll survive.

The designers used two planting systems on the roofs at Olive 8. The largest expanse is composed of Green Grid trays (www.greengridroofs.com) holding nine types of sedum in varying colors for a patterned effect. The mechanical pop-up is greened with a German system called Xero Flor (www.xeroflora.com) which is a pre-grown fibrous mat that rolls out like sod. Morley recommends starting with small, densely planted sedums for fastest coverage. Hard to argue with that theory, considering the Olive 8 roof was planted in January and has filled in nicely in just a few months.

Project manager Derek Janke doesn’t seem perturbed that green roofs cost two or three times what a regular roof would cost. He ticks off the benefits: The green roof looks great; hotel guests are enjoying it. It extends the life of the roof, reduces maintenance and provides sound insulation for the ballroom directly beneath it. As storm water-impact fees go up in the city, the green roof’s ability to soak up 60 to 90 percent of the rain that falls on it will pay off.

“The green roof is the most commented-on part of the building,” says Janke. “It’s emblematic of all the green elements, most of which don’t show.”

It doesn’t hurt that the green roof sports a sign large enough to read from the many floors above, “Native and adapted plants provide crucial habitat for birds, bees and butterflies and promote biodiversity.” How good is that message tucked in among Seattle’s tallest buildings?

Valerie Easton is a Seattle freelance writer and author of “A Pattern Garden.” Check out her blog at www.valeaston.com. John Lok is a Seattle Times staff photographer.