A Well-Watered Green Roof

We’ve become accustomed to rooftop gardens—several stories up. When you walk through the “rooftop” garden at the Bookmen Stacks loft building in Minneapolis, you’re on ground level. It’s a garden planted on the roof of an underground parking ramp.

Bookmen Stacks Garden and Building

Bookmen Stacks Garden and Building

The area has very little greenery—the building itself faces a highway overpass. The project’s developer, Steve Frenz, wanted some green space and was open to new ideas. He’d worked with the landscape architects, Oslund and Associates, before, and asked them to come up with a plan.

Creating a rooftop garden had a few challenges. One was structural—working with the weight of the roof, and compensating for the fact that the roof slopes. Bookmen’s immediate neighborhood has no storm sewer, so managing stormwater was another challenge.

Tom Oslund, principal of Oslund and Associates, needed a way to combine storage and irrigation. He found it in a system manufactured by a Minneapolis company, RESI Inc. The system has three parts: a liner to store water, a chamber to control the water flow, and a sand fill.

In the Bookmen Stacks garden, the system stores stormwater in cisterns buried on the roof, and draws it through the sand as needed by relying on capillary action. Oslund says, “It is able to irrigate more efficiently, and you don’t see any of it. The surface becomes very durable and we’re not casting or using water. It’s all done through wick irrigation.”

The sand fill is good for plants, especially turf. Oslund says, “The biggest challenge presented by turf areas is compaction. It kills the root structure. With sand as a matrix, it doesn’t compact. It stays firm and the roots survive.”

Bookmen Stacks Garden, Aerial View

Bookmen Stacks Garden, Aerial View

The system has other advantages. One is that gardens that rely on it need less soil than other rooftop gardens—6 to 8 inches as opposed to 12 to 14. The design of the parking ramp didn’t need to factor in the additional weight, which saved on construction cost. The system has no moving parts—the only element that moves is water. Oslund says, “It’s so simple that there’s not much that can go wrong.”

Planting the garden was really not different from putting plants in a pot. And it let Oslund plant taller plants—ornamental grasses and larger plants in pots. Oslund says, “It has the appearance of something that’s not on the roof. You’re hard pressed to realize it isn’t growing out of the ground.”

Bookmen Stacks Garden, Front View

Bookmen Stacks Garden, Front View

The result was, Oslund says, “an open space in an area surrounded by access ramps to the freeway. It’s a respite spot.” The building’s dogs like it too—it’s a good place for them to run and play. The garden looks and feels like a private park for the people who live in Bookmen Stacks.

All images courtesy of Oslund and Associates.


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