A garden grows on a Minneapolis rooftop

Architects, designers, and new geosynthetics build a high-rise green space.

By Mason Riddle

There is mesh and then there is mesh.

For a downtown Minneapolis 10-story penthouse, the proper mesh was critical to the installation and maintenance of a rooftop garden and terrace, which, like the owner’s living quarters, affords impossibly spectacular views of both the Mississippi River and the downtown skyline.

The loft conversion residence occupies the entire top floor of the 1914 Washburn Mill warehouse building, a national historic monument once integral to a thriving milling industry. Now it is a revitalized citizen of Minneapolis.

Nick Winton, of Anmahian Winton Architects in Cambridge, Mass., designed the dwelling proper. Landscape architect Tom Oslund, of Oslund and Associates in Minneapolis, designed the garden. The two worked hand in glove from the beginning to create a seamless aesthetic passage between the two unique spaces: the expansive minimalist loft-space interior to the reductive roof garden exterior, rooted in Japanese tradition. One is aware not only of a strong spatial linkage but also one of materials that are drawn from the rich but spare urban vernacular of concrete, steel, and gravel.

For Oslund, the challenge was to create a space that could withstand the extreme conditions of Minnesota summers and winters. The trick was how to sustain the lawn year-round. Oslund consulted with Mike Kelly of Rehbein Companies in Blaine, Minn., who has developed and implemented a new layering system to support this type of green space. Currently awaiting patent approval, it is called the Muellner System.

According to Kelly, the issue with rooftop lawns is that grass needs a 30.5 cm profile for a proper air-to-water ratio, one that allows the roots to grow aggressively. This requires not only the proper growing matter, but also an environmentally sound, low-maintenance irrigation and drainage system.

In using the Muellner System, first an ethylene propylene diene monomer (EPDM) liner was installed on the roof to correspond to the measurements of the lawn, essentially creating a “big bathtub out of the whole area,” Kelly explained. On top of this was the next layer, an Evaporative Control System (ECS) liner. This system of chambers provides adequate moisture to the root system of the grass, bypassing the need for irrigation sprinklers. This self-irrigating system uses rainwater stored in the subsurface reservoirs that soaks upward, or what Kelly calls “capillary rises.” Five cm of crushed rock was then placed on top of the ECS, followed by 30.5 cm of sand, in which grass was seeded or sodded.

What makes this layering system work, however, is Netlon Turfguard, a tough, flexible, extruded mesh manufactured from polyethylene. Thousands of these small interlocking mesh elements were incorporated into the sand base creating a dense, bird’s nest-like structure. The roots of the grass penetrate down through the mesh to form a deep, anchored root system. The Turfguard allows for enhanced water drainage and infiltration properties, and also makes the grass more durable against the protrusion of lawn furniture or general wear and tear. Installed in 2003, the thriving green lawn is testimony to the Muellner System’s ability to maintain a sustainable lawn subject to 50°C days under baking sun, high punishing winds year-around, and sub-zero temperatures in the winter.

Another material critical to the success of the rooftop garden was Cambridge Architectural Mesh, a metal mesh that covers the Corten steel pergola above the 36m² patio. It is a commercial grade mesh often used for improving acoustics in theaters. Here, its intent is to dissipate the sun’s rays without creating a permanent shade. Sold by the square foot, the mesh comes in prefabricated rolls, which Oslund had cut to specifications for the pergola. The result is a visually poetic grid that throws dappled light onto the patio but still allows for an “en plein air” experience.

The compelling 279m² outdoor space was conceived as a series of rooms comprising the raised lawn, a Corten steel waterfall with an Ikebana shelf, the concrete slab terrace beneath the pergola, a walkway, a system of square and circular Corten steel planters, and the austere meditation pavilion. For Oslund, the guiding aesthetic was that the entire garden terrace appears to float above the roof plane of the old warehouse, which was covered with a deep gray Dresser trap rock.

In the end, it is these highly developed systems and materials such as Netlon and Cambridge mesh on pergola that allow for a once viable industrial building to overcome its own obsolescence and be transformed into a spectacular but sustainable urban living experience.

Mason Riddle is a Minneapolis-based freelance writer who writes frequently about design and art.