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Green Roofs: Patterns and Practices

By Elaine Kearney, LA; Technical Director, Columbia Green Technologies





Landscape architects are increasingly taking advantage of the possibilities presented by green roof installations beyond their purely practical aspects. The green roof at the Turtle Bay Resort on the island of Oahu, Hawaii, highlights and extends the pattern of waves created with the ballast rock on the other roof levels, augmenting the view of the ocean. The sedums are planted in tiles, cuttings, or plugs atop the growing media and underlying green roof structure.


The increasing popularity of vegetated roofs, or green roofs, in the urban environment can be traced to their widely recognized environmental contributions, including stormwater retention, reduction of the urban heat island effect, provision of habitat and increased building envelope efficiency. Their implementation as a low impact development technique has been encouraged by the establishment of favorable public policies in Chicago, Washington D.C., Toronto and Seattle, just to name a few.

Landscape architects are also taking advantage of the design possibilities presented by green roofs in the skyline, beyond their purely functional aspects. Materials and plant selection can be employed to create legible patterning on extensive green roofs. The following strategies and innovations will allow designers to make full use of expanded palettes, and showcase projects that feature successful implementation.

 

 




The same pavers that form the pattern from above at the Horizon House are used to create a welcoming pathway when viewed at eye level. These paths are also functional and allow access for maintenance. Ornamental Japanese maples add seasonal interest to the roofscape.



Color Blocking
Extensive green roofs, which are usually planted with sedums - also known as stonecrop - can be constructed from tray or layered systems. A planted-in-place tray system ensures invisible tray lines and allows for an integrated irrigation system, which safeguards plant health and a lasting pattern. One of the most obvious ways to create a high-impact graphic is by planting vegetation in contrasting colors, as seen in the roof of the Turtle Bay Resort on the North Shore of Oahu Island in Hawaii. The green roof is used to highlight and extend the pattern of waves created with the ballast rock on the other roof levels. The sedums are planted in tiles, cuttings, or plugs atop the growing media and underlying green roof structure.

An important factor in the implementation of a color block strategy is that reliance on a single-species sedum monoculture is too risky for long-term viability. Using a collection of 3-4 plants with a similar coloring and temperament in each 'block' of color will ensure long term vigor and the survivability of the pattern itself; the intended color pattern will remain in the event any individual species fails to thrive.

Pre-grown sedum tiles are a great option, as they are available in a wide variety of textures and colors using proven performers. The use of accent plants such as drought-tolerant ornamental grasses or perennials can create contrasting texture and height. At its logical extreme, this technique can be used to create a recognizable symbol or logo, such as was done for the Alpha Resources headquarters in Bristol, Va.

 




The green roof at the Aleutian Spray Fisheries headquarters in Seattle blooms in a vibrant yellow, contrasting with the blue summer sky. Metal edging was used to create clear separations between planting zones, and defines locations for maintenance personnel to remove spent blooms from certain swaths of sedums.



Operating at Multiple Scales
One of the most appealing aspects of green roofs is they are an asset not just for the occupants of the particular building, but also become highly visible "borrowed" landscapes for anyone else who happens to look down on the roof. In addition to a typical rooftop site analysis, which takes into account sun/shade and moisture levels, it is important to anticipate the scales at which the roof will be experienced. Will it be accessible to people who can view the delicate textures up close, will it be primarily viewed from afar, or both? Successful green roofs take these objectives into consideration from the outset and frequently operate at multiple scales. Berger Partnership, designers of the green roof on a high-density housing community called Horizon House in Seattle, Wash., created a pattern using pavers, ballast rock, edging and other components to create a striking pattern when viewed from one of the residences high above; the pathways bridging the various materials are both functional and interesting when experienced at eye level. The fall color of the ornamental Japanese maple in the intensive planters adds seasonal interest to the roofscape.

 




The Horizon House vegetated roof in Seattle, Wash., reveals a striking pattern from above. The designers created a pattern using pavers, ballast rock, edging and other components.



Evolution
Like all plantings, a green roof will evolve and mature, so the owner should be prepared to expect a softening of the pattern over time as the sedum colonize their preferred microclimates. This evolution and growth is part of the beauty of living architecture.

 




In winter, the roof at Aleutian Spray is selectively deadheaded to create a higher contrast pattern of both color and texture. Selective removal of spent flowers on every other planting band is used to creative effect during those colder months.



Functionality and Seasonal Interest
As mentioned above, a great strategy is to make functionality a part of the design by incorporating ballast rock, or pavers of different colors, for enhanced legibility throughout the year. It is important to consider from the outset that roofscapes need to coexist with mechanical equipment, vents, drains, window-washing davits and other paraphernalia that require separation from vegetation. These so-called vegetation free zones can consist of roof ballast, river rock or architectural pavers. They provide points of access for personnel to service equipment or perform regular landscape maintenance activities, but they can also be used to enhance subtle planting delineations. This strategy is used to great effect at Horizon House.

Like all landscapes, a green roof will change throughout the seasons, offering opportunities for discovery and surprise. Many sedum species go dormant during the winter months, so choose a mix of evergreen and perennials. Bulbs, such as Allium, can be incorporated into green roof profiles of four inches or more of growing media, allowing for an extra pop of color in the springtime.







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August 25, 2019, 5:40 am PDT

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