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American Society of Landscape Architects

 

October 2006 Issue

Shades of Green
Green roof or rooftop garden, this landscape brings a high-mountain meadow to downtown Salt Lake City. 

By Jan Striefel, FASLA

Moving Beyond Mies Photography by Craig Widmier

Green roofs are increasingly popular in the United States, and as energy costs and temperatures soar, they seem likely to become even more widely used. But just what is a “green roof”? As roof garden genres multiply, the definition of the lightweight, environmentally focused green roof is getting very fuzzy indeed.

The rooftop landscape on the Church of Jesus Christ of Latter-Day Saints (LDS) Conference Center in Salt Lake City, comprising more than eight acres, is stunning. Conifer-forested terraces rise from the north and east sides of the structure to the rooftop and then sweep off toward a meadow on the west, framed by mountain views, and hanging gardens along the south and west building ledges. The effect is dramatic, with flowing water cascading over the building’s south facade and broad views across the valley that pull in the surrounding landscape of mountains, streams, and the Great Salt Lake. But is it a “green” roof?

While a lot of green roof construction is driven by ecological considerations, this design had nothing to do with sustainability and everything to do with aesthetics and image. It’s definitely a rooftop garden. Should it be identified as a green roof?

Early Teamwork Helps a Building Sit Comfortably in Its Landscape

The project, designed by the Olin Partnership, was completed in 2000. The huge building—it can hold 21,000 people—was designed by Portland architect Robert Frasca. By all accounts it was a close and fruitful collaboration by two firms that had worked together previously and valued interdisciplinary collaboration. “This was an extraordinary example of architects and landscape architects merging to integrate a project into the wider natural landscape,” says Susan Weiler, ASLA, of the Olin Partnership.

Early in the design process Frasca and Olin agreed that the huge structure needed a strong, overriding landscape presence that did not overpower other buildings and plazas nearby, or it risked becoming a massive box covering almost 10 square acres in downtown Salt Lake City. Church officials also wanted to quell concerns about the impact of the building on the neighborhood. Their solution was to enrobe the building with the landscape, using the site and the regional context to inform the structure of both the building and the landscape. They worked closely with Salt Lake City’s Planning Division to identify a design that would complement other urban design elements in the downtown while also reflecting the local landscape with native grasses, shrubs, and trees as well as nonnative ornamentals. The landscape architects wanted the effect of the planting to be subtle rather than heavily symbolic, allowing for individual interpretations.

While not overtly religious, the evocation of what the region’s earliest settlers—Mormons coming west—had seen when they first arrived in 1847 was appropriate to the site and to the character of the project as a gathering place for the faithful. The design concept, however, especially the wild and grassy meadow, differed significantly from gardens around other church buildings, which tend to be formal, flowery, and manicured. Peter Lassig, who had served as the head gardener at nearby Temple Square, says the church leadership was initially very skeptical of the native plant concept and anything resembling a xeriscape. But Olin and Weiler eventually brought them around with photographs of high-mountain meadows and Weiler’s vision of re-creating, in a sense, the views seen by early Mormon pioneers.

Church leaders also knew the approach laid out by the architects and landscape architects would be an enormous commitment of resources, but that was something they were willing to undertake, says Weiler. And, as she told Landscape Architecture, while clients pay a premium for landscapes built over structure, a collaborative approach such as this one, with all systems integrated from the start, is a better guarantor of success than planning a landscape apart from the building process. This sort of collaboration, says Weiler, helps everybody.

The architects and landscape architects, working with civil and structural engineers, took their cues from the site, which covers an entire city block. It slopes over 70 feet from the northeast corner to the southwest corner. Massive terraces reflect the mountain landscape and envelop the building with shelves of plants. The terraces buttress and support the structural elements underpinning the vast interior without columns that might interrupt views from the large assembly space on the northeast corner of the building.

The northeast corner is where the roof structure is strongest (supporting 527 pounds per square foot) and the soils are deepest (averaging 4 feet), giving ample space for the root systems of large trees. Shallow soils, averaging 18 inches, and a much lighter roof structure support the grasses and wildflowers in the meadow on the south and west parts of the site. (For comparison, a typical lightweight “green” roof in a temperate climate is about 2 or 3 inches thick and weighs about 13 to 20 pounds soaking wet.) Beneath the soil surface, chunks of lightweight material such as Styrofoam are used to fill space where plants’ root systems need less soil depth.

The multilayered topography of the building is essential to its landscape. Visitors walk up paths along the terraces and can enter the conference center at various points along the way. When they reach the top, looking out over the meadow to the mountains beyond provides a sense of the vastness of the western landscape, but the space is designed so that visitors don’t have a sense they are standing on top of a 21,000-seat auditorium. “We had to create spaces big enough for thousands of people and small enough for 10 people,” says Weiler.

Incorporating water elements was challenging, says Weiler, but the landscape architects felt it was important to their program to provide a focal point for reflection, a refreshing element in a hot environment, and the aural benefit of moving water. The project’s structural engineers also liked the idea, since water weighs less than saturated soil. Water runs throughout the landscape, from the “source basin” on the uppermost part of the roof down to two basins at the lower-level conference center entrance. The “waterfall” cascades over the south-facing building facade and falls two stories into one of these basins. This wall of water, and the glass bridge over this pool, are among the most striking aspects of the building.

The hot, dry western climate was also a challenge in terms of plant selection. Rooftop plantings in the intermountain West are subjected to stressful conditions—high heat, severe cold, and drying winds. Weiler and her colleagues, working closely with Lassig, chose plants that could survive in the difficult environment. In some cases this meant native plants; in others, nonnatives or cultivars bred for heat or drought tolerance or other important characteristics.

They didn’t seek to re-create nature—that would be impossible in such an unnatural built environment—but instead sought to evoke it. On lower levels, trees with a horizontal branching habit, such as koelreuteria, were selected to create a bosque effect. Trees such as aspen and fir in the stepped planters rising up the sides of the building relate aesthetically to vegetation on nearby hillsides even if all of the species are not identical.

Given the aesthetic concerns of the church leadership, planting the meadow required particular attention. Lassig, who as head gardener would be caring for the plants, was concerned that the seed mix initially proposed was too limited in species and would be brown and dry all summer long, causing consternation among church leaders and visitors. He suggested using as models the tallgrass prairies of Kansas and Utah’s own high alpine meadows, since they stay green all summer. In the end, Weiler asked Lassig to develop the seed mix.

“I used Ian McHarg’s theory and identified seven different microenvironments on the roof: runoff areas, run-on areas, full sun exposed, full sun with a south-facing reflective wall, full sun with a south-facing wall that is not reflective, shade from equipment, and shade from trees,” he says. Lassig also let nature take its course, using 20 different grass species as 80 percent of the mix, on the theory that about 10 would eventually take hold and thrive. Though he’s retired now, he visits the garden frequently. “Initially the Canada wild rye was the most dominant species, but now you don’t even see it,” he says. “These kinds of landscapes evolve and, given enough time, they will work things out on their own.”

The meadow changes with the seasons, and those closest to it, including the gardeners who care for it and church officials, appreciate it in all of its stages. In April and May, the meadow is abloom with wildflowers, looking very much the way the mountain meadows at high elevation do in July and August. In July and August, when there are few blooms, visitors comment on the contrast between Temple Square, below and across the street, and the conference center roof. At this time of the year, the unkempt look of the meadow contrasts with the manicured and maintained appearance of the church’s other properties in downtown Salt Lake City. Once visitors are reminded by the ever-present volunteer guides of the design concept replicating a mountain meadow, however, they seem to be more accepting and appreciative of its sometimes shaggy appearance. When fall approaches, another visual dimension emerges with the golden shades of the grasses and foliage. Songbirds abound on the roof, and raptors have been spotted swooping down for mice.

Low Maintenance? No Way

Traditional “green roofs” built to meet ecological objectives usually require minimal maintenance once established. So-called intensive green roofs, with deeper soil and a wider range of plants, often need more. In this case, however, the high expectations of visitors and church leaders do not allow for anything less than constant good looks and the impeccable care needed to achieve them.

This roof garden requires a lot of hands-on maintenance. In 2004, six gardeners were assigned to the conference center block; in 2005 the number of full-time gardeners was reduced to three, with occasional temporary help. These three gardeners are responsible for the roof, as well as all of the other landscaping on the block—most of which is built over structures such as parking garages.

Even the meadow, usually a low-maintenance type of landscape, is spray irrigated twice a week during the hot summer months and mowed in the fall. At first it was mowed uniformly like a football field; now it is mowed when seedheads mature, leaving islands of grasses and forbs. Staff say the meadow takes far less maintenance, fertilizer, and water than other gardens such as the LDS Church Headquarters garden, which is built over a parking garage, or the at-grade and over-structure gardens in nearby Temple Square.

Experimentation with watering cycles has taught maintenance personnel that too much water invigorates aggressive plants, whereas just the right amount allows others to compete equally. The same watering experimentation has been beneficial to trees, allowing them to survive with limited root space where they will never achieve their true genetic size and will likely remain smaller as many urban trees do.

In addition to mowing and watering, weeding and the removal of aggressive vegetation in the meadow occur about four times a year. At other times, gardeners are busy pruning, deadheading, cleaning, adjusting, and repairing sprinklers and other equipment, removing snow, and doing whatever is necessary to assure the garden’s good looks.

The most aggressive plant in the meadow is Poa compressa, a native bluegrass species that out-competes other wildflowers and grasses. When it gets out of hand in a particular location, it is removed and the area is reseeded with other species. Another aggressive species has been Artemesia schmitiana ‘Silver Mound,’ a variety of sage that is not native to the region but is adapted to rooftop conditions and has very ornamental foliage. Gardeners will try to control, rather than eliminate, the artemesia to preserve the effect of its attractive silvery foliage, but if possible the bluegrass will be eliminated.

Eldon Cannon, who oversees maintenance on LDS church headquarters properties, expects that the meadow will always need to be managed and maintained. Simply allowing nature to take its course is not an option, because the aesthetic appeal of the project is critical. Lassig told church leaders that while ideally meadows are burned for maintenance every three to five years, a thorough pass with a mulching mower at the same interval would promote the work of natural bacteria and oxidation.

Snow in winter—Salt Lake City sometimes gets four or five feet per year—is removed from other parts of the roof and stored on the meadow until it melts. In summer, heat generated by the granite pavers transfers into the planters, heating up soils and requiring more watering in some areas to assure plant survival. The special soil mix, a local, lightweight, expanded shale aggregate used for its loose quality and resistance to compaction, also generates heat, exacerbating the problem. Maintenance personnel find that because of its dark color, the material absorbs heat that dries soils quickly. Typically, the shale material is mixed with peat moss to form the basis of the growing media. Some was added at the time of planting, but maintenance personnel would like to have seen more organic material incorporated into the mix and are adding it in the hopes of mitigating heat and evaporation.

Expanding the “Green Roof” Paradigm

How should we think of this project—is it a green roof, a rooftop garden, a landscape over structure, a living roof, intensive, or extensive? Weiler doesn’t like the term “green roof,” finding it too limiting. Neither does she particularly like the terms coined in Germany referring to “intensive” or “extensive” roof gardens, again because they do not really describe the design intent, and they don’t convey a clear sense of the roof’s function.

Weiler prefers to refer to the functional, shallow-soil, thinner-profile variety as “living green roofs” and more complicated approaches with deeper soils, more varied plants, and features such as paving as “landscapes over structure. One isn’t better than the other,” she says. “They just have different applications.” The former is better for stormwater mitigation and management and has helpful secondary effects of decreasing costs for cooling and heating buildings. These roofs are not meant to be accessible, and they are typically not irrigated once established. The available plant palette is limited owing to the depth of soil and harsh conditions.

Landscapes over structure allow greater diversity of plants, often have a greater impact on microclimate because larger materials like trees can be planted, provide space for people to reflect and socialize, and incidentally provide ecological benefits such as stormwater containment and insulation. Weiler notes that these two types are not mutually exclusive—some projects have characteristics of both. In Salt Lake City, the meadow section is closer to the former style, while other parts of the conference center roof are clearly in the latter camp.

Other experts on green roofs and rooftop landscapes agree that the existing vocabulary is insufficient. Engineer Charlie Miller, principal of Philadelphia-based Roofscapes, suggests four broad categories of on-structure landscape:

  • elevated, exuberant landscapes such as the conference center
  • at-grade urban landscapes over buried structures such as Chicago’s Millennium Park
  • intensive elevated green roofs—thinner landscapes that support a wide variety of perennials and require significant maintenance
  • extensive elevated green roofs—veneer landscapes, typically thinner than six inches, that require less maintenance and, usually, no irrigation

Sustainability and energy conservation are often the driving forces behind the choice of a green roof, but that was not the case in Salt Lake City. No monitoring is done to determine energy or water savings, so there is no way to substantiate any tangible benefit from the rooftop landscape. Certainly there is a benefit derived from the captured rainwater available to the rooftop plantings that does not contribute to urban runoff, and there is some insulating value in the landscaped roof, and perhaps improvement to air quality. But the enormous inputs of irrigation water in Salt Lake City’s hot, dry climate may well offset any environmental gains. Church officials and on-site gardeners anecdotally report savings on water, fertilizers, and general maintenance compared to other gardens on church-owned facilities downtown, though no detailed data is gathered.

Does the increased maintenance required of a rooftop garden cancel out any energy and resource savings? Probably. The LDS Conference Center block, all of which is over structure, requires regular maintenance, supplemental water, and a long-term commitment to resources. It will probably never save enough energy or water to offset its high initial cost or the costs of day-to-day maintenance. And only the LDS church or a similarly endowed institution is able to commit the necessary resources to maintain such a landscape. But if it were not on the rooftop and instead occupied other urban space, would it be more sustainable? Probably not.

Miller says that on-structure landscapes such as this one can provide significant benefits, but trying to pin down the benefits of various green roofs is “a mess. There is little attention being devoted to investigating the variables that control the potential benefits,” he says. “Furthermore, the role of context is rarely discussed. The same green roof will provide different benefits depending on the climate in which it is installed, the elevation of the structure, whether or not it is irrigated or fertilized, and so on.”

Even more important, says Miller, is the method of measuring the benefits, since quantifying such benefits is essential to promulgating effective rules for constructing green roofs and making them attractive to builders and clients. “For instance, traditional requirements for stormwater analysis, such as control of the 25-year 24-hour storm, end up measuring little benefit from extensive green roofs,” Miller says. “Likewise, a green roof’s ‘effective albedo’ will be very different in a temperate climate with regular rainfall than in an arid climate. Irrigation can affect thermal mass, albedo, stormwater management, and other factors. But green roofs are rarely, if ever, evaluated at this level of detail. We have some pretty good tools for making these assessments, at least for temperate climate zones. Oddly, it is extremely rare that a client is interested.”

As an engineer, Miller associates green roofs with building performance. “I would like to see green roofs and green facades, as well as ground-based landscape management practices, come of age as building systems.” When this happens, there will be space for both the humble sedum green roof and the lush and flamboyant landscape in Salt Lake City.

Jan Striefel, FASLA, is president and founder of Landmark Design in Salt Lake City, with more than 25 years of experience. She is a recognized urban designer and landscape architect, with particular interest and expertise in regional landscapes that fit into and complement local climatic, environmental, and cultural environments.

PROJECT CREDITS Client:
The Church of Jesus Christ of Latter-Day Saints Conference Center, Salt Lake City. Client representatives: Leland A. Gray, senior design architect; Tom Hanson, project manager; Kerry Neilson, project architect; Mark Williams, ASLA, project landscape architect. Architects: Zimmer Gunsul Frasca, Portland, Oregon (Robert Frasca, design partner; Robert Packard III, partner in charge; Joseph Collins, project manager; John Thompson, senior designer). Landscape architects: Olin Partnership, Philadelphia (Laurie Olin, FASLA, principal landscape architect; Susan Weiler, ASLA, principal in charge; David Rubin, ASLA, project manager; Leslie Bishop, Leslie Hey, and Kim Douglas, project landscape architects). Fountain consultant: CMS Collaborative, Carmel, California. Irrigation consultant: AIC Irrigation, Los Angeles. Structural engineer: KPFF, Portland, Oregon (Art Johnson, principal in charge; Nathan Charlton, project manager). Theater and media facilities design: Auerbach + Associates, San Francisco (S. Leonard Auerbach, principal in charge; Steve Pollock, theatrical designer). Acoustics: Jaffe Holden Acoustics, Norwalk, Connecticut (Christopher Jaffe, principal in charge). Lighting designers: Auerbach Glasow, San Francisco (Patricia Glasow, principal in charge; Richard Osborn, exterior lighting designer; Susan Porter, interior lighting designer). Consulting architect: Gillies Stransky Brems Smith, Salt Lake City (Michael J. Stransky, principal in charge; Jonathan Bradshaw, project manager; Jim Neilson, job captain). Electrical/mechanical/plumbing: CHP and Associates, Houston. Civil engineer: Stantec Consulting Inc., Salt Lake City. General contractor: Legacy Constructors, Salt Lake City (James Peterson, project manager; Harvey Wright, construction manager).

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