[Landscape Northwest]
March 31, 1998

Captured space: urban rooftop environments


It's no surprise that in recent years landscape architecture has become a major force within industrialized nations and urban areas throughout the world. Our hospitals, schools, universities, public facilities, and private sector developments are expanding within increasingly limited building and land envelopes, while environmental concerns are demanding more green areas and sensitivity to both developed and natural landscapes.

For landscape architects and environmental planners this typically translates into the rehabilitation of degraded areas, park and nature preserve planning, the preservation of rural, historical, and cultural landscapes, and site planning and design. In urban areas, it also translates into the greening of building interiors, and the design of rooftop gardens.

While each of these project types has its own set of issues and complexities, one of the most challenging design assignments a landscape architect can undertake is that of a rooftop garden.

The historic precedent is one of the Seven Wonders of the World -- the Hanging Gardens of Babylon. These gardens did not actually "hang." They were a series of ziggurat terraces that were irrigated by pumps from the Euphrates River.

Today, there are virtually acres of roof area in any major city. These areas represent one of our greatest resources for urban greening, environmental quality, recreation, socializing, outdoor classrooms, health care therapy, and capturing space that might otherwise be overlooked, inaccessible, or underutilized.

Canal City Hakata

Canal City Hakata, Fukuoka,Japan.
Photo by Dixi Carrillo

Various public, institutional and private sector clients are quickly realizing there is considerable value in rooftops as usable areas and amenities to their capital improvements and commercial projects. Building users, buyers, and tenants are also becoming more sophisticated -- demanding innovative planning, architecture, and internal space layouts that work for them while providing meaning and value in green space and landscape development.

Along with this heightened environmental awareness there is also a rise in regulations and incentives for developing rooftops, for making them something more than desert-like expanses of gravel or tar with drains, vent stacks, and clusters of mechanical units. For example, according to Canada's Office of Urban Agriculture, recent regulations in parts of Germany require green roofs on all new industrial buildings. In Switzerland, new construction must transfer the green space displaced by its footprint to the rooftop. In urban centers across the United States and Asia, rooftops are used as a leverage point for city planners to offer higher density incentives and bonuses to developers in exchange for green roof area.

Design Challenges

The design and development of these rooftop spaces go beyond the term roof "gardens." They are better defined as rooftop environments and can be unlimited in design or programmatic applications. Almost without exception, they become a dynamic and integral part of the structure they occupy, occurring on, over, or within a structure that is capable of supporting them -- in high rises, on bridges, over parking structures, and at building terraces.

Their form can range from open, paved malls and plazas to intimate courtyards, lush gardens, conservatories, or food production areas. Rooftop environments also offer a compelling design opportunity to move beyond the ordinary or anachronistic solution that simply supports the architecture or fulfills a functional part of the building and site program.

Making the move to design a truly remarkable rooftop environment that is somewhere between architecture, art, and landscape is the primary design challenge facing the landscape architect. Articulating a balanced relationship between the architectural framework of columns, structural slabs, roof membranes, parapets, mechanical and electrical systems, and the natural environment composed of plants, water, sunlight, microclimatic influences, and people can be a meaningful response without being contrived or artificial.

Over the last 30 years, hallmark projects such as Robson Square in Vancouver B.C., the Oakland Museum, Seattle's Freeway Park, Forecourt Fountain in Portland -- and most recently the Getty Center near Los Angeles -- are powerful examples of this balance, thereby creating a contemporary legacy of urban rooftop environments.

Technical and functional considerations

Rooftop environments require a distinct set of materials and systems to sustain the designers' original vision while addressing functional and technical issues.

Loading capacities, soil depths, drainage, waterproofing, horticultural requirements, microclimatic influence, accessibility, safety, and maintenance all have to be addressed at the front end of any new construction that includes rooftop development. It is here that many of the rules landscape architects apply to on-grade site development and construction conditions change.

While it is certainly not beyond good planning and design for a landscape architect to creatively address these issues, it is important to point out the interdisciplinary effort necessary to ensure the successful implementation of a rooftop environment.

Structural and mechanical engineers, architects, and waterproofing specialists are all key players in the collaborative planning and design process. Manufacturers and suppliers are also responding to the market in terms of materials, specifications, and technical assistance for rooftop environments. Everything from specialized bituminous membranes, elastomeric coatings, and filter fabrics to high density protection board, drain mats, and lightweight planting soils are available for a vast array of applications.

Soil, paving, decking, water features, planters, stone surfacing, trees, and people in a rooftop environment all have a direct impact on the structural capacity of the roof and other parts of the structure. Designing for the distribution of these loads on new construction is a considerable advantage over a retrofit of an existing roof.

Rooftops represent one of our greatest resources for urban greening, environmental quality, recreation, socializing, outdoor classrooms, and health care therapy.

In either case, a full analysis by a licensed structural engineer should be conducted to determine the feasibility and limitations of roof loads. According to Jerry Griffith, structural engineer with Skilling Ward Magnusson Barkshire in Seattle, "Some older structures may even have less capacity than expected."

He goes on to say, "A seismic analysis as well as a gravity load analysis may also be required by the Building Department when you are making major improvements. Who knows, the structure may even have a greater capacity than you expect and would allow for rooftop enhancements."

Harrison City Hospital

Harrison City Hospital
Photo courtesy of EDAW.

Typically, 1 cubic foot of fully saturated, lightweight planting soil weighs about 75 pounds, while gravel and stone weights average about 125 to 150 pounds per cubic foot. Soil depths can range anywhere eight inches for shallow rooted plants to 5 feet or more for trees. In certain cases, the soil depth can be reduced by using fillers such as foam board. Heavier components such as walls, planters, large trees, water features, and sculptures can be strategically positioned over columns, bearing walls, or structural slab areas that have additional reinforcing.

Rooftop environments occur over office space, museums, operating rooms, laboratories, classrooms, computer mainframes, and parking areas. But protecting the occupied space and structure below from the invasion of water is paramount to the design of a rooftop. This requires a continuous, unbroken seal over the structural slab and up the base of parapets, around standpipes, drains, and other penetrations. The membrane is then overlaid with a built-up section consisting of a drainage medium such as drain rock or matting, protection board, filter fabric, and finally the upper, exposed topping or wearing coarse that may consist of paving, decking, planting, or other surfacing. To ensure adequate drainage, the roof slab and any concrete paving courses should be a smooth surface with a minimum slope of 1/4 inch per foot to a roof drain. Roof drains should be adequately distributed and sized to accommodate the roof runoff within the building mechanical system. It is also important that the roof drains are visible

and accessible for regular maintenance.

Rooftop planting differs from planting on grade. Temperature, reflected heat, and wind can be extreme. Despite these elements, there is still a wide range of plant material that can thrive in this environment. The primary criteria for plant selection, besides adequate soil depth, include: microclimatic factors such as wind, sun, and drought tolerance; regional hardiness; root structure; and the anticipated level of maintenance.

To support the plantings, a fully automatic, low volume irrigation system with pop-up type heads can be installed below the surface of the soil. Porous pipe and drip irrigation systems are also very effective in rooftop applications. The irrigation system should be capable of accommodating hose bibs for maintenance and drainage to prevent freezing in the winter.

Building type, occupancy, structural loads, and access in rooftop environments have a big influence on how health and safety codes interpret rooftop environments. These factors as well as practical considerations for exiting, maintenance and operations, and roof repairs vary with each project. Two constant considerations, however, are compliance with the Americans with Disabilities Act (ADA) and an effective parapet or guardrail height of 42 inches.

Sustaining the original concept of the rooftop environment through maintenance measures is probably one of the most important aspects of a successful design. In part, this is also the responsibility of the designer. This means informing owners, facility managers, and staff of any special requirements for maintenance prior to the time they 'inherit' the project.

The profession of landscape architecture has its own set of rules. Many times these rules are outside the bounds of architecture and engineering and perhaps more akin to art. At the same time it is a very inter-disciplinary profession that thrives on collaboration. In examining the future and opportunities for the profession -- its contribution to the vitality within urban areas -- the planning and design of rooftop environments are key factors in quality of life and sustainability issues.

Robert Shrosbree, landscape architect and director of design for EDAW Seattle. His recent rooftop environment designs include the Harrison Hospital Memorial Garden, the East Capitol Campus Redevelopment, the Seattle Center Flag Pavilion, and Kookmin Plaza in Seoul, Korea.

Copyright © 1998 Seattle Daily Journal of Commerce.