By James Hencke, Portland, Oregon, 1-503-478-2353, hencke@pbworld.com

Green roofs have been around for hundreds, if not thousands, of years. They can be traced back to the ancient ziggurats¹ of Mesopotamia, the hanging gardens of Babylon, and cities of 16th, 17th, and 18th century Europe and Mexico. In recent years, green roofs have become common throughout northern Europe, particularly in Germany, Austria, Switzerland and Scandinavian countries where their growth is directly related to supportive government policies and programs, and concerns about environmental degradation.² Now a number of public and private market drivers are encouraging the growth of the green roof industry in North America.

Green roofs involve the creation of a contained green space (vegetation) on top of a human-made structure. The elements of such a roof (see box) can be installed separately or as part of a modular system.

Green roof types range from extensive to intensive. Extensive green roofs tend to be thinner, lighter, lower cost, and lower maintenance but less ornamental (Figure 1). Intensive green roofs tend to be thicker, heavier, more costly, and higher maintenance, but typically offer the benefits of accessibility and use (Figure 2). Generally, there are no climate-related restrictions.

Many variables influence the capital and maintenance costs of green roofs. As a general rule of thumb, capital costs of extensive green roofs are in the range of $10 to $15 per square foot for installed elements above the water proof membrane. Maintenance costs range from about $1.25 to $2.00 per square foot for the first two years, although the size of and access to the roof can have a big impact on cost. Intensive green roofs can be considerably more expensive. Costs continue to fall, however, as the market for green roofs matures.

Design Principles

A core principle of green building is to use an integrated design process. Having multi-disciplinary teams working as a group is the best way to assure that green roof technologies are incorporated into a project in an efficient, beneficial and cost-effective manner. For example, a decision to incorporate an intensive gree n roof into a project can require substantial strengthening of structural members and impact decisions such as the location of columns. Making this decision early in the process allows team members to work together to explore cost saving strategies and make coordination between disciplines clear and efficient.

Figure 1: Extensive (lighter) and less ornamental green roof at Derby Multifaith Center, UK prior to opening.

First order design decisions include identifying the type of green roof and its primary functions and size. Next, specific details are explored, including:
• Integration with building systems
• Structural loading
• Accessibility and inaccessibility
• Drains and drainage layers
• Waterproofing
• Growing medium and substrate
• Vegetation
• Irrigation
• Joints and borders
• Design-related maintenance.
• Roof replacement strategies

Good drai nage and snow loading is an issu e for any roof in a cold climate. Green roof plantings tend to be very hardy to alpine conditions (sedums can endure freezing weather for months). Expansion and contraction of growing media can be a consideration if high organic content growing media is used. Gravel around the edges of planted areas can help. Some advanced designs coming out of Germany use higher mineral/low organic content growing media.

Planting design and drainage are perhaps the most challenging technical aspects of green roofs, but according to Tom Lipton, a green roof specialist with the City of Portland Bureau of Environmental Services, the biggest current challenge is to successfully convey costs and benefits to clients. Green roofs are still outside of most developers “comfort zones,” but good information is being generated by an increasing number of built projects and monitoring programs, such as those undertaken by Portland State University and Michigan State University.

Figure 2: This green roof on people's Food Coop in Portland, Oregon is somewhere between Intensive and Extensive because it has deeper soil depth and more ornamental plantings than most extensive roofs, yet ist is not so "intensive " that you can go out and walk around on it, like a roof garden.

Figure 3: Dark areas indicate green roofs planned for West Hyattsville transitoriented development.

PB’s Involvement

We are witnessing an increasing interest in green roofs among public and private sector clients, making expertise in green roof technology an emerging and valuable service offering. One of PB’s notable green roof projects is the Derby Multifaith Center in the UK (Figure 1), for which we served as structural engineer. This unique, 2-story conference, event and educational facility that serves all faiths incorporates a unique modern design and energy saving design features.

The project utilized an extensive Hydrotech Garden Roof® produced by Alumasc. The assembly provides a lightweight, low profile system that can be installed on roof and plaza decks not engineered to handle the heavy load requirements of traditional (intensive) green roofs. Developed in conjunction with ZinCo GmbH of Germany, this assembly is backed by more than 35 years of combined experience in premium waterproofing and green roof technologies.

According to Mike Goldsmith in PB’s Bristol UK office, the Multifaith Center structure is irregular and varied, and comprises in-situ and pre-cast concrete, steel frames, timber and masonry. The building fabric was designed to provide a long, maintenance-free life and low-energy environment. Concrete and dense masonry interiors with temperature regulated automatic opening clerestory windows were designed to modify daytime and nighttime temperatures with minimal energy input. High insulating materials were used in the exterior cladding material to minimize heat loss/absorption. Heavy solar shading and the green roof ameliorate interior temperatures and enhance the “hewn” look of the building.

The success of the Derby effort has led to another green roof project that the Bristol office has just begun—the Exwick Heights Primary School in Exeter, England.