The humble, workaday green roof—flat, thin, and supporting a limited palette of plants—increasingly is a part of mainstream construction in North America. But sometimes both greenery and slope are desirable on top of a building. Pitched roofs are usually more visible at grade, and unusual topography can make a strong design statement. There are, however, unique concerns to consider.
The first priority in designing such a project is holding the assembly in place. The critical pitch at which shear force becomes an issue on a green roof is generally considered to be 10 degrees.
Different blends of growing medium will have different coefficients of friction. Some are engineered specifically for sloped applications. Often the medium is held in place by a perforated plastic honeycomb-like grid or similar material. That’s the case for a new hyperbolic paraboloid roof at Lincoln Center in New York with slopes of up to 18 degrees. Architect Heidi Blau of New York–based FXFowle says the plants’ roots will grow through the perforated polyethylene cellular Geoweb, which is attached to the building perimeter with stainless steel tendons, and knit the system together.
In some cases, the necessity of stabilizing the medium begets a feature. At the California Academy of Sciences, the green roof has seven domes to reflect San Francisco’s seven hills, with the steepest slopes approaching 60 degrees. Holding the medium and biodegradable planting modules in place without connections, pins, curbs, or cleats that could damage the waterproofing membrane was a challenge, says landscape architect John Loomis of SWA Group. Their solution included a network of narrow gabions laid out in 576-square-foot grids. Intersections and sloped areas were reinforced with rebar and the same kind of polyester cord straps used to secure shipping containers.
According to engineer Charlie Miller, president of Philadelphia-based Roofscapes, on a failed green roof the slip surface is most likely to lie underneath the medium. Designers must have a solid understanding of all system interfaces, he says, and stability analysis should be based on interface friction coefficients only. Since these systems are intended to last a long time, designers should not depend on the bonding strength of adhesives between the layers of the green roof assembly, including the one that attaches the waterproofing membrane to the roof deck. The shear stresses in the membrane should be reduced as much as possible. For installations that exceed the critical pitch, stabilization systems that can transfer downslope forces to the roof ridge or eaves, or to battens installed at intervals across the roof deck, will be required. These supports must be designed to avoid compromising the waterproofing system.
Slope greatly affects retention, so pitched green roofs are not ideal for stormwater management. In addition, especially on large roofs, the amount of water moving through the system during a storm can potentially overwhelm its ability to drain properly.
The roof on the Haworth furniture company headquarters in Holland, Mich., vividly illustrates the challenge. It’s big—45,000 square feet—and its west end slopes down six stories to grade while also narrowing in width from 60 feet at the top to 24 feet at the bottom. When the manufacturer of its aluminum edging expressed concern about hydraulic pressure during severe storms, horticulturist and system designer Dave MacKenzie of LiveRoof shared the concern with the design team, which prompted an engineering review. In a collaborative effort, modules were modified for increased subdrainage and three elevated steel barriers were installed to help regulate flow. Larger drain holes also were drilled into the edging at the base. The roof has performed well during two 100-year storms since its installation in 2007.
On the California Academy of Sciences roof, the gabions were designed to provide drainage as well as stability. The open grades between the coarse black basalt stones that fill the wire baskets promote fast movement of water off slopes.
Sloped roofs are challenging horticultural environments. The area close to the ridgeline will usually be especially hot and dry, while lower areas can be quite wet. While these situations should be accounted for when designing drainage, they also present opportunities to take advantage of microclimates. Even if tried-and-true green roof plants such as sedums comprise the primary plant palette, more sheltered spaces might accommodate flowering perennials or grasses.
Most sloped green roofs are extensive, meaning they host at most 6 inches of medium. But on the accessible Lincoln Center roof, an average depth of 9 inches supports a carefully selected lawn mix of turf type tall fescue and Kentucky bluegrass. The adjustable irrigation system was designed with seven zones to make sure water is correctly distributed, and a series of check dams directs excess water under the surface to drains that are strategically placed to provide even distribution.
Linda McIntyre is a freelance writer based in Washington, D.C., and New York. Her book, The Green Roof Manual: Define, Design, Install, and Maintain, written with Ed Snodgrass, president and founder of Emory Knoll Farms Inc., will be published by Timber Press later this year.