Green roofs on a slope

I posted another note on the Eco Brooklyn webpage this evening, this time about installing green roofs on a pitched slope. Here it is:

In this post, I’m going to diverge from the format of my past several entries a bit and explore strategies for installing green roofs on pitched surfaces.

The International Green Roof Association (IGRA) states that roofs with up to a 30 degree pitch can be outfitted with a green roof, and that “traditional” green roof components can generally be safely and effectively used on roof slopes up to 10 degrees. Beyond the 10 degree threshold, however, installation becomes significantly different from a traditional green roof and requires a different approach.

Though some problems typically associated with green roofs can be mitigated when installing on a sloped suface (e.g., drainage is better and water does not typically pool as frequently), other stressors arise. Modifications to structural components, growth medium, and vegetation must be made to prevent roof failure. I will explore challenges and techniques for installing green roofs on sloped roofs in more detail below.

To start, the IGRA wrote a brief but helpful introduction to pitched green roofs (scroll to the bottom of page 2). The article points out several important challenges to establishing a vegetative community on a pitched roof. First, shear forces threaten to pull the build-up, substrate, and plants off of the roof. Second, the substrate is more highly susceptible to erosion. Third, plants are often subjected to more intensive drought stress.

Other sources, including a Zinco planning guide, highlight some of the other important challenges to establishing sloped green roofs. For example, pitched roofs with southern exposure are typically subjected to more intense solar radiation than flat roofs, which can dessicate plants more rapidly. In addition, installing independent root barriers is often difficult or impossible since build-up materials may slip more easily against root barrier materials than against waterproofing membranes, so waterproofing membranes must be root-proof. Third, access for routine maintenance on sloped roofs is more difficult, but must be incorporated into the design. Lastly, to prevent erosion, vegetation must be planted and cultivated to cover the roof as quickly as possible, and coverage must remain high year-round. And as roof slope increases, these challenges typically become more severe.

Although establishing a pitched green roof can be more challenging than installing a flat roof, steps can be taken to ensure that a sloped green roof functions as desired. allbusiness.com and the Pomegranate Center provide good perspective on establishing sloped green roofs in an article and a homeowners’ guide, respectively, that I borrow from below.

First, a varety of structural and build-up components can be installed to hold the roof in place. Safeguard produced a good visual how-to on installing pitched green roofs; it includes information on installing front edging boards and free-standing timber frames and can be found here. Green roof component producers make build-up materials specifically for use in pitched roof situations, as well. For example, Zinco’s Floraset FS 75 and Georaster provide additional reinforcement to the substrate layer, thereby reducing erosion. Designers of one of the steepest green roofs in North America used Roofscapes, Inc.’s Roofmeadow Type I, a full green roof system intended for pitched roofs, which includes an integrated “slope stabilization system”, in their build-up. American Hydrotech produces GardNet, and Presto Products Corp. produces Geoweb, components that divide the roof into cells and thereby help to reduce erosion on sloped roofs. Colbond makes Enkamat, a 3-D mat designed to act as a synthetic root structure and quickly and permanently prevent soil erosion. In lower-slope applications, jute, straw, or wood erosion mats laid top the substrate might suffice to reduce soil movement and loss.

Second, design of nonproprietary components (e.g., growth media, plant type, etc.) can be modified to address concerns associated with pitched green roofs. Growth media depth can be increased (depending upon the roof’s load bearing capacity) to retain additional moisture, or irrigation can be built into the roof design for use in serious droughts. Growth media formula can be tweaked to ensure greater water retention. Thicker water retention mats can be used to retain additional moisture, as well. Vegetation must be chosen carefully to ensure that the plant community grows quickly enough to cover the roof withtin the first growing season and survives prolonged periods without water. In addition, it is important to consider that water will flow down the roof, affording rooftop plants the least amount of time to absorb water and those plants along the bottom edges of the roof more time. In some cases, it may be beneficial to increase growth medium depth at the rooftop to allow additional water storage. It might also be beneficial to plant vegetation that can tolerate drier conditions on roof peaks and vegetation that grows best in wetter conditions along the gutters.

The bottom line, therefore, is that with careful planning it is possible to sucessfully install a pitched green roof. It is crucial, however, that builders make considerate design choices and source appropriate materials.

The next steps in outlining a design for a pitched green roof include identifying which particular materials will be used to build the roof, identifying appropriate growth media composition and depth, and choosing vegetation. Part of the decision-making process with respect to build-up components will depend upon cost and availability. Build-up components will, to an extent, influence nonproprietary components. Getting prices and information on whether these materials can be shipped to Brooklyn, then, is the next priority. This process can sometimes be tricky, however, as green roof materials (and their prices) are generally available only through company sales representatives. I’ve begun contacting folks at the companies I’ve listed above to get a feel for what these products will cost and will post that information when it becomes available. Check back!