Root barriers for green roofs

I just put a new post examining root barrier materials for green roofs up on ecobrooklyn.com. Check it out here and below:

In my last two posts, I examined the types of membrane available to waterproof the roof of the Eco Brooklyn show house. First, I identified EPDM as a good alternative and researched the cheapest provider in Brooklyn. Then I looked at conflicting information on the suitability of EPDM for green roof applications and determined that (1) Firestone RubberGard 60 mil. EPDM is FLL-certified for green roofs and (2) if we ensured that seams were properly sealed, and we installed a root barrier, an EPDM roof is likely to perform well. In this post, I will analyze the root barrier products available and identify the most suitable options for the show house.

Essentially, root barriers can be used to create either a physical or chemical boundary between plants and the waterproofing membrane. Physical barriers are made from dense inorganic materials that will not break down due to bacterial activity or allow roots to penetrate. Chemical barriers contain a compound that stunts root growth on contact and therefore do not allow roots to penetrate the surface of the waterproof membrane.

A number of materials are used to block root growth, including:

1. plastics, including high-density polypropylene, polyethylene (also: here, here, and here), high-density polyethylene (also: here and here), and polyester and PET/mylar geomembrane and sheet-based systems;

2. mebranes that contain copper compounds, and;

3. copper foil, lightweight concrete, or rigid insulation.

Product use and availability
In my research I’ve found that root barriers installed on green roofs are most frequently made of plastics and that polyethylene and high-density polyethylene are the most commonly used materials.

HDPE makes up 47% of the material in all plastic milk, water, and juice bottles in the U.S. It is fully recyclable, and is a good alternative for green roof root barriers because it withstands impact, is strong, is lightweight, absorbs little moisture, is heat weldable, root resistant, pliable, and withstands temperature extremes, and is tear and puncture resistant. It is susceptible to degradation by UV, but can be treated to better withstand radiation. Since the root barrier will be fully covered, however, UV stress is not a major concern.

The American Society of Landscape Architects recently installed a mixed extensive/intensive green roof atop their headquarters in Washington, D.C. The design used high-density polyethylene sheeting to protect the roof from construction work, foot traffic, and root damage. To accommodate areas with higher traffic and plants with deeper and/or more aggressive root structures, a thicker HDPE sheet was used. Given that we plan to use the show house as a demonstration platform and will have visitors on the roof to observe it, the ASLA model could serve as a good blueprint for the current project.

Build it Green provides a green product directory we can use to find appropriate materials for our root barrier. Using this directory, I found that ZinCo, American Hydrotech, and Tremco make HDPE products that would be suitable for the show house roof. Xero Flor produces root barriers from low-density polyethylene that could also be used on the show house.

Polypropylene, in the form of non-woven geotextile, is sometimes used as a root barrier as well. It is waterproof, strong, flexible, lightweight, resistant to breakdown by bacteria, recyclable, resistant to tears and punctures, chemically inert, does not leach, and is resistant to acids and bases. In my research I found a number of polypropylene-based root barrier products. Some rely upon chemical means to prevent undesirable root growth while others do not. Grace Construction, for example, produces a polypropylene product that incorporates copper hydroxide, a root inhibitor, into a drainage layer. Bakor, Henry, Sealoflex, and American Hydrotech make products that function similarly, incorporating water retention and drainage layers into a root barrier incorporating copper hydroxide. Finally, Carlisle makes a 40 mil heat-weldable polypropylene geomembrane designed for belowgrade applications.

Since I found just a single reference to the use of copper foil, concrete, or insulation as root barrier layers, I opted not to investigate these possibilities. It is possible that these materials have been successfully used to prevent root damage to waterproofing layers but the vast majority of the literature that I read suggested using plastics for this purpose.

Environmental concerns
When constructing a green roof for the show house, one of our primary considerations is minimizing the environmental impact of construction by carefully choosing the products we use. In investigating the environmental impacts of copper hydroxide, I found literature that suggests that copper in tree root barriers mobilizes readily, and that the compound can be ecologically detrimental, even at low concentrations. A review of studies of the impacts of copper on fish, wildlife, and invertebrates by Ronald Eisler of the U.S. Geological Survey states that copper is among the most toxic heavy metals in freshwater and marine organisms. Although it is unclear to what degree the copper hydroxide contained within the root barriers considered here would mobilize, it is clear that there is a chance that copper would leach out of the membrane. This would be both detrimental to the effectiveness of the root barrier and potentially bad for the environment. I therefore will eliminate the copper-based root barriers from consideration and weigh only the aformentioned HDPE, LDPE, and Carlisle polypropylene geomembrane for use on the show house roof.

As part of my effort to identify the most suitable material for the show house root barrier, I’ll examine the environmental impact of the plastics from which the root barriers are made. When investigating the environmental impact of plastics, there are a few key questions to consider. First, does the manufacture and/or disposal a given plastic result in the production and/or release of harmful contaminants? Second, how much energy is consumed during production of the material? Third, is it practical to recycle the product when the roof is refurbished?

Research on the manufacture, chemical properties, and disposal of polyethylene and polypropylene suggests that these are relatively benign compounds from a toxic contaminant standpoint. The Sustainable Design Resource Guide claims that polyethylene compounds (HDPE and LDPE) are “considered to less harmful than other types of plastics.” healthandenvironment.org states that, unlike investigations into the characteristics of PVC, polystyrene, and other plastics (#7), most research on HDPE, LDPE and polypropylene compounds has not shown leaching of carcinogens or hormone-dirsrupting compounds form these plastics. Build It Green notes that HDPE is chlorine-free and that its manufacture does not result in dioxin production. Last, the Tellus Institute conducted a study of the environmental “cost” of a variety of plastics based on individuals’ willingness to pay to prevent associated contaminants from entering the environment and found that HDPE, LDPE, and polypropylene fared roughly equally, though HDPE came out ahead. These studies suggest that, among the plastics manufactured today, the three under consideration for use in the show house are relatively innocuous.

Other questions to consider in assessing the environmental impact of the plastics under consideration are the embodied energy and the greenhouse gas release associated with production of the product. In a 1997 study examining the embodied energy of a variety of commonly used building materials, researchers from New Zealand found that it takes 103 MJ/kg to produce HDPE and LDPE and that it takes 64 MJ/kg to make polypropylene. Another New Zealand-based study from 2003 (conducted by one of the authors of the 1997 study) found that the embodied energy of HDPE and LDPE is actually closer to 50 MJ/kg, but did not reexamine the figures for polypropylene. The Lighthouse Sustainable Building Center found that HDPE and polypropylene consume roughly the same amount of oil (in raw materials and energy) to produce (1.75 kg), and that LDPE consumed slightly more (2 kg). Lastly, EPA conducted a review of the greenhouse gas implications of the manufacture of a variety of products, including HDPE and LDPE, and found that the production of one ton of LDPE resulted in the release of roughly 20% more greenhouse gas than the production of one ton of HDPE (polypropylene was not included among the products reviewed). The majority of the available literature, therefore, suggests that the production of HDPE and polypropylene is slightly less energy-intesive than production of LDPE. Given the conflicting results and limited information, however, further review would be necessary to identify the least energy- and GHG-intesive material with certainty.

A third important consideration regarding environmental impact is recyclability. Maryland’s Department of the Environment assembled a breakdown of the recyclability of a variety of common plastics; according to the analysis, HDPE is more commonly recycled than either LDPE or polypropylene.

Conclusion
Based on my analysis, it appears as though HDPE may be the best alternative for the show house root barrier for the following reasons: (1) there are a number of HDPE products made for this application; (2) the substance is not associated with toxic byproducts; (3) HDPE does not leach contaminants after production; (4) its production is less energy-/GHG-intensive than production of LDPE (and roughly equivalent to polypropylene); and; (5) it is widely recyclable. In my next post, I will investigate the availability of HDPE root barriers in the New York City area and identify the least-cost supplier. Considering that polypropylene and HDPE had many of the same desirable characteristics, however, I will also investigate the availability of the Carlisle copper-free barrier; if cost is significantly lower than for HDPE barriers, we may choose to use this product.