Green roofs use an engineered substrate, rather than natural soil, to support plant growth.  There are a number of commercial varieties available, but these are often shipped considerable distances from supplier to consumer and can be expensive.  To reduce the cost of our roofs, it would be best if we could produce our own green roof soil.  In this post I will examine information available from a variety of roof soil producers and independent studies to explore the feasibility of making our own green roof substrate.  (n.b. Substrate formula differs based on the type of roof being installed (i.e. extensive vs. intensive); since Eco Brooklyn focuses mainly on extensive roofs, I will concentrate on soil formulas for extensive roofs only.)

The planting substrate used in a green roof installation is crucial to the success and longevity of the roof itself.  If the soil mixture does not support plant growth while simultaneoulsy allowing the roof to function as a traditional roof would (i.e. shedding water), then the installation will fail.  Therefore, the standards to which green roof soil will be held are extremely high, and we need to do our best to get the formula right!

Steve Skinner, a green roof products manager with American Hydrotech and soil scientist, writes that the most important green roof substrate considerations include:

• grain size distribution
• density
• water & air management
• pH, lime & salt content
• organics
• nutrients
• cation exchange capacity (CEC)

Since the modern green roof movement began in Germany, the Germans have a head-start on establishing standards for successful green roofs.  The FLL is held as the authority on green roof planning, execution, and upkeep, and has established guidelines for individual components, including substrate; these guidelines can be downloaded here.

Before we dive into FLL guidelines, let’s check out some of the commercial soils and identify relevant attributes.

In our most recent green roof installation, we used Gaia Soil for Green Roofs, a locally-produced, lightweight (10 lbs./cu. ft. dry, 30 lbs./cu. ft. saturated) substrate made from expanded polystyrene (modified to improve water-holding and cation-exchange capacities), clay, and finshed compost.  We liked that it was made close to New York City, that it was light, that it retained water well (200% of dry weight!), and that it uses recycled content.  However, the soil was expensive and tricky to pick up and transport.

The makers of Live Roof, a modular green roof system, have developed their own brand of green roof substrate which aims to increase soil lifespan and reduce substrate shrinkage by incorporating a high percentage of inorganic material (94+%) and avoiding the use of peat moss, compost, and perlite, all of which can break down and erode after exposure to the elements.  In addition, the system uses clay to bind nutrients, a buffering agent to reduce the impact of acid rain, and a disease-supressing organic element.

G-Sky produces two blends of Roof Soil from pumice, well graded sand, and aged bark mulch compost.  The manufacturers claim that, given appropriate installation, these soils can be used with little or no structural support on roof slopes of up to 35 degrees.

Rooflite Extensive MC uses a blend of mineral lightweight aggregates like HydRocks, a calcined clay, and organic components to achieve FLL certification.  Detailed specifications for the product can be found here.

The most important commonality among these substrates is that they include a high percentage of low-density inorganic particles.  These elements, whether pectin-coated expanded polystyrene, pumice, or calcined clay, play an important role in air and water management: they allow the soil to drain quickly during heavy rains and, because they do not break down, maintain interstitial space over the long-term.  Each blend also incorporates an organic element to absorb water and provide a nutrient reservoir.  The organic component within roof soils, however, typically represents a lower proportion of the soil mass than would be found in a natural system and CEC (and nutrient availability) suffers as a result.  Some companies, therefore, use calcined clay or add clay to their soils to increase CEC and bind essential plant nutrients like potassium, calcium, magnesium, and ammonium.

Independent studies are a good source for information on substrate formula, as well.

A recent Michigan State University study looked at the ability of a variety of substrates to support Sedum and native plant growth on rooftops in Michigan.  In their study, Rowe et al. found that at a depth of 10 cm., a soil mixture of 80% heat-expanded slate (PermaTill, from Carolina Stalite), plus USGA grade sand, aged compost, and peat, amended by approximately 50 grams of controlled-release fertilizer per sq. m per year enabled a Sedum community to establish itself (in about a year) and thrive with minimal decomposition of organic material and nutrient-laden runoff.

Another study by Oberndorfer et al. suggests that organic components typically comprise about 10% (by weight) of green roof substrate and notes that crushed brick can be substituted for expanded (calcined) clay, depending on the load-bearing capacity of the roof.

In his 2007 dissertation for Ohio State University’s Horticulture and Crop Science program, Reid Coffman notes that that extensive green roof soil is typically engineered to be 75% inorganic and 25% organic and closely mimics the conditions found within the rock outcrop ecosystem.  In a 2006 article, Lundholm notes that the most popular plants used in green roofs are naturally found in these outcrop systems.  Though I will not do so in this post, study of productive rock outcrop ecosystem soil may provide additional insight into optimal green roof substrate composition.

Lastly, Skinner’s article provides very helpful information on green roof substrate composition.  He notes that extensive green roof soils are typically between 3% and 6% organic by mass, but that additional organic matter can be added initially to help to establish young plants.  The organic component, however, should have a low carbon to nitrogen ratio (i.e. be highly decomposed) so that plants can easily extract nutrients and to prevent the severe shrinkage and accumulation of toxic compounds that can accompany rapid decomposition.  Humus made from composted straw, sawdust, wood, leaves, grass clippings, agricultural waste, biosolids, and/or animal carcasses, separately or in combination, could be used as the organic component in green roof media and will initialy provide the macro- and micronutrients necessary for plant growth, though later it may be necessary to amend the soil with fertilizer.  The inorganic component, Skinner states, could be comprised of synthetic expanded slate, clay, or shale or of natural materials like pumice.  Sand, perlite, scoria, or pumice fines are often used as secondary inorganic components.  Skinner advises that soil pH be maintained between 6 and 7.5 and that the substrate contain carbonate levels sufficient for plants to readily absorb nutrients and water (though architect Allan Wingfield suggests that carbonate levels be kept in check since high levels (above 6 grams/liter) can lead to corrosion of roof drains and outlet pipes).

Based on the general formulas outlined here, it seems reasonable for us to develop our own Eco Brooklyn substrate blend for our extensive roof installations.  The next step would be to source materials, mix a trial batch and, if we choose to try to meet FLL guidelines, perform an empirical comparison to the FLL figures.  For example, FLL places emphasis on grain size, organic content, permeability, water storage, air content, pH, and salt and nutrient content, and all of these parameters must be within a specific range for the soil to be certified FLL compliant.  Most of these characteristics could be easily measured, and our formula tweaked to ensure compatibility with the FLL guide.

Without undertaking an effort to guarantee FLL certification, however, I believe that it is possible to mix a good quality green roof substrate.  To begin, I would advocate the use of calcined clay or expanded slate as our primary inorganic substance, as both have reasonably high CEC.  Pumice, repurposed polystyrene, and lava rock (with some clay mixed in) would work as well, and might lower the embodied energy of our substrate mix (although calcined clay is produced as a byproduct of cement production in Brazil).  Depending on the load-bearing capacity of the roof, crushed brick could be added to the mix, too.  Cost will be a major factor in determining our ultimate formula.  To the clay/slate/etc., I would recommend adding well graded sand and finished compost or humus.  I would aim for 10% organic material initially, with the expectation that the percentage will fall to closer to 5%.  I would plan to amend the substrate with fertilizer annually, and water the roof if it has not received precipitation for two weeks.

In an ideal world, we would work to get our soil to FLL spec, and periodically test the product to ensure that it is appropriately mixed.  For now, however, the time that it would take to test and reformulate the soil is not a luxury that we have.  Still, based on my analysis of the green roof studies and substrate formulas available, I feel reasonably confident that my recommendations will produce a soil that will green a roof and support plant growth for years to come.

I wrote last week about root barrier options to protect the Eco Brooklyn show house roof from root intrusion and/or bacterial degradation, which could potentially lead to leaks.  Since Eco Brooklyn focuses on green building, I laid out some of the characteristics associated with the most commonly used root barrier materials, including manufacturing byproducts, potentially harmful compounds associated with root barrier leachate, embodied energy, and recyclability.  I identified HDPE as the best among the alternatives analyzed and mentioned that I would follow up with information on pricing and availability for potentially suitable materials.  I’ve contacted sales representatives at Zinco, American Hydrotech, Tremco, and Carlisle, though I have not heard back from all of them.

From the research I’ve done, these barriers range from about $0.50/sq. ft. to $2.00/sq. ft. and are generally available via mailorder only, through company sales representatives.

One of the suitable root barrier alternatives I identified in my initial post was Tremco’s Root Barrier VR.  It is made from 40-mil (1 mm) HDPE and, according to the representative I spoke with, works well with EPDM roofs.  The material comes in 4′ x 100′ rolls and can be in Brooklyn in 2 business days.  In addition, the gentleman that I spoke with works from Long Island and offered to travel to the job site with a green roof “expert” to consult on the roof and additional items, including PV, daylighting, LEED accreditation, etc.

Zinco produces two root barriers suitable for our use, as well.  The WSF 40 is a 0.4 mm-thick HDPE sheet specifically designed for use in extensive green roof systems and comes in 1,000-sq. ft. rolls.  The WSB 80 root barrier is FLL-certified, heavier-duty than the WSF 40, and is rhizome-resistant (though not resistant to bamboo).  According to the rep., it is typically not used for extensive/perennial roofs and the seams must be hot air-welded.  It is more expensive than the WSF 40 barrier and is available in roughly 430-sq. ft. rolls.

I will pursue the representatives from American Hydrotech and Carlisle and will post additional information as it becomes available.

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.

One strategy for addressing impacts associated with urban development is to reduce the functional area of developed surface.  The European community, and especially Germany, has opted to do so by re-vegetating roofs.  Green roofs have been associated with a variety of environmental and economic benefits, including the ability to retain precipitation, filter pollutants, reduce maximum rooftop temperatures, insulate buildings, and reduce the urban heat island effect.  Interest in green roof development in the United States has recently spiked, and the industry continues to grow consistently.

Investigations into the effectiveness of green roofs, however, have shown that not all green roofs perform equally.  In order to ensure that a green roof functions as desired, it is important that the designer considers several factors.  These include:

- desired endpoint (e.g., is roof being constructed to provide habitat, buffer stormwater, beautify, etc.?);

- type of roof (e.g., whether an intensive roof — park-like, interactive, relatively high maintenance, and typically heavier — or extensive roof — non-interactive, low maintenance, lighter — is appropriate for the functions desired);

- whether structural modifications are necessary to handle additional weight associated with the green roof;

- overall composition of roof system (e.g., whether to include water retention mats, depth of substrate);

- proper substrate formula;

- which mix of vegetative species will grow well together and perform expected functions, and;

- time to maturity and establishment method for vegetation (obtaining sod-like pre-grown vegetation mats, planting on-site as plugs, or planting on-site as shoots).

In my next several posts, I will explore these questions and others as I identify methods for installing a green roof system for the Eco Brooklyn show house.  I will review primary literature, information available from practitioners, and identify supply chains in the New York City area.  The project will culminate with the installation of the roof.