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Learning how to make the right material choices, without compromising sustainability.
By Bruce Dvorak
Roofscapes across North America are beginning to turn green, not just because of their hue, or sustainable fabrication, but “green” in an organic sense of the word. Rooftops are blooming in the form of green roofs and roof gardens. A green roof is a system of materials designed to support a thin carpet of vegetation living on rooftops to mitigate environmental stresses; a roof garden is an accessible garden on a roof deck for entertainment and enjoyment. Though the technology is not new, it is now gaining in popularity in metropolitan areas across North America. A recent survey by Green Roofs for Healthy Cities (a not-for-profit green roof advocator) shows that green roof construction across North America continued to grow by more than 30% in 2007 (Cities 2007) to increase the total area of green roofs constructed since 2004 to almost 630,000m2.
The contemporary materials and construction methods used to construct green roof technology can be traced back to Germany during the 1980s, where researchers experimented with and developed lightweight green roof materials for a broader application of green roof technology. Fabrics played a key role in the development of this emerging technology. In this article, we will explore how fabrics are used in green roof construction and key issues green roof designers need to know when designing green roofs. (Several methods to construct green roofs will not be considered in this article, such as the use of prefabricated and prevegetated green roof modules. Some of these systems use fabrics, but the focus will be on custom designed green roofs.)
Application
Prior to the 1980s green roofs were known to be heavy as they were constructed from gravel, sand and topsoil. With the development of lightweight aggregates, the weight of a green roof was reduced and drainage characteristics were significantly improved. It was discovered that by separating growth media from drainage media, more efficient and low maintenance systems could be designed. Fabrics are used in these granular-based drainage systems to filter and separate materials. Selection of the specific type of fabric to use is determined by a number of factors.
First, there are several categories of fabrics. Geosynthetics is a family of fabricated materials typically used for landscape applications. Geosynthetics most commonly found on green roofs include geotextiles, geogrids and geocomposites. There are some new geosynthetics on the market developed specifically for the green roof industry, but there has been much carryover from the existing palette of geosynthetics to use on green roofs.
Geotextiles
Geotextiles are permeable, synthetic fabrics designed with physical and engineering properties that are used to either enhance or improve structural performance of soils in the landscape. Geotextiles are typically constructed with one of two methods: woven, which looks like a mailbag or a potato sack, or nonwoven (needle punched), which is a spun fabric that looks like felt. Another type of fabric used is a modification of a nonwoven fabric, such as a heat bonded material. These are nonwoven fabrics that have been heated and bonded at high temperatures to achieve special characteristics. Woven fabrics are typically stronger than nonwoven fabrics, but they have inferior ability to perform desirable green roof functions such as filtration and water management.
Geotextiles have four properties that are important to understand for their use on green roofs: tensile strength, elongation, puncture resistance and resistance to ultraviolet light. Tensile strength is the amount of force the material can withstand when being pulled (such as a rope’s resistance or breaking point). Elongation is a material’s ability to stretch. Ceramic, for example, has very low elongation properties, metals have moderate elongation properties, and some plastics can have high elongation properties. Puncture resistance is the ability of the geotextile to resist being punctured. Resistance to ultraviolet light is important only where a geotextile is exposed to the elements. Fabrics on green roofs can be subject to all of these stresses. Tensile strength and puncture resistance are perhaps the most critical properties of fabrics to investigate prior to their proposed use and specification.
Geotextiles can be tested in accordance with one of two common measures: the American Society for Testing and Materials (ASTM) has standard tests to perform as does the Geosynthetics Research Institute (GRI). In addition, the German based Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V. (FLL Guidelines) has a chapter on the application of fabrics on vegetated roofs. The ASTM standards have been updated to include standard tests and guidelines for green roofs, but have no specific tests for fabrics on green roof applications.
Geotextiles have a number of uses on green roofs. Most commonly used functions include drainage, filtration, reinforcement, separation and protection. There are two broad categories of custom designed systems — synthetic and granular drainage designs — that make use of layers of materials as well as modular systems.
Drainage
A green roof’s drainage is as important as having an adequate water supply. Where not enough water can cause wilt and death, too much water can drown plants. Green roof habitats need to be predetermined and the drainage needs to be designed to support the desired plant community. Drainage on green roofs can be accomplished through the use of geocomposites or geotextiles. A geocomposite is the combination of a plastic drainage material with a nonwoven geotextile fabric. The fabric holds the soil and moisture in place while the plastic design allows water to drain freely underneath and off the roof deck (Fig. 4). Nonwoven geotextiles can be used at the bottom of the green roof profile as a water management blanket to manage and slow water and evenly distribute moisture. This also helps to prevent the undermining or erosion of media at the roof deck.
Filtration
Nonwoven geotextiles can be used to filter materials by holding them in place. Typical green roof growth media contains less than 12% organic matter and very few fine particles. Skewing the growth media design toward courser materials allows for adequate drainage and helps to prevent clogging of fabrics and drainageways. A fabric used to filter green roof growth media should hold and sustain at least 90% of the media in place (FLL 2002).
Reinforcement
Geogrids include a variety of polymer designs that can be used to structurally support and retain soil on the landscape or growth media on sloping rooftops. According to FLL Guidelines, vegetated roofs on slopes up to a 36% gradient or 20 degrees, do not typically require additional support measures to prevent growth media from sliding off of roof decks. Structural or geotechnical engineers should be consulted on sloped green roof applications to engineer the slope stability, even on projects where slopes are less than 36%. On roof decks sloping more than 36%, geogrids can be used to retain growth media and plants in place. When fabrics are used in conjunction with such sloped roofs, the texture of the fabric is critical. The selected material needs to aid the retention of growth and drainage media and not aid susceptibility to slippage. Since nonwoven fabrics tend to have rough textures and heat bonded fabrics tend to have smooth textures, nonwoven fabrics can be used with less reinforcement on sloped roofs than heat bonded fabrics. Create mock-up sections of steeply sloped roofs to test their stability prior to their specification on a project. Of course, structural engineers should be consulted with any green roof project at all phases in accordance with the green roof design process (Dvorak 2008).
Separation
Nonwoven geotextiles are available in a variety of thicknesses and designs that can be used as separation fabrics. These materials separate growth media from drainage media and have many possibilities for use. In addition to a fabric’s ability to filter is the ability to allow rainwater to pass through. Most geotextiles manufacturers can provide test results of permeability or water flow through rates for their fabrics. Once placed on a vegetated roof, however, one can expect that the flow rates will be reduced as fines can reduce flow rates over time. Organic content should be kept below 12–20% in northern climates to prevent a variety of potential environmental problems with green roofs such as resistance to clogging, fire and long-term stability of the growth media.
Protection
An often secondary function of a geotextile is its ability to protect waterproofing and root barriers after they have been installed on the roof deck. Nonwoven fabrics can provide excellent protection against puncture during the construction process. Care should be used, however, during construction to avoid damaging the fabric and preventing its proper function. Wind erosion protection is another use. When lightweight growth media are used in exposed and windy sites, biodegradable wind erosion prevention blankets can be used (Fig. 3). Polymer-based materials can also be used when they have the ability to decompose or break down over one growing season so not to interfere with plant growth or weeding. Other natural blankets are available constructed from coconut fiber or hemp, but these stouter materials may take one or two growing seasons to decompose.
Characteristics of green roof fabrics
When selecting geosynthetics for green roof applications, compare a fabric’s characteristics with several factors specific to green roofs. FLL Guidelines communicate some of the following observations. Geosynthetics should be compatible with the environment, meaning that it should not create environmental toxicity where humans come into contact with them. They should pose no risk to phototoxicity. Fabrics should be tested to ensure that they have no residual or long-term toxicity to green roof plants. Fabrics should also be tested for their behavior under fire. Generally, green roof growth media can prevent or dampen heat transfer. Therefore, vegetation and growth media are recognized for their ability to protect waterproofing and roof decks from fire danger. However, where growth media has an organic content above 20% internal heat combustion and smoldering type fires are not out of the realm of possibility (Bruening 2008). In green roof designs, where organic content is above 20%, a fabrics resistance to fire becomes critical.
Fabrics such as geotextiles are typically segregated by their type of construction, such as woven and nonwoven. Another distinction and method to specify their use is weight. According to FLL Guidelines, minimum recommended weight is 100g/m2 and recommended range places them between 100 and 200g/m2, where growth media is expected to be up to 25cm deep. Another method to distinguish use is by thickness. The designation of mils is common in North America where 1 mil is one-thousandth of an inch.
A fabric’s weight can add measurable density to a roof deck. Some water management blankets, composed of nonwoven geotextiles, can add as much as 4.88 kg/m2 (1 lbs/ sq. ft.) to a green roof. Where lightweight green roof applications are desired, type and use of fabric should be carefully considered. Fabric manufacturers provide dry and saturated wets of materials and should be consulted.
Additional fabric considerations on green roofs include their cut through resistance or ability to cut through the material. When separating materials, for example, the cut through resistance should not be low, but when using a fabric as a wind blanket, cutting through would be desirable to install plants when establishing vegetation with plugs. Resistance to weathering and soil-borne solutions, micro-organisms and insects should be considered as well.
During the construction process, be aware of several technical considerations. First, rooftops are often exposed to wind velocities that are at a much greater frequency and velocity than are found on the ground. Handling and laying out of fabrics can be challenging, as fabrics should not be laid on the roof until they are covered or properly secured with temporary measures.
When handling and applying fabrics onto roof decks from rolls or sheets as a filter material or separation course, they should have a minimum overlap of 10cm. They must also be brought up beyond the edges of growth media boundaries to retain the growth media and prevent it from eroding off the roof (Fig. 5). Second, the layering of fabrics should mimic that of shingles on a roof. The downslope edge of the fabric should overlap on top of the upper slope side of the fabric placed beneath it. In this way, growth media can be placed and water can move across the fabric without disturbing the underlying layer.
Summary
Fabrics play a critical role in the design of many types of green roofs. It is important to be aware of several important considerations. Geotextiles can be used to drain, filter, reinforce, separate and protect elements of green roof designs. When used, fabrics should be checked for their ability to retain at least 90% of growth media in place, the permeability or flow through rate of rainwater, the weight of the fabric when saturated, the texture of the geotextile and its resistance to fire. If you cannot find test results for a fabric you would like to use, consult with green roof system providers or consultants who have used the same fabric in landscape applications. Designing mock-up sections of green roofs is another alternative to investigate the performance of a custom application.
With the changing dynamics of the building environment, architects, landscape architects and engineers will continue to need to learn about and help bring about green building innovations. As green building continues to increase in popularity, green roofs are likely to continue to grow in popularity as well. Green building recognition programs such as LEED, have already bumped up a green roof’s contribution from 1 point toward certification in LEED version 1.0 to up to 15 points possibly contributing to a project now with LEED version 2.3. In the future, we can only anticipate the critical role that fabrics play in the function and design of green roofs will continue to sustain and support the emerging culture of green roof technology.
Bruce Dvorak, ASLA, RLA, an assistant professor of landscape architecture at Texas A&M University, School of Architecture, and is a contributing editor to Fabric Architecture magazine.
References
Bruening, Jorge. (2008). Fire and Wind on Extensive Roofs. Greening Rooftops for Sustainable Cities Conference. Baltimore, Maryland, Green Roofs for Healthy Cities
Green Roofs for Healthy Cities (2007). Final Report Green Roof Industry Survey 2007: 5.
Dvorak, Bruce. de la fluer, Marcus (2008). “Seeing Green Up.” Fabric Architecture January/February 2008: 6.
Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V. (2002). Guideline for the Planning, execution and Upkeep of Green-roof Sites.