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Green roofs 101: From bottom to top
By Allan Wingfield
Introduction
Most green roofs use a variety of geosynthetic materials to support healthy and sustainable installations. Geosynthetics contribute in many ways to the operational success and long-term sustainability of green roof systems.
Because they are lightweight, they reduce green roofs’ structural loading on buildings and also reduce the energy required for transportation and installation of green roof assembly components. Many geosynthetics can be made of recycled materials and using them can contribute to LEED credits.
The needs specific to each project determine the appropriate selection and application of geosynthetics. Examples include:
- minimizing weight while retaining good drainage properties.
- strength under construction duress.
- climate considerations and rainfall patterns.
- slopes susceptible to erosion and slippage of the growing media.
- Leadership in Energy & Environmental Design (LEED) certification requirements.
A job for every layer
While the finished, and beautiful, vegetated field is what catches our eye in project photographs, the casual viewer may be unaware of the many layers of geosynthetics and other materials that contribute to the completed green roof.
Typical green roof systems, whether assembled as separate components or combined into a single product offering, are located above the waterproofing membrane and its substrate. These include a root barrier, drain and filter layer, water retention layer, slope stabilization and root reinforcement layer, soil media and plants (often a prevegetated mat), and wind blanket (Figure 1). Each layer fulfills an important role in the overall functional success of the roof design, sustainability objectives, and healthy, ongoing life of the vegetation.
From bottom to top
Positioned directly on top of a roof’s waterproof membrane, the root barrier is typically a durable synthetic membrane layer that prevents root encroachment and protects the roof’s waterproofing membrane, which may be made of polyvinyl chloride (PVC), polypropylene, or polyethylene. In some cases the root barrier is a nonwoven geosynthetic fabric impregnated with copper sulfate or other root deterrents. These deterrents may also be embedded in some membranes such as PVC or thermoplastic polyolefin (TPO), eliminating the need for a separate root barrier layer. However, in those cases where a root barrier is not required, it is still advisable to add a layer of nonwoven fabric as extra protection for the membrane.
The drainage layer performs multiple functions and is essential to optimal performance. Rainfall, whether limited in arid climates or abundant in temperate, humid climates, must be managed properly to ensure that erosion or ponding does not occur, damaging the plants. The drain must effectively remove surface water from impervious surfaces, such as hardscaped areas on the rooftop, as well as subsurface and vegetated areas.
By providing a void space and slope to transport water to roof drains, gutters, and scuppers, the drainage layer conducts excess water away from the plants and off the roof. In some instances, another function of the drainage layer is to supply water to a retention layer that affords the right amount of moisture to the roots above.
Another necessary component above the drain layer is a filter layer. The drain’s integrity is protected by preventing fine planting media particles from washing into the drain cavity, eliminating the danger of clogging or reduced drain capacity. Typically, this geotextile is a nonwoven fabric, but woven fabrics are also used.
Balancing the need for keeping the planting media out of the drain core while still allowing water to flow away from the plants and growing media and into roof drains is the most important challenge. Thus, the filter layer must have openings large enough to allow the unimpeded flow of water; otherwise, hydrostatic pressure will build up and the media will not drain. This important fabric also must be durable to withstand the construction process. Fabric tears or openings created between woven strands allow media to wash away, impeding drainage of the system and harming the plants.
Geocomposites—best practice for drainage
The geotextile layers that provide protection, filtration, and water retention may be combined with a polymeric drain core to create what are referred to as geocomposite drains.
The core composition can vary in design and material and the geotextile may be applied to one or both sides, using heat bonding or adhesives. These composite products are manufactured either in sheets or rolls with edges that are overlapped or joined during installation. There are three basic types of geocomposite drain cores: an entangled filament in either net or structured waffle shape, a cuspated dimple drain, and a geonet grid. All three compare favorably to a traditional granular or aggregate drain layer.
Geocomposites weigh significantly less (14 to 33 oz/yd²), have tested flow rates, and are much thinner while offering superior draining. In addition, the filter is integral to the product. In contrast, granular or aggregate drains weigh 40 to 50 lb/ft³, have less predictable flow rates, and must be deeper or thicker to equal the flow rate of a geosynthetic. Plus, a separate filter layer is required, increasing the likelihood of failure due to puncture or “filling in” of crevices created by the aggregate edges.
While the roles of protection and filtration may seem obvious, the functions of holding water while at the same time moving water away may seem at odds. However, especially in hotter climates that may experience flash rainstorms and extended periods of drought, the water retention layer benefits both storm water management by reducing runoff and plant health by storing water for use by the plants, maintaining optimum growing conditions. Three basic types of manufactured products are typically used to retain water: rigid, molded, plastic “cup” sheets; recycled foam mat; and nonwoven fleece fabrics. These fabrics, with recycled content, are thick and super absorbent.
The retention fleece, a nonwoven fabric, goes on or is attached to the top of the drain mat. It serves as a filtration barrier against media erosion and facilitates distribution and temporary storage of rainwater. In most prevegetated green roof installations, additional growing medium, a lightweight mix of porous mineral aggregate and composted organic material, is spread out over the retention fleece before the prevegetated mats are rolled out.
Geosynthetics—stability and protection
While waterproofing and drainage technology is at work underneath the vegetative layer of green roofs, it is important to also consider what is happening on top. Here, the exposure to wind (perhaps, even hurricane-force winds), rainfall (hydraulic loading), and even the construction process itself can take a toll on fledgling plants during the green roof establishment phase.
Root reinforcement, sometimes referred to as turf reinforcement, provides a stable environment for roots to hold on, multiply, and develop a strong base for the plants, even to withstand hurricanes.
At the same time that roots are being supported, this geo layer also prevents erosion of the growing media and holds both soil and plants in place, even on slopes. This is critical during heavy rainfall or excessive irrigation. An unsecured slope allows erosion, slippage, and movement of both media and vegetation, which is not conducive to plant life or green roof stability. Particularly recommended for slopes greater than 2:12, the typical geosynthetics used are geocells or turf reinforcement mats (TRMs), which are 3-D entangled filament mats. For low slopes, impermanent erosion mats made of jute or straw are sometimes used.
Another option to consider for ensuring healthy plants is the use of prevegetated mats (Figure 2). These mats have multiple advantages to help ensure the success of the plants and green roof installations. Cultivated off-site in controlled fields, the plants are grown in an engineered growing medium incorporated into a permanent geosynthetic carrier that does not decompose over time.
This durability makes the process of temporary overburden removal more efficient and less costly. If facility managers need access to the underlying roof for inspection or repairs, specific sections can simply be rolled up and then rolled back into place rather than digging out, discarding, and then replacing entire areas of the green roof.
Transported from the farms and delivered in rolls, much like sod, the mats roll out across rooftops for easy installation. Their light weight (as light as 8-10lbs/sf) make mats a good option for retrofits and other projects where roof load is limited. Prevegetated mats also have the advantage of immediately providing a fully greened vegetative field that looks beautiful while minimizing media erosion and the encroachment of weeds. These mats can avoid a potential problem with in situ planting that may take several growing seasons to get established, spread, and create full cover.
Wind blankets are still another layer used to protect fragile media and plants during the installation process itself, especially for green roofs installed without full-grown plants and extensive vegetative coverage. Wind can quickly cause growing media to blow off, scour, or shift to other parts of the roof, possibly causing damage to adjacent structures and exposing roots or even uprooting plants. Typical wind blankets are made of woven or nonwoven fabrics or 3-D entangled net mats secured by anchors. In some applications, a bio- or photo-degradable mat is acceptable if complete vegetative coverage is achieved before they lose strength.
Geosynthetic materials have played an important role in successful green roof projects, both large and small, around the world. The uniquely tailored properties of lightweight geosynthetic materials for green roofs (including long-term strength, durability, tear and puncture resistance, proficiency to both drain and retain water, and the ability to stabilize sloped green roofs) enhance the long-term performance of modern green roof systems. Geosynthetics provide solutions to keep vegetation thriving and give designers many options to create functional and beautiful rooftops that reduce the energy use of buildings and reducing stormwater runoff.