Building owners are always looking for new ways to bolster their green credentials, and many have turned to vegetative roofs as a means to improve building performance and display an environmental consciousness. Green roofs will do just that, but they come with a set of distinct challenges for retrofits. Would your building be a good fit for a vegetative roof?
Components of a Vegetative Roof
Before addressing any of the benefits and challenges of vegetative roofs, it is important to identify their components. Vegetative roofs require adequate structure to support the roof’s weight and specific materials to properly maintain the ecosystem and capitalize on the main benefits for a long period. No matter the type of application, you need to consider these components of a vegetative roof.
- Starting at the bottom is the structural roof deck. It’s most likely metal, but it also could be structural concrete, which might be better because of the weight of the vegetative roof.
- Above the roof deck, you need a waterproofing membrane that is covered by other materials and potentially in contact with water a high percentage of the time. This will often be different from a conventional roof membrane, although many of the same products are used for both applications. Use fully adhered or mechanically attached membranes that are reinforced so you won’t have ballooning issues that can displace the vegetative materials. You can choose between PVC, TPO, liquid-applied (rubberized asphalt), asphaltic (BUR, modified bitumen and rubberized asphalts) and EPDM.
- The root barrier is required in some systems and includes not only the barrier to keep roots from damaging the membrane but also insulation. It is optional depending on membrane type, with asphaltic materials requiring some sort of a root barrier.
- The retention or drainage sheet is basically a thin plastic molded sheet that is dimpled and is similar to waterproofing in walls. The retention/drainage sheet can retain rainwater for plant use on drier days, as it will fill up in the low spots and water can later diffuse into the growing media.
- Above the retention/drainage sheet is the filter layer or a filter fabric that keeps the growing medium above it from clogging the drainage system.
- The growing media is not soil, so you shouldn’t dig up top soil from the ground and bring it up to the roof to grow your plants in. This is a manufactured blend of materials to let the plants grow in.
- Of course, the vegetation is on top.
Improving Your Building and the Environment
Beyond the clear aesthetic benefits to vegetative roofs, they provide extensive benefits to facilities. The first and perhaps most important advantage they can provide for facilities is better stormwater management. In highly populated urban areas with limited stormwater capacity, stormwater management is a key concern. To address these issues, conventional roofs are designed to let rain water run off of them with slight slopes, and we commonly see specialized roof drain covers that restrict water flow to the drains so that water can be held in a reserve capacity and dribble into a stormwater system to prevent overflow problems. Vegetative roofs instead absorb and hold water from precipitation to naturally delay the flow of water to the drains.
Another environmental concern vegetative roofs address is reducing the urban heat island effect. Dark roofs and pavement radiate solar energy back into the environment so that it is several degrees warmer than the surrounding suburban and rural areas. Although they aren’t as effective as a highly reflective roof, the plants and growing media in a vegetative roof absorb less heat than the dark roof would and therefore radiate less back. Part of the energy that would normally be radiated back from a dark-colored roof is used by the plants, which also evaporate water, thereby cooling the air and helping bring urban heat temperatures down.
In addition to absorbing more sunlight, the plant materials and other components of vegetative roofs increase overall roof life. Because the roof membrane is covered and protected from ultraviolet light, it generally has a greater life expectancy. The materials above the roof itself also provide lower temperatures for the membrane, which slows down its aging process.
With the extra protection that the plants and materials can provide, vegetative roofs provide energy savings. In the summer, the plants can offer shade and lower the temperature of the membrane, which will limit the amount of heat that flows into the building. Moreover, the effect of the evaporative cooling lowers the temperature and amount of heat moving into the building. It is a little more difficult to save energy with them in the winter, but there are some benefits because the added air films prevent heat loss to the environment.
Plants also improve air quality by removing carbon dioxide from the air and lowering temperatures. Additionally, the growing media and plants can trap dust and some types of pollutants, which lowers some of the concerns about the environment.
Other benefits include the restoration of natural habitats for birds (though they also attract insects and other vermin) and the creation of a minor space for recreational activity or to simply go outside, depending on the space. If they can be seen and used, vegetative roofs provide aesthetic and psychological benefits for building occupants.
Issues to Consider for Retrofits
Beyond the costs that can add up when implementing a vegetative roof, there are other important considerations for retrofits. First, you need to address the extra load you will put on the building. If you had a ballasted single-ply roof in the past, you’re looking at 10-12 pounds per square foot of extra weight over your membrane. With a vegetative roof, it is 15-30 pounds. Once the structural engineer has approved the retrofit, you need to determine whether the permanent extra load will cause ongoing deflection of the roof deck. Concrete and steel will cold flow, and that extra load can cause the roof deck to deflect and turn a draining surface into a retaining surface.
The demands of a vegetative roof will clearly require structural and waterproofing expertise, but it is important to also consult with experts about the vegetation itself. You are not likely to find an expert in both, but it is ultimately best if both are knowledgeable about local issues to make sure everything is compatible.
Retrofitting a vegetative roof is no place for shortcuts, so do not use the existing membrane. It wasn’t designed to be buried under things and in potentially constant contact with water. Instead, put in a proper waterproofing system that is essentially bulletproof because the cost to find and access leaks is high. Be sure to flood the test membrane before installing the plants because the cost of replacing the membrane is much lower than moving all the materials once they are in place.
Other shortcuts will also be off the table as more codes for vegetative roofs are put in place. In addition to any local codes, the International Code Council has a code for vegetative roofs, which will need to meet fire codes in 2018. Even though growing plants don’t burn well, dormant and dead plants do, so you will need to test to make sure you don’t have dormant plants during the winter. Moreover, many roof designs use rock perimeters to meet fire codes.
Be sure not to overlook the effects of wind during the design process. When wind goes over the roof wall, you have areas of wind vortex and negative pressure that could displace plants and the growing media if not properly secured. The solution for this is to use a paver or ballast area around the perimeter of the roof. Using rock works because it typically isn’t disturbed much by the wind and there is less displacement.
Be sure to consider any additional costs and maintenance that a vegetative roof will require but a conventional roof might not. You will likely need to irrigate the plants and winterize the system during the winter in cold climates. Properly maintaining the roof by weeding and controlling insects and vermin is key to the survival of both the plant life and the roof. For improved safety, you may need to add safety railings (at least 3 feet in height) and better access to the roof.
Ted Michelsen is President of Michelsen Technologies. He is a roofing educator who has served as the Executive Director of the Roofing Industry Educational Institute and worked with BURSI, the Better Understanding of Roofing Systems Institute. His extensive background in roofing products includes 26 years at Johns Manville, where he served as Vice President for Technical Services and Research in the Roofing Systems Division.