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Is a Vegetative Roof Right for Your Building?

Jan. 19, 2018

Understand the challenges of installing and maintaining a green roof.

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.

Tech Deck in Mountain View, CA, received a 2016 Award of Excellence for its intensive
green roof from Green Roofs for Healthy Cities, a non-profit organization that supports
the green roof industry in North America. | MERIKO REED

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.

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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.

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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.

Selecting the Appropriate Waterproof Membrane

Each type of waterproof membrane for vegetative roofs comes with its own pros and cons. Consider these attributes of different membranes for the right fit.

■    Welded seams
■    Most common sheet used for green roofs in Europe
■    Higher cost
■    Uses 60- to 80-mil sheets

■    Welded seams
■    Costs less
■    Needs thicker sheets
■    Confirm whether manufacturer has approved use for green roofs

Rubberized Asphalt
■    Good waterproofing
■    Designed for waterproof applications
■    Monolithic
■    Needs root protection
■    Higher cost

BUR and Modified Bitumen
■    Good waterproofing
■    Need root protection
■    Require more plies

■    Water-resistant sheet
■    Low cost in loose laid systems
■    Seam concerns
■    Leak finding system does not work with EPDM

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.

Vegetative Roofs: 3 Tips for Planning Maintenance

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.

Types of Vegetative Roofs

Based on your aspirations, budget and the structural demand for a vegetative roof, you need to consider the different types that range widely in cost and style.

Intensive Vegetative Roof
When you think of green roofs, you are likely picturing an intensive vegetative roof, which is the most attractive and park-like. A lot goes into these roofs. They can have trees put in them, but they need special pits that are predesigned for trees and large plants. That’s a specialty that requires a lot of planning.

Intensive vegetative roofs use a wide variety of plants, shrubs and trees. However, it is important that plants that are selected don’t have aggressive root systems, as plants with tap roots or any other invasive root system can damage and penetrate the roof membrane.

Intensive projects require considerable irrigation and maintenance, as trees and shrubs require trimming. They require growth media that is deep (8-24 inches) and heavy (60-200 pcf when saturated).
Because of the structural demands they place on buildings, intensive vegetative roofs are not practical for existing buildings with very few exceptions.

For an intensive roof, you are looking at costs anywhere from $30-220 per square foot.

Extensive Vegetative Roof

The extensive vegetative roof is still green and has a lot of the same benefits as intensive vegetative roofs, but it is not necessarily park-like in vision. They use simple plants, typically sedum-type varieties. These have some small flowers and color, but their main benefit is having non-invasive root systems. Compared to intensive vegetative roofs, they are much more affordable at $10-25 per square foot depending on elements like railings, irrigation systems and waterlines.

The growing media tends to be 2-4 inches thick weighing 15-30 pcf. While you can get by with 2 inches of growing media, it is much harder to support plant life with thinner media because there is less room for the root systems to grow, less nutrients for the plants and less ability to hold water, meaning you will need to
irrigate more often.

Like intensive projects, these typically require irrigation. One problem in the past has been the false belief that you don’t need to irrigate outside of the initial stages when the plants are developing. In areas with natural and regular precipitation, you might not need to continue to irrigate except during extended dry periods. In other parts of the country, particularly in drier climates, irrigation is an absolute necessity.

Whether you irrigate or not, you will definitely need to do regular maintenance. If you’re on a 50- to 60-story building, you won’t have many weed seeds getting up there, but if you are on a 1- or 2-story building, the potential for weeds is high. Weeds by their very nature have aggressive root systems, so you’ll need to do something to control them before they can penetrate your roof system.

Weights for extensive vegetative roofs fall in a range that an existing building or roof system can handle, and you have two options for planting vegetation. One is in plastic containers that hold the growing media and plants. With containers, you can pre-start the plants before installation so they get used to the environment and have more developed root systems. They are easier to remove but provide a less flexible layout with their rectilinear shape.

The other option works without containers, as you bring the growing media to the rooftop, level it out and begin planting. Plants can be somewhat mature and grown, but they have to then acclimate to the environment and growing media as they put their roots out. There will be a development time for them to become accustomed to their conditions. If you have to move material to get through the roof system for any reason, you’re going to have to move the plants and the growing media and start over again in those areas. It’s much more labor intensive and more flexible in design, but not necessarily less expensive.

Semi-Intensive Vegetative Roof
Looking for a happy medium? The semi-intensive vegetative roof is a hybrid system that generally weighs between 25-40 pcf, which typically surpasses the weight capacity for existing roofs. You can include a more diverse mixture of plants.

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