Residential Green Roofs – Why Not at Your House?

‘Going green’ isn’t just a passing trend or catch phrase any longer in our rapidly changing world. Everywhere you look, in nearly every industry, changes are occurring which reinforce this green movement. From the reusable cloth bags at the grocery store, to the broadening recycling programs in nearly every community across the country, everyone has noticed or should notice the importance of this catch phrase.

Can you imagine our world today if we had continued to produce and consume as we did up until 15 years ago? I envision looking out my window and seeing the peak of the local land fill in the distance like you would view the Rocky Mountains in Colorado from a great distance. I can also envision putting on a portable respirator to go out and mow the lawn to offset the poor air quality, or having to filter my drinking water through a complex contraption in my basement just to make it safe enough for consumption. Fortunately, these dramatizations are not our reality yet, but very well could be if we do not continue to embrace this ‘going green’ movement and expand and improve upon it in every way we can.

As a landscape architect, I pride myself on trying to do my part to further the green movement. Sure I design gardens and we plant trees, shrubs and perennials every day to improve air quality, stabilize slopes, reduce erosion, filter the water, and so many other things. But these efforts are not enough, at least not enough for me. The green roof concept has fascinated me for years and I truly believe in this technology. My hope is that someone reading this article 10 years from now will find its content obvious and outdated because green roofs will be as common as the bright blue recycling bins placed at the curb on the second and fourth Tuesday of every month.

The commercial side of the green roof industry is far ahead of its residential counterpart in this country. Many cities and municipalities already have building codes which require commercial buildings and renovations above a certain square footage to incorporate a percentage of the roof surface to a green roof. It’s only a matter of time before these types of codes are standardized throughout the county and hopefully across the world.

The residential side is clearly behind at this juncture, which creates an opportunity. An opportunity to begin to educate people on the importance of proper storm water management, reduction of urban heat island effect, air purification, and decreased heating and cooling costs. These are just a few of the issues green roof technology clearly addresses. Once the homeowner sees the impact that a green roof can have, even on a residential scale, the technology nearly sells itself.

In order to fully understand current green roof technology, I offer this brief description of the two main types of green roofs. The first type of green roof is called extensive. Extensive green roofs have a growing medium depth of 6” or less. Sedum and Delosperma species dominate the plant pallet for the typical extensive green roof. Their unmatched drought tolerance, coupled with their fibrous root systems, cause few problems in the long run in terms of compromising the waterproof membrane layer of the system. There are many other grasses, perennials and groundcovers suitable for an extensive green roof system. However; the succulent varieties seem to be most commonly utilized in this type of green roof system. More and more data is becoming available for successfully constructing green roofs in the U.S. More research is showing that utilizing more native plants in the green roof system may be another way to further improve upon the technology.

The second type of green roof system is called intensive. Intensive green roof systems have a growing medium depth of 6” or greater. This type of green roof system offers a much greater plant pallet due to the thicker growing medium layer and ability to hold more water within the system. Trees, shrubs and a myriad of perennials and groundcovers have been successfully incorporated into intensive green roof systems all over the world. Landscapes have been designed, complete with water features and outdoor seating and entertainment spaces, within the green roof. Intensive green roofs require a much stronger load capacity than extensive green roof systems and typically are considered more appropriate for huge commercial and public projects.

There is no question that the initial cost of a green roof, when compared to a standard asphalt or rubber roof, is higher; approximately twice the cost of a conventional roof. Studies have shown, however, the average life of expectancy of the waterproofing membrane in a typical extensive green roof system is twice as long as a traditional flat roof system lasting from 20 to 30 years. Combine the longevity of the lifetime of the waterproofing membrane with the savings in heating and cooling and the higher initial cost of the typical extensive green roof is not a bad investment.

There are several components to a green roof which are typical of both extensive green roof systems and intensive green roof systems. Starting from the bottom up, we first must consider the sub structure of the green roof. Due to the increased weight of a green roof, the sub structure must be properly engineered and able to withstand the required live and dead load of the system. Typically an extensive green roof system must be able to support 10-50 Lb/SF which is a general estimation of the weight of a fully saturated extensive green roof. Consult with local building code officials and a structural engineer before attempting to construct or retrofit any green roof system.

Above the sub structure or structural deck is the waterproofing membrane. This layer can be hot applied rubberized asphalt, ethylene propylene diene membrane (EPDM), polyvinyl chloride (PVC), modified bitumen or built up bitumen. Selecting the proper membrane is critical in a successful green roof project. Final decisions should be made with the aid of a reputable roofing professional who takes all aspects of the project into consideration: existing roof membrane, structural loading capacity of the roof, size of roof, pitch and drainage.

Once the waterproofing membrane has been installed, it is critical to take measures to protect the membrane through the remainder of the green roof construction and test for leaks before installing the growing medium and plants.

Above the waterproofing membrane, a root barrier is applied to help protect the waterproofing membrane from roots, which can compromise the system over time. Above that is the drainage layer. This layer is critical to provide quick, efficient drainage of water, which cannot be held within the growing medium. There are thousands of different drainage layer products available and the appropriate one should be selected based upon pitch of roof, water retention needs for vegetation, and growing medium depth.

Above the drainage layer is a filter layer, which helps filter the water and prevents the growing medium from clogging the drainage layer over time. Many of the drainage mats available have the filter layer built right in to eliminate the need for a separate filter layer.

Directly on top of the filter layer is the growing medium layer. This portion of the green roof is absolutely critical to get right. Otherwise your green roof has no shot of long term success. Green roof growing medium is a very highly engineered mixture which is composed of organic and inorganic material. Inorganic material may include expanded shale or slate, coarse sand, volcanic rock or other aggregate to improve drainage and reduce weight. The organic material may be composted material such as leaves, grass, or saw dust along with peat moss or manure. The percentage of organic material may vary dramatically based on drainage requirements, plant needs, soil depth, or roof slope. Have your green roof plants selected and know all the project parameters to specify the correct growing medium. It is important to note that over time fertilizers or additional organic matter may need to be added to the growing medium to replenish depleted nutrients.

The final component of the green roof, and undoubtedly the most interesting in my mind, is the vegetative layer. Remember, most extensive green roof systems utilize succulent plant varieties such as sedums and delosperma species. Plants must be carefully selected to meet the individual site requirements; taking into account aspect, temperature, moisture requirements and overall green roof performance. The vegetative layer, if designed properly, will provide aesthetics, insulation, storm water management, air filtration and oxygen production, and in some cases fire retardation.

Having gone through the basic components of a typical green roof system, my focus now is to look at how this technology can be implemented on a smaller scale over a broad range to eventually make a great impact on the environment and further enhance the whole ‘going green’ movement.

At Stoney Bank Nurseries, Inc., we are striving to further the green roof industry, educate our clients and design and construct green roof systems for our residential clients. Looking back on the many projects done over the years, I find myself realizing a green roof on a client’s pool house, garden shed, front porch or rooftop addition would have made so much sense. I see more opportunities in residential applications to work with the project architect and client to develop these ideas and implement them in the real world.

The amazing benefits of reducing the storm water runoff by 60-65%, reducing the clients’ heating and cooling costs, and the overall environmental benefits of air purification and even habitat creation are what really strike me as important issues.

We have constructed several green roof projects so far in the residential sector. We’ve even done a green roof on a portion of our office building, complete with rain barrels and gray water drip irrigation, to sustain the plantings around the structure.

So I ask you, as I do my clients when we begin a project, “Why not at your house?”