Is a Green Roof Right for Your Home?
Weigh the Pros and Cons of Adding a Green Roof to an Existing House
Oct 15, 2008
Green roofs are ever more popular in European and Canadian residences, but in the US, they are still thought of mainly as an option for large commercial and public buildings. A green roof may be the right choice for your existing home.
Green Roof Basics
Many people think that a residential green roof requires structural reinforcements, but many existing houses can support a 3” green roof without major structural changes. A reputable green roof company will initially consult with a structural engineer to make sure of this, and will then thoroughly test your roof for weak spots and possible leak sites before moving forward with an installation.
Three-inch green roofs are typically planted with low-growing plants such as creeping sedums and mosses, which need no supplemental irrigation. To grow larger perennials and/or crops, the green roof typically needs to be a minimum of 6” deep, up to 9” or more. A green roof of this depth may need structural reinforcements on residential homes. Depending what plants you choose for a larger installation, you may need to add an irrigation system, increasing costs. Many green roof installation companies encourage irrigation systems that cache rainwater rather than use tap water.
TPO and PVC roofs don’t need an additional roof membrane installed before a green roof is added, but if a roof has shingles, they will need to be removed before the green roof installation can begin. Most green roof companies recommend that the entire roof be covered by the green roof installation.
A green roof will dramatically extend the life of your roof, making it last a minimum of 2-3 times longer. There is a green roof on a factory in Switzerland that was installed in 1914 and has never needed replacement. Green roofs limit roof damage, help prevent deterioration from UV rays and provide better insulation, limiting the weather-variable expansion and retraction that significantly lessens the lives of residential roofs.
Additional Benefits and Considerations
Green roofs improve energy efficiency, particularly with cooling costs, but the extent of the benefit varies depending on the shape of the building. The shorter and squatter the building, the higher the energy savings will be. A two-story house that is wider than it is tall will reap more savings than a thin eight-story building. In the latter, the energy savings will mostly be seen on the top couple of floors. The difference in temperature on the roof will be dramatic, however; in summer, a black roof can get to 160 F or more, whereas a green roof is about 70 F.
In addition to the shape of the building, consider the shape of the roof. The more dramatically sloping the roof is, the more likely the installation will require special extra materials to secure the green roof in place. If the roof is to be accessible to people, it may also require extra materials to make pathways and other accessibility points. Also find out what your local storm water requirements are, as they may impose certain requirements on the project.
Green roofs provide new amenity space to homeowners, improving the property’s value. This is especially beneficial in cities, where green spaces are at an especially high premium. Green roofs also provide additional habitat for wildlife, which is not just an added bonus to many homeowners, but may be an extra selling point to potential buyers.
Consult with a Specialist
Since each existing building has unique characteristics, it is impossible to estimate how much a green roof will cost. If you are considering adding a green roof, consult with a green roof specialist. One way to tell how reputable they are is whether they calculate roof load based on “saturated weight,” the weight the roof will be when fully soaked (and, if in a cold-winter area, topped with excess snow).
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