Whether you’re a homeowner in need of a new roof or an intrepid architect seeking out unique roofing details, you’ll have your choice of materials. The standard for most homes in the U.S. is asphalt shingles, followed by clay tiles, wood shakes and metal roofing. One medium often not considered in North America is living vegetation.
Though “green roofs” or “living roofs” have been found in Europe for at least 40 years, the trend to treat roofs as yet more substrate on which to grow plants has only sprouted up in North America over the last 10. To learn more about living roofs, engineering.com has decided to explore the topic in depth.
What Is a Living Roof?
There are three main types of green roofing, determined by the depth of the planting and the level of maintenance they require: intensive, semi-intensive and extensive. While extensive roofs can handle 10 to 25 pounds of vegetation per square foot, intensive roofs aim for 80 to 150 pounds, and semi-intensive fall between those two ranges.
An intensive green roof is more like a rooftop garden, with a great deal of labor, irrigation and feeding required to maintain plants with roots that grow deep into 7 to 24 inches of soil. Some even include water features, such as small ponds. Extensive roofs, in contrast, are meant to be self-sustaining with 1.6 to 5 inches of soil and the need for only once-yearly weeding. If the roof is not easily accessible, an extensive roof is likely the best bet.
Generally, an intensive living garden is more likely to decorate the roofs of commercial and municipal buildings, like the Chicago City Hall, while extensive roofs are more often found atop a residence. Usually, a roof slope of 40 degrees or more is required for runoff, or else the living roof might end up in the home’s living room.
A green roof typically begins with a layer of a water- and rot-proof membrane, such as a heavy-duty pond liner, stretched across the area of a roof leading up to the gutters, where drainage can run off. This is topped with layers for water and soil retention, made up of mulch, pumice, peat or clay.
Because topsoil is too heavy for this application, a root-growing substrate layer made up of a mix of organic and inorganic material is added. This might consist of shale, peat moss, perlite and coconut husk. Finally, plants are added, with most living roofs featuring local, drought-resistant plants to minimize the need for maintenance.
Why Living Roofs?
Sure, green roofs can serve an aesthetic purpose, but they serve a number of practical purposes, as well. Two of the most notable ones are for stormwater management and insulation.
Due to the natural absorption of water by soil and plants on a living roof, a large portion of stormwater can be absorbed. Penn State estimated that 50 to 60 percent of rain water in Pennsylvania can be captured, freeing city storm systems from runoff that could potentially cause flooding, pollution to the local water supply, and soil erosion.
The benefits of a living roof. (Image courtesy of British Columbia Institute of Technology.)
This last detail contributes to further benefits for the surrounding community by reducing the urban heat island (UHI) effect. Due to the amount of sheer pavement sprawling over urban environments, along with waste heat generated by energy usage, cities tend to have much higher temperatures than surrounding areas, thus exacerbating impacts such as warm summers and heat waves, among others.
In New York City, the most vegetated areas show a 35.6°F difference in temperature compared to the least vegetated areas, suggesting that living roofs can be used to reduce the UHI effect. This may also further impact global temperatures, which are increasing due to climate change, by offsetting some of the loss of albedo caused by the loss of Antarctic glaciers.
Green roofs are also said to have a lifespan of two to three times that of traditional asphalt shingles, with some lasting over 60 years. While providing shelter to residents inside, greenery atop a building also provides shelter and habitat for birds and insects.
Another added benefit not immediately considered is that living roofs provide sound insulation, reducing sound levels by 10 dB. Vegetation also serves as a natural filtration system for air pollutants, cutting air particles and sulfur dioxide in the air around the roof by 6 percent and 37 percent, respectively. When widely implemented, this can provide an entire city with a reduction in air pollution.
Why Not Living Roofs?
The biggest disincentive for the use of green roofs is the up-front costs. A green roof can cost between $10 and $20 per square foot, which is dramatically more expensive than your standard $1.20/square foot shingle.
Whereas a conventional roof may require little maintenance until the end of its life, maintenance for living roofs are essential in the first five years and likely over the life of the structure. Cost of maintenance can range from $0.10 to $1.00 per square foot after the first five years.
Depending on the extent of a green roof project, it may be necessary to add more structural support to the roof, as green roofs are heavier than other roofing options.
Proponents of living roofs argue that the cost of installing a living roof can pay for itself over the long haul, due to the overall lifespan of the roof combined with the energy savings achieved through improved insulation. You can calculate the energy and cost savings of installing a green roof using a calculator developed by Arizona State University.
One metanalysis of green roofing found that, if the practice were widely adopted throughout Helsinki, cost savings would actually add up to €1.9-3.4 per square meter. This is due to the benefits storm water runoff would have on the city’s sewage system, which would otherwise need to be repaired and expanded over time.
The same metanalysis determined that private installation of living roofs is currently not very feasible economically for most building and homeowners. For that reason, the authors argue for government subsidizing of green roofs, thus bringing costs down and increasing adoption.
In places like Norway, green roofing has been around for over a thousand years. The modern tradition of living roofs, however, can be more closely linked to Germany, a long-time leader in green roofing having established the practice more than one hundred years ago. Roughly 10 percent of the roofs in the country are green. In Stuttgart, that number jumps up to 25 percent for flat roofs, due to regulations, tax abatements and government incentives.
Laws related to living roofs are increasing worldwide, including in the U.S., where Denver, Portland, San Francisco and New York are among the cities requiring green roofs on new constructions. Toronto was the first city in North America to mandate green roofs on new builds, including additions, over 2,000 squaremeters in size, starting in 2008. In France there is a law requiring either living rooftops or solar panels on new builds—though green roofs and solar panels are said to be complementary, due to the fact that the vegetation decreases the temperature of the area, preventing the panels from overheating and thus ensuring optimal working temperatures.
When there aren’t specific mandates in place, there are often incentives. This includes cities like Austin, Chicago, Milwaukee, Minneapolis, Philadelphia, Seattle, Syracuse, Vancouver and Washington, D.C. Most of the aforementioned cities that have been rolling out green roofing laws are offering incentives in the meantime.
Historically, the rise in human populations has come at the expense of flora and fauna populations. With the human population projected to continue its growth, such initiatives will be crucial to preserving nature both for its own sake and for the sake of maintaining the ecosystem surrounding humanity itself.