Take a look at some of the most exciting, sustainable buildings around the world, and you may notice something they have in common: green roofs, or rooftops covered with a thin layer of soil to allow for grasses and other low-lying plants to grow.
Green roofs provide major benefits, are relatively straightforward to install, and can be added to existing buildings as well as new construction projects. However, easy installation doesn’t necessarily come cheap: depending on a variety of factors, the costs for installing a green roof can be quite high. Read on as we discuss the many advantages of green roofs and delve into some of the budgeting and cost issues that you might come across if you’re thinking of a living roof for your own home.
Although green roofs have existed for thousands of years, they’ve become increasingly popular in the past few decades as interest in sustainability has grown. As the largest horizontal surface on a typical building, roofs are vulnerable to both cold and hot temperatures, and other weather conditions. Green roofs help counteract these issues by reducing heat gains in the summer (and therefore saving money on air conditioning), increasing the thermal mass of a building, and acting as insulation during cold winter months. The significant reductions in energy bills based on these factors are usually enough to convince most home owners to install green roofs.
However, the benefits of green roofs go beyond energy savings: their density and depth on a rooftop can provide acoustic insulation, protect the roofing membrane below to increase its lifecycle and longevity (often to more than 50 years, up from the typical 15 years), and help earn points towards LEED certification. In fact, according to the Michigan–based network of green roof professionals LiveRoof, the biggest financial benefit of a green roof isn’t money saved on electrical bills, but rather not having to replace and regularly maintain a roof. Because green roofs extend the lifetime of a roof between 200 and 300 percent, owners are able to save significant costs in the long run.
For the broader community, green roofs are aesthetically pleasing, adding a bit of greenery to what is often an otherwise gray or black cityscape. They also add to urban biodiversity and typically use local plants that are native to the area. Most companies, like Portland, Oregon–based Columbia Green Technologies, select plants that are "drought tolerant and appropriate for the depth of the system and project climate."
Other community benefits include reduction of stormwater runoff that would often go into overflowing municipal systems, and improvements to air quality; the living roof actively absorbs many pollutants and passively filters and directs airflows. Because green roofs tend to absorb radiation from the sun rather than reflect it onto its neighbors, green roofs also mitigate the heat island effect in cities.
However, all of these environmental, community, economic, and aesthetic benefits come at a cost. This can depend on a wide range of factors including the type of roof system used, where the building is located, the availability of a labor force that is experienced in green roof installation and maintenance, and the need for additional structural support.
Today, green roofs comes in two primary forms: intensive (which can hold up to 150 pounds per square foot and typically support more diverse types of plants—but also requires more maintenance), and extensive (able to support up to 25 pounds of vegetation per square foot and usually grown naturally and without restriction, needing only yearly weeding and fertilization). These two systems are significantly different in their weight, and therefore also associated costs with construction, installation, and maintenance.
Factors like location, quality of the roofing membrane, roof accessibility, structural load capacity, and ease of material conveyance to the roof affect the price, explains Recover Green Roofs, a Boston area–based design-build firm specializing rooftop gardens and green roofs in New England. Based on these considerations, Recover Green Roofs has found that extensive roofs can cost about $10 to $50 per square foot, while intensive roofs can cost from $20 to $200+ per square foot.
Baltimore, Maryland–based Green Roof Technology, a company that specializes in the specification and design of green roofs, notes that increases in costs can also depend on growing media depth, desired water storage, and plant material. However, they point out that green roofs can be considerably less expensive—up to 50% cheaper—when they cover more than 10,000 square feet because of economies of scale.
Interested in installing a green roof but want to do your own research before reaching out to a professional or consultant? You’re in luck: in 2010, the University of Arizona put together a handy calculator tool that allows you to calculate the annual energy performance of a building with a vegetative green roof compared to a traditional dark or white roof. More helpful resources including design standards, conference recordings, and an open forum webinar can also be found on the Green Roofs for Health Cities website.