Green Roofs (Minus the Greenery)

A short brief on popular sustainable roofing techniques that do not involve grass: Green Roofs

When most people encounter the term green roof, they picture rich, lush grasses carpeting the top of a sustainable development. While they certainly would not be wrong, to cast eco-friendly roofing into such a narrow and specific type would truly fall short of revealing the variety of sustainable solutions available. In fact, sustainable roofs often come in any number of arbitrary color designations. There are “green” roofs composed of actual living plant life, “white” roofs that reflect the sun, and “blue” roofs that collect rainwater. There are “cool” roofs, “reflective” roofs “energy efficient” roofs, and “low-albedo” roofs. There are roofs shaded with trellises, mounted with solar panels, covered in high solar reflectance index shingles, or composed of sheets of galvalume. The truth is, the number of strategies available when it comes to “green” roofing is quite remarkable – there needn’t be any greenery involved.

Cool Roofs

The term “cool roofs” is an apt description for a diverse group of roofing techniques and material applications that focus on one thing, reflecting sunlight and thereby reducing heat gain – hence the name. A cool roof is defined by the U.S. Green Building Council as a roof that reflects 70 percent of sunlight. This can achieved with a variety of techniques that are often combined. A straightforward solution popular in the nation’s sunbelt involves the use of all white or light-colored materials in order to shed the sun. These are known as white roofs and are inappropriate in northern climates where residents may actually appreciate some heat gain during the winters.

Roofs that employ metal, as opposed to traditional shingling, are also often lumped under the same category as they function by reflecting away sunlight and heat. Metal roofs have the added benefit of being significantly easier to clean and maintain, harvest rainwater cleanly and effectively, and lend themselves easily to the installation of photovoltaics without roof penetrations. In general, cool roofs employ materials with high SRI’s. SRI, or solar reflectance index, is a key indicator of how well a material will reflect light, and thus reduce heat gain. The ratings index, based on a complex formula, generally assigns standard black materials a value of zero and standard white materials a value of one hundred. That means that some especially reflective roofing materials can score over one hundred. To help homeowners determine a good SRI, the U.S. Green Building Council’s LEED rating system recommends an SRI of at least 29 for sloped roofs and 70 for flat or shallow roofs.

Energy Efficient Roofs

Roofs are a big deal. Because they tend to receive the most exposure to sunlight, they also tend to be a major overheating concern for the inhabitants. Heat from a sunbaked roof will work its way into interior spaces through radiation, conduction, and to a lesser extent, convection. On the flip side, as much as 45 percent of a home’s heat is lost through the roof during the winter. Roofs are a point for combating unwanted heat gain during the summer and heat loss during the winter. A very common energy efficiency measure includes combining metal roofs with spray foam insulation. Metal roofs themselves act as an effective radiant barrier by reflecting upwards of 90% of the sun’s radiant heat. The addition of spray foam on the underside of the roof decking provides a very efficient roof system, preventing heat from ever entering the attic space where the equipment used to deliver cold air in the summer is commonly located.

Rainwater Collectors

Like sunlight, rain water is ubiquitous to roofs, in that roofs keep the water off of our heads. But with much of the west and midwest still experiencing drought, there’s no reason not to harvest that water. That’s where roofs, combined with rainwater collection system, are key. The systems themselves could range from comprehensive water management systems to a simple barrel beneath a downspout, but the intent is the same. Considering that water from the sky is free, there’s almost no reason not to collect and use it, whether for watering plants, recreation, or with proper water treatment equipment, even drinking. If potable rainwater collection is planned, a metal roof is preferred to avoid any contaminants which could come from petroleum based asphalt shingles.

Rooftop Solar

Your roof certainly needn’t be covered in tall grasses to qualify as a hyper-green roof. Rooftop solar applications can not only be designed to provide shading, but also to convert that unwanted sunlight into clean power that can then be diverted for other uses, including heating or cooling.

Lifecycle Cost Analysis

There are many other factors that come into play with regards to sustainability. Often times, energy calculations on paper, or the final balance sheet at the end of the day don’t give a comprehensive and complete picture of a home’s sustainability. Concerns about lifecycle costs, water conservation, and even social ramifications all underscore the fact that roofing sustainability is more than just about energy efficiency. For example, a home roofed with high SRI shingles that must be constantly replaced may not be sustainable, or cost effective, at all.

The truth is, each home’s roof is subject to unique conditions and factors that should ultimately determine the solutions considered. A grassy green roof may be a great idea in Houston, but a terrible idea in arid West Texas. Likewise, rooftop solar may very well be far less cost-effective and sustainable than simply adding more insulation in cloudy Seattle. The beauty of sustainability is that in essence, it’s about working with nature, not against it. In that sense, roofs across the nation should be as varied and different as the houses themselves – green, white, or blue.