When you're thinking about what goes into a green roof, the terminology can really get in the way. Technically speaking, the term "green roof" applies to a roof upon which living plants have been planted and nurtured. Think the hanging gardens of Babylon.
Oddly enough, though, this style of green roof isn't a cool roof. And to be really eco-friendly, you want your roof to be cool. So here's another way to look at it. Some of the great producers of the greenhouse effect are the roofs over our heads. The sun hits the roof, and the roof reflects light back into the atmosphere. The intensity and angle of that reflected light create the so-called "heat island" effect in cities and can erode the atmosphere. Further, if the roofing material doesn't have insulating properties, it can create a secondary environmental issue by requiring the inhabitants of the building to crank up the climate control.
So to be green, you have to be cool. What are some of your options when it comes to roofs? Read on to learn about 10 of them.
If you want a roof that will last up to a hundred years, you might want to look at tile. It's favored in warm climates, and the key to its status as a green material lies in its composition, its shape and its ability to be recycled.
"The curved shape helps with ventilation," says Jessica Clark, marketing liaison for the Cool Roof Rating Council.
Roofing tile comes in a wide range of colors, including those lighter colors that reflect less heat into the atmosphere. As an added eco-bonus, they're heavy: That superior thermal mass keeps the cool air inside on a hot, hot day.
Basically, a single-ply thermoset roof consists of a roof membrane derived from oil and natural gas that's cured and bonded to roofing materials. These roofs are well-sealed and insulate the buildings they cover.
They're constructed in the factory as opposed to roofs that are constructed and sealed at the site. According to the Single Ply Roofing Association, they have a more consistent quality because they're not constructed on site.
According to the association, thermoset membranes are good at withstanding the damaging effects of sunlight. Usually the membranes are black, but manufacturers also make them in white, which is the best color for a cool roof.
Thermoplastic membranes have a different chemical composition from thermoset, which means that they don't necessarily have to be bound to the same type of material. They're welded shut and can contain fire retardants and materials to absorb ultraviolet light.
According to manufacturers, this type of roof is a good choice for restaurants and businesses that push oils out through ventilation systems because the membranes are highly resistant to oils, animal fats and bacterial growth. The membranes can also be installed over older types of roofs when a full tear-down isn't practical.
Typically, these rolls of plastic come in the cool colors gray or white, and that slick single surface improves the roof's ability to reflect light.
Shakes and shingles are generally the most popular types of roofing materials, but without the right engineering, they're also the least cool of the cool roofing options. Or, to be more precise, they're the least reflective. And you want your roof to reflect, not collect, the heat.
To improve the green qualities of these roofs, consumers can request that they be treated with reflective pigments. But even that treatment doesn't solve all the problems: A smooth roof reflects more light than a bumpy roof. A bumpy roof reflects light in many directions, and with shingles, you've got bumps all over. To help make them greener, paint them bright white.
Pavers are actually paving tiles designed for a roof. They're thick -- more than 2 inches (5 centimeters) -- and the cool varieties reflect a minimum of 78 percent of ultraviolet light. And they have all the great features of roofing tiles, to boot.
A roof entirely composed of pavers is as heavy as a stone roof and can weigh more than 23 pounds per square foot. But in a building that's designed to carry that load, the heavy roof can have tremendous insulating benefits. If you don't have a home that can hold that kind of weight, you can install pavers on smaller areas, like balconies.
Again, when you're thinking cool and green, the hot tin roof is not necessarily the first material that would come to mind. But then, this isn't the '50s, and we're not just talking about regular metal. "It's not bare," Clark points out. "It's a coated metal."
You start with a coated metal and paint it a light color or add some light-reflecting pigment, and you've got a cool roof. You can even install it over an existing roof. "It's really durable, and a lot of times you can recycle the material," Clark says. You'll also have the benefit of a unique roof that stands out in a sea of shingles.
You probably wouldn't associate a cauldron of hot tar with a cool roof, but this is not your highway department's tar. The key lies in the modification. "It's modified with a plastic, and it's layered with reinforcing material," Clark says. "What really makes it cool is that it's a cap sheet."
The technology can be used to top off an insulated roof, or it can be used as the primary roofing system. Some cap-sheet technologies require torching or hot-mopping the surface to seal it, and some modified bitumen products come with attached adhesive, like a big contact sheet. By eliminating the hot-mopping, the material can be installed without generating fumes.
Some products are fire- and wind-resistant, and with the right surfacing, this material can be a bona fide cool roof, as well.
This is a material that can be applied to virtually any type of existing roof -- even metal. It's either applied at the factory or on site. Generally, the products are bright white or contain reflective pigment. "Half of the solar energy coming in is the infrared light. That's energy we can't see," Clark says. "Those pigments can reflect the infrared."
The National Roof Coatings Institute reported that after the roofs on an entire city block in Philadelphia were coated with "highly reflective white coatings," the surrounding air temperature dropped by 1 degree in summer heat.
This type of roof is usually more of an industrial application. You start with a flat roof, then build it up with layers of asphalt and material coated with asphalt. It's cool because of the top layer, which can be made of something like white-colored gravel.
According to the National Roofing Contractors Association, this type of roof has been used in the United States for more than 100 years, and it's commonly known as a tar and gravel roof.
It's considered "built-up" because it's made up of at least four alternating layers of tar and reinforcing material such as glass fiber mats or organic mats. Because of the layering, this type of roof is very insulating, and with careful selection of materials, the roof can be a cool roof as well.
Seal up the roof, put in some planters, pick some hardy plants, and you've got a living roof. That's right -- plants on the roof.
It's a great concept, but it's not exactly cheap and easy to maintain. For one thing, you'll need a structural evaluation to assess how much load the building can take. Then you make decisions from there.
The National Association of Home Builders, in an evaluation of the roofs, determined that they do have thermal resistance and insulation properties. But you can't just plant it and forget it -- it needs constant maintenance, just like a garden. And whatever you do, don't try to grow an oak tree up there.
To read more articles on roofing and green building, check out the links on the next page.
Green Roofs And White Roofs: Low Tech Ways To Save Tons Of Energy. Keep reading to learn about Low Tech Ways To Save Tons Of Energy.
- Beacon Commercial Roofing. "Modified Bitumen." (May 3, 2011)http://www.beaconroofingsupply.com/commercial-roofing/modified-bitumen/default.html
- Clark, Jessica. Marketing liaison, Cool Roof Rating Council. Personal interview. April 22, 2011.
- Ludowici Roof Tile. (April 26, 2011)http://www.ludowici.com/color_choice/go_green
- NAHB Research Center. "Residential Green Roof Systems." (April 28, 2011)http://www.toolbase.org/Technology-Inventory/Roofs/green-roofs
- National Coatings. "Reflective Roof Coating." (May 3, 2011)http://www.nationalcoatings.com/Reflective_Roof_Coating.html
- National Roofing Contractors Association. "Built-up roof (BUR) membranes." (May 3, 2011)http://www.nrca.net/consumer/types/bur.aspx
- National Roofing Contractors Association. "Thermoplastic membranes." (April 28, 2011)http://www.nrca.net/consumer/types/thermop.aspx
- National Roofing Contractors Association. "Thermoset membranes." (April 28, 2011)http://www.nrca.net/consumer/types/thermos.aspx
- RSI Magazine. "Ballast and paver systems save as much as cool roofs." May 28, 2008. (May 3, 2011)http://www.azcoolroof.com/downloads/Resources/Ballasted%20System%20-%20Energy%20Efficient.pdf
- Single Ply Roofing Industry. "Single Ply Roofing in Perspective." (May 3, 2011)http://www.spri.org/perspective/