The most essential feature of any roof is dependability since it must keep the weather out. Roofs should also provide a barrier against fire, protect the house below from excess heat, and add aesthetic appeal.
But a truly green roof can be much more than just a shelter. Truly green roofs can collect water for gardens. They can supply the energy for your electricity needs. They can even cool cities and provide a habitat for plants and animals…
Let’s start with tips for more conventional roofing, since not every roof will suit every climate. If you have greater cooling than heating needs, you’ll want to make sure your roof is a light color to reflect the sun’s heat and reduce the energy costs of air conditioning. Correctly installed roof vents will ensure good air circulation, critical to reducing heat build-up and eliminating interior moisture problems.
Roofing is often heavy: always consider the environmental costs in addition to the monetary costs of shipping long distance, and purchase locally produced materials if you can. Six other sustainability considerations for commonly used roofing materials include:
- Durability—The longer it lasts, the less roofing demolition waste and new resource use. Reroofing accounts for about ¾’s of roofing in the U.S.
- Weight—Heavier materials require a stronger underlying structure, which usually means increased use of resources.
- Appropriateness for rainwater collection—Does it leach toxic products?
- Appropriateness for adding photovoltaic cells and other solar collection equipment.
- Resource cost of manufacture—How much energy is required? What about other environmental costs such as toxic byproducts? Is there recycled content?
- Waste-stream issues—Can the material be reused or recycled after its lifespan is up?
Residential Roofing Options
15-year composition asphalt shingles: Short-lived asphalt shingles are one of the least-green roofing choices. They consist of an asphalt-impregnated cellulose-fiber base surfaced with mineral granules. While some are made with some recycled content, they also utilize petroleum products, and the roofs themselves are rarely recycled. Because of their short lifespan and their popularity due to lower pricing, shingles and shakes make up a significant portion of all building material waste.
Toxic VOC’s off-gas from asphalt in the sun, and toxic chemicals leach from it in the rain. Because the base underneath is black, even light-colored composition shingles offer poor heat reflectance compared to other light-colored materials.
50-year composition asphalt shingles or shakes: Although they have the decided advantage of being on your roof so very much longer than their inexpensive relations, they share all of the other environmental disadvantages.
Recycled synthetic shingles: These are made from plastic or rubber, mixed with wood waste (much the same formula used for composition decking), and are shaped to mimic wood shakes. Lighter-weight than many alternatives, they use otherwise waste materials, and are UV-resistant, fire-resistant, and long-lasting. Some are comparable in lifespan to 50-year asphalt shingles. Though they generally cannot be recycled, due to their inseparable mixture of biological and plastic content, recycled synthetic shingles are still a greener material than real cedar shakes.
Cedar shakes: Wood makes a poor roofing material in most circumstances. Timber shakes are biodegradable and beautiful, but they are so flammable they are illegal in some fireprone areas. Cedar or oak makes a short-lived roofing material that has to be replaced regularly. The best shakes are produced from old-growth cedar, a non-renewable resource.
Concrete or cement tile has a relatively long track record, as it was first made in the 19th century. Both concrete and cement tiles are durable, and fire and insect proof. Inert and stable, they don’t leach or off-gas. However, tiles yield one of the heaviest roofs, have some significant environmental costs in manufacture, and are costly to ship. Both concrete and cement tiles must be specially formulated if they are to resist breaking down in freeze-thaw cycles of cold climates.
Fiber-cement composite tile is composed of concrete, clay, and wood fiber. This mixture, both durable and fireproof, is often formed to look like shakes. Fiber-cement tiles are not as heavy as regular concrete tiles so they don’t need extra-heavy roof structures. While they don’t perform well in northern climates, and you can’t walk on them, these composite tiles can be recycled, are non-leaching, and make good water-collection roofs.
Clay tile can be as low-energy-input as the old Spanish mission tile made from clay dug on site, curve-formed over the thighs of the workers and low-fired in wood-burning kilns. Modern clay tiles often have cement added and/or are high-temperature fired for strength. They can be glazed or unglazed. Like cement tiles they are available in a wide range of colors, with the lighter colors having greater capacity for solar reflectance.
Clay tiles are very heavy, requiring beefed-up roof framing, but they are non-combustible and extremely durable. They are non-leaching, and make a good base for water collection. Certain styles of tile roofing have better air circulation than most other roof types, which increases their cooling capability. If a local source is available, clay tiles are definitely one of the greenest roofing choices.
Slate tile: If there is a source of slate in your area, this can be an excellent sustainable roofing choice. Quarrying and splitting slate tile has little environmental impact compared to the production of other roofing products. Although very heavy, a slate roof is non-leaching, will last for hundreds of years, is easily repairable, and easily recyclable as there is a thriving demand for used slate tiles. Because slate is often a dark color it isn’t recommended for high-heat locations.
Metal roofing appears to have a lot going for it in terms of sustainability, but there are some serious downsides to consider. Steel, copper, and aluminum roofing are durable materials, fireproof, and lightweight, making them an especially good choice in fire prone areas and in earthquake zones where heavy roofs are a liability. Its light weight also reduces how much gas it takes to ship it to you.
Metal roofing comes in large pieces and is easy to install. It usually contains a large percentage of recycled material, and can be recycled indefinitely, as these metals are intensively recycled. Since the mining and smelting of metals has a very high environmental
cost, it’s especially important to ascertain the percentage of recycled
metal used. Metal roofing can be painted a light color for solar reflectance.
While this type of roofing is excellent for rain catchment, debates swirl about the cleanliness of rooftop runoff and the impact of this stormwater drainage on surrounding ecosystems, especially concerning roofing materials that include copper and galvanized metal (steel with a coat of zinc oxide to guard against corrosion).
Watershed protection groups and agencies monitor the impact of stormwater pollution by conducting various studies. The research varies widely in terms of toxicity levels present in the water, which is measured during various cycles, or “flushes”. This discrepancy aside, many studies support the conclusion that copper and galvanized roofing materials are, in large part, responsible for the alarmingly high levels of zinc and other metals present in the urban landscapes.
Flat Roof Systems: Flat or nearly flat roofs are waterproofed in various way. Roll asphalt, single-ply membranes, and multiple-ply (also called Built-Up Roofing or BUR) are the most common. If you need to waterproof a flat roof, EPDM (ethylene propylene diene monomer) rubber is probably the best commonly available system. Unlike asphalt-based systems, it doesn’t release odors and fumes during installation.
New Green Roofing Ideas
Even if you don’t want solar tiles, photovoltaic systems are worth considering. They can provide a silent, non-polluting, infinitely renewable source of energy to anyone with an unshaded south-facing roof. Although PVs can be used to charge batteries for use during those dark and rainy times in an off-grid system, many PV users stay on-grid and use that system as their back-up electricity source. Without government incentives, photovoltaic systems, including solar roof shingles, can be expensive—about twice as expensive as grid electricity.
The Department of Energy maintains a website that details whether or not incentives apply to your area. The good news is that, with new technologies and better manufacturing techniques, prices are currently falling by about 5.5 percent annually. It is predicted that PV systems will become fully competitive with utility power within a decade or so.
Green Roofs are vegetated roof covers constructed on top of a roof deck. Sometimes called an ecoroof or sky garden, this burgeoning building technique is the ultimate in greenness and sustainability.
There are two basic types: extensive and intensive. The extensive green roof is a self-sustaining biotic community, colonizing the exposed, dry, and shallow-soil environment of a roof. They are seeded with alpines, succulents, herbs, and grasses appropriate to the climate, and left nearly alone to develop naturally. They are not usually accessible for use.
The intensive green roof is a more involved and expensive affair with deeper soil which can accommodate larger plants, even trees. They are designed for recreation, and differ from roof gardens in that the whole surface of the roof is covered with soil and plantings; it is not just containers of plants.
Green roofs have become increasingly popular, especially in commercial urban architecture, because they ameliorate loss of greenspace, absorb carbon dioxide, reduce building heat gain, and recreate natural habitat. They are particularly remarkable in their ability to filter and control stormwater runoff: green roofs can absorb anywhere from 50 to 95% of the rainwater that falls on them.
Green roofs offer one big piece of the solution to Urban Heat Island syndrome. Modern cities, with their huge production of waste heat, and replacement of cooling, sun-reflecting vegetation with heat-absorbing asphalt and concrete, are hot spots on the planet’s surface which even create their own unnatural weather. Perhaps one day our city roofs viewed from the air won’t look like geometric scars on the landscape, but patterns of waving greenery.