Historically, copper has served as the ultimate roofing material of choice. Unsurpassed in durability, beauty and value, copper is the only roofing material that actually improves with age. This "aging", or patina process, not only enhances its appearance, it also protects the metal from further oxidation. Copper's longevity is legendary. Almost all of the copper produced throughout time is still in use. And, in addition to being 100% recyclable, most architectural copper produced today has a recycled content of nearly 75%. Thus, not only is copper the best choice to protect a structure, it's also the best choice to protect the environment. The only drawback to using copper is the initial cost involved.
There are a variety of choices when it comes to metal roofing. Material choices range from galvanized steel to zinc to aluminum. The seaming or type of roofing is also another consideration. Metal roofs can be found in a standing seam installation or made to resemble
wood shakes, clay tiles, shingles, and Victorian metal tiles.
Standing-seam metal roofs are used with many types of high-pitched expanses of roof that can be enhanced with strong definition of the architectural lines. Standing-seam roofs are also very prevalent in mountainous regions where the need to shed snow is important. In addition, standing-seam installation is sometimes easier than other roofing styles.
Roofing tiles can be ceramic (e.g., clay fired at a high temperature) or fabricated from cement concrete. Some of the lighter types use fibers (e.g., cellulose) added for strength. The color of a tile may be dispersed throughout, or it may be applied in the form of a coating. Perhaps the most venerable type of roof tile is the Spanish style red barrel tile made from fired clay. The modern version of this tile is sometimes a cement tile with a suitable coating. In either case, the red color is due to the ubiquitous iron oxide material, hematite.
Roofing tiles are available in a wide range of colors; more data on the solar reflectance properties is needed. A starting point for the estimation of the solar reflectance would be to use the reflectance of a paint coating of similar color.
Advantages of tile include fire safety, as they are non-combustible, and durability. Disadvantages include increased weight and cost compared with low-cost asphalt shingle roofs.
Tile roofs often have enhanced air circulation compared to other roofing types because ambient air can circulate below as well as above the tile. (Wood roofs also have provision for air circulation below the roof, to make sure they always remain dry.) This enhanced air circulation helps the roof shed solar heat more readily. The temperature rise figures in the table below are conservative in the sense that we have not attempted to account for this uncertain benefit of enhanced convection.
Asphalt shingles are a very economic roofing choice, which have a large share of the market, including most houses with sloping roofs. From a reflectance point of view, they are very similar to rolled asphalt roofing, which is often used as the top layer of low slope roofing. These materials are composed of asphalt saturated mats made from organic felts or fiberglass. The asphalt is protected from the sun's uv light by roofing granules pressed into the shingle while it is hot (and soft). The roofing granules are 1 millimeter-sized stones (e.g., of crushed granite), which are coated with an inorganic silicate material. The coating contains microscopic pigment particles, similar to those used in paint, to provide color.
The solar reflectance of all commercial asphalt shingles is rather low (see table). Premium white shingles are only about 30% reflective, and other colors reflect less. (Incidentally, white roofing materials usually contain additive to inhibit biological growth, to avoid roof discoloration.) The low solar reflectance can be attributed to several factors. First, there is a limited amount of pigment in the granule coating. Also, the roughness of the shingle contributes to multiple scattering of light and thus to increased absorption. Finally, the black asphalt substrate is not 100% covered, and reflects only about 5% of the light which strikes it. ISP Minerals of Hagerstown, MD has developed a whiter white roofing granule which can be fabricated into shingles with reflectance exceeding 50%. It is hoped that this granule will become available commercially.