have a high surface roughness. To decrease this, other granule materials are being investigated, such as flat rock flakes, which could reduce the reflectance inefficiencies due to surface roughness. Another alternative is to coat the granules using a dual coat process: the outer coating would have the desired color pigment, though it may not be very reflective, while the inner coating is a highly reflective titanium dioxide coating. Natural white gravel covering can be seen as an alternative option to obtain cool roofing and cool pavements. The highest SRI rating, and the coolest roofs, are stainless steel roofs, which are just several degrees above ambient under medium wind conditions. Their SRI's range from 100 to 115. Some are also hydrophobic so they stay very clean and maintain their original SRI even in polluted environments. [A] Coated roofs An existing (or new) roof can be made reflective by applying a solar reflective coating to its surface. The reflectivity and emissivity ratings for over 500 reflective coatings can be found in the Cool Roofs Rating Council. Blue and red roofs Researchers at the Lawrence Berkeley National Laboratory have determined that a pigment used by the ancient Egyptians known as "Egyptian blue" absorbs visible light, and emits light in the near-infrared range. It may be useful in construction materials to keep roofs and walls cool. They have also developed fluorescent ruby red coatings which have reflective properties similar to white roofs. Green roofs Green roofs provide a thermal mass layer which helps reduce the flow of heat into a building. The solar reflectance of green roofs varies depending on the plant types (generally 0.3–0.5). Green roofs may not reflect as much as a cool roof but do have other benefits such as evapotranspiration which cools the plants and the immediate area around the plants, aiding in lowering rooftop temperatures but increasing humidity, naturally. Moreover, some Green roofs need maintenance such as regular watering. Disadvantages A 2011 study by researchers at Stanford University suggested that although reflective roofs decrease temperatures in buildings and mitigate the "urban heat island effect", they may actually increase global temperature. The study noted that it did not account for the reduction in greenhouse gas emissions that results from building energy conservation (annual cooling energy savings less annual heating energy penalty) associated with cool roofs (meaning that one will need to use more energy to heat the living space due