Cool roofs are the roofs that can deliver high solar reflectance (the ability to reflect the visible, infrared and ultraviolet wavelengths of the sun, reducing heat transfer to the building) and high thermal emittance (the ability to radiate absorbed, or non-reflected solar energy). The benefits associated with cool roofs include reduced cooling energy load, reduced air pollution and greenhouse gas emission, and improved human health and comfort. Cool roofs may extend the roof service life and help mitigating the urban heat island effect. Cool roofs achieve cooling energy savings in hot summers but can increase heating energy load during cold winters. Therefore, the net energy saving of cool roofs varies depending on climate. However, a 2010 energy efficiency study  looking at this issue for air conditioned commercial buildings across the USA found that the summer cooling savings typically outweigh the winter heating penalty even in cold climates near the Canadian border giving savings in both electricity and emissions. Without a proper maintenance program
to keep the material clean, the energy savings of cool roofs can diminish over time due to albedo degradation and soiling.
While it is true that cool roofs are mostly associated with white roofs, they come in a variety of colors and materials and are available for both commercial and residential buildings. Note that today’s “cool roof” pigments allow metal roofing products to be EnergyStar rated in dark colors, even black. They aren’t as reflective as whites or light colors, but can still save energy over other paints.
Most of the roofs in the world (including over 90% of the roofs in the United States) are dark-colored. In the heat of the full sun, the surface of a black roof can increase in temperature as much as 50 °C (126 °F), reaching temperatures of 70 to 90 °C (158 to 194 °F). This heat increase can cause negative effects on cooling energy use and environments.
Cool roofs, on the other hand, offer both immediate and long-term benefits including:
- Reduced building heat-gain, as a white or reflective roof typically increases only 5–14 °C (10–25 °F) above ambient temperature during the day
- Savings of up to 15% the annual air-conditioning energy use of a single-story building
- Extended service life of roofs
- Improved energy efficiency of roofs, especially when there isn’t adequate insulation provided in the roof envelope
- Improved thermal comfort in buildings that do not have air conditioning
- Reduced air pollution and greenhouse gas emissions, as well as a significant offsetting of the warming impact of greenhouse gas emissions.
Research and practical experience with the degradation of roofing membranes over a number of years have shown that heat from the sun is one of the most potent factors that affects durability. High temperatures and large variations; seasonally or daily, at the roofing level are detrimental to the longevity of roof membranes. Reducing the extremes of temperature change will reduce the incidence of damage to membrane systems. Covering membranes with materials that reflect ultraviolet and infrared radiation will reduce damage caused by u/v and heat degradation. White surfaces reflect more than half of the radiation that reaches them, while black surfaces absorb almost all. White or white coated roofing membranes, or white gravel cover would appear to be the best approach to control these problems where membranes must be left exposed to solar radiation.
If all urban, flat roofs in warm climates were whitened, the resulting 10% increase in global reflectivity would offset the warming effect of 24 Gigatonnes of greenhouse gas emissions, or equivalent to taking 300 million cars off the road for 20 years. This is based on the fact that a 1,000-square-foot (93 m2) white roof will offset 10 tons of carbon dioxide over its 20 year lifetime. In a real-world 2008 case study  of large-scale cooling from increased reflectivity, it was found that the Province of Almeria, Southern Spain, has cooled 1.6ºC over a period of 20 years compared to surrounding regions, as a result of polythene-covered greenhouses being installed over a vast area that was previously open desert. In the summer the farmers whitewash these roofs to cool their plants down.
When sunlight falls on a white roof much of it is reflected and passes back through the atmosphere into space. But when sunlight falls on a dark roof most of it is absorbed and converted into much longer wavelengths that we call “heat” which cannot pass back through the atmosphere because they are absorbed by the greenhouse gases. The atmosphere is transparent to sunlight but opaque to heat, which is why white roofs help cool the planet and dark roofs warm the planet.