A cool roof is a roof which is designed to stay cool in hot weather, rather than absorbing heat and growing warmer as the day progresses. Cool roofs are useful for a number of reasons, ranging from building efficiency to durability, and they are especially common in tropical and subtropical areas, along with desert regions. Residents of temperate climates can also benefit from using cool roofs in building construction.
Two mechanisms are at work in a cool roof. The first is that the roof is made or coated with a highly reflective material which allows the sun's energy to bounce off the roof, rather than allowing the roof to absorb it. The second is that cool roofs are able to release tremendous amounts of heat, so even when the sun's energy does penetrate the roof, it can't stay and heat up the roof. White and other pale colors are classic for cool roof construction, because they are highly reflective, and usually the surface of the roof is cool enough to touch comfortably even in the hottest weather. Vegetated roofs can also double as cool roofs by providing a layer of insulation and trapping the sun's energy.
For the building itself, a cool roof is very beneficial. Cool roofs are more durable in the long term because they break down and wear more slowly in hot weather than roofs made from other materials. Having a cool roof can increase the sale value of a property, for example, as potential buyers will know that the roof will not need to be replaced in the immediate future. Cool roofs also cut down on cooling costs, and they can eliminate the need for insulation in hot climates, making a building less costly to construct.
Cool roofs also benefit their surrounding communities. In warm climates, urban areas tend to create what is known as a heat island, in which the temperature goes up radically because there are so many substances which absorb and hold heat, like roofing and paving materials. Cool roofs can offset this effect, keeping a city cooler and making it more comfortable to live in in addition to reducing pollution.
Existing roofs can be converted to cool roofs with a membrane or skin, which makes the retrofit less expensive since the roof will not need to be stripped and reinstalled. It is also possible to use cool roofing which has been designed to be architecturally appropriate for historic homes, allowing people to retain the character of older homes without needing to sacrifice energy efficiency. New homes can also be constructed with cool roofs installed from the start, a decision which is often made for green housing and structures in hot climates.