Get a little misty-eyed
If you'd rather enjoy the benefits of a cool roof without the addition of new surface materials or coatings, consider a roof mist cooling system. Also called an evaporative cooling system, a roof misting system lowers surface temperatures by spraying an extremely small amount of water across the roof, allowing the water to cool the roof as it evaporates.
At its lowest-tech, a roof misting system can be little more than a garden hose set to the "mist" setting and aimed across the roof. (The same method will also quickly and effectively cool your stone patio or wooden deck on a hot day.) Of course, more sophisticated roof mist cooling systems are much more practical for applications such as office buildings and industrial space, as well as for regular residential use. Using sensors to measure roof temperature and moisture, these systems can lower rooftop temperatures by as much as 40 to 60 degrees Fahrenheit (4.4 to 15.5 degrees Celsius) on a hot summer day, while consuming less than one pint (473 mililiters) of water per square foot per day [source: Abernathy]