I don't know if any of you are quite as crazy as I am and dream of someday owning a house with a green roof (and I mean an actual growing, living, planted, green roof), but if you're not quite ready to make that leap, then maybe your green roof could be white instead...
When the sun is shining and you're headed to the beach do you wear dark or light colors? Of course, you grab that light colored shirt because the dark one would be too hot. The light shirt is cooler because it has a high albedo, or reflectance. By reflecting the heat of the sun you stay cooler.
Your roof is no different. When the sun shines down on your roof it absorbs heat, and dark roofs absorb much more heat than light ones. I'm sure you've been in an asphalt parking lot in the summer time and noticed how hot it is. Many roofs are made of similar materials, and they get just as hot. These hot roofs clustered together in dense urban areas contribute to what's known as Urban Heat Island Effect (UHI), which is a warmer zone typically found around cities.
Now, the point of all this is that when your roof absorbs lots of heat energy, two things happen: First of all, the inside of your building heats up, and the air conditioning has to work harder to keep it cool. This uses more electricity, releases more green house gasses from power plants, and runs up your electricity bill. Not to mention, contributes to global warming. Second, by not reflecting heat, the building retains it, and we experience UHI. Now that the city is even warmer we need to run even more air conditioning, and we're back to the beginning. Are you starting to see a connected cycle?
The solution to this issue may be simpler that you think. Applying a white elastomeric roof coating, or installing white shingles will raise your albedo considerably, and provide a lasting solution. These are great options when it comes time to replace a roof, but until then, you can make a change right now for a whole lot less money. Get up there on that black roof with a couple buckets of exterior grade white paint. It will provide a temporary solution and reduce your immediate solar heat gain until it's time to make a more permanent fix for your roof.
Image via Flickr member Klearchos Kapoutsis licensed under Creative Commons.