In the northern tier of the United States,
one might think that a black roof would provide a winter heating benefit to building owners. However, there are several factors that make any potential heat gain relatively immaterial:
Laws of Physics
The laws of physics dictate that hot air will always rise. Thus, any heat that is transferred to the interior of a building structure from the outside will remain at the top of the structure, providing minimal heat savings.
In all parts of North America, there are fewer hours of sunlight to affect energy costs. In fact, in some areas, there is a greater than a six-hour difference between peak-summer and peak-winter sunlight, meaning there is less sunlight available to contribute to a building's potential warming. Plus, the angle of the sun is less direct, which also helps to minimize warming potential.
In many areas, roofs are covered with snow for much of the winter, turning them "white" and eliminating any potential black roof heat gain.
Energy in the Summer
The energy required to air condition a building in the summer is usually considerably greater than the energy to heat it in the winter, making the potential for summer energy cost savings much greater with a highly reflective white roof than winter savings with a heat absorbing black roof.
Aside from energy savings, a white roof saves money on roof repairs. Since black roofs become overheated in the sun, they are prone to warping and cracking. This means more roof repair. A white roof saves money on roof repair because it is more durable. A white roof can also extend the life of a buildings cooling equipment by decreasing use.
Though NYC is a northern city, it's a huge Urban Heat Island. Therefore, it would still see a net gain from white roofs!