Did you ever wonder what to do with all the space your roof occupies? Chances are, there’s a lot of empty space up there that could be put to better use than reflecting the sun—assuming you used a white surface to reflect the heat back into the sky, rather than absorbing the heat and adding cooling to compensate.
Why not put a photovoltaic (PV) array up there and make some electricity? Not only could you use it for cooling and lighting, you could also use it to run equipment, and if there’s any power left over, sell it back to the electric company. But, you say it’s not practical because you’re in a northern climate.