Dear Jancy:  The greatest number of unused 'billboards' in any city are blank rooftops. If you were to draw a 5 mile radius around any airport you would clearly be in view of the surrounding rooftops. This means thousands of people every day, in every major city, look out airplane windows at blank billboards! In a time when every conceivable space is converted to profit (including above the urinals in some restrooms!), hockey boards, stair risers, etc. I wonder why this possibility has not been explored.
It seems to me these ads could either be a deluxe permanent display for a given product (eg Pepsi) or a shorter term project for special events (eg The Royal Winter Fair). The concept would be to lease the roof from the building owner and sell the space to advertisers. The options could include either nighttime illumination or a more limited and less expensive unlit daytime exposure only. In this day of heated roadways and solar panels, the challenge of melting the elements can be met and if the ads were elevated (say 12 inches above the surface) they would overcome such interference.

On some occasions (eg Atlanta) I have noticed individual businesses that have painted their names on their roofs. in such instances there is a clear 'message to the sky' that is clearly communicated.

Anyway, I am curious to hear your reply. If this is doorknob thinking I know you will give it to me straight!!!! On the other hand, who could be our first customer? Mississauga is loaded with flat roof buildings!