If an asphalt (ie, flammable) roof is a must, then might look into roof sprinkler systems. They can be as simple or complicated as desired, and can be home-made as well as the commercially-available systems. They can also be laid out to cover not just the roof, but to help create somewhat of a perimeter around the entire house as well. If danger of freeze is a concern, they can have a simple, ground-accessible drain valve to empty the lines in the winter.
A side benefit is that on hot summer afternoons, you can run the sprinklers for even just a few minutes, drastically reducing the temperature of your roof and positively impacting your cooling bills. We don't have a roof sprinkler largely because even though our roof is shingle (for cosmetic & 'wife' reasons), the house structure it sets on is steel. So while the roof could possibly burn off, the structure won't burn down. In the summer, I'll sometimes go out with a hose and spray the roof. On a hot, sunny day, it will take literally 2-3 minutes before any water runs off; initially it steams off, the roof gets so hot. And when the water does finally start to run off, it's hotter than what comes out of the water heater - literally scalding. Just a few minutes of water-spraying removes a HUGE number of btu's.