Too much heat makes the roof go up in flames

Two weeks ago, I was coming back from the mailbox one morning as I was about to go to work. I noticed an unusually large number of people milling about the building to the south of mine. Then I noticed there was an alarm sounding. Then I noticed there were fire truck sirens off in the distance. Then I recalled there had been a fire in that building just a few years ago, and I broke into a sprint through the gangway to the back yard.

Were those wisps of smoke rising from the roof? It only took a few seconds for the tiniest signs of smoke to develop into impressive flames. Then, when the fire truck came, it only took a few seconds to douse them. The fire was apparently confined to the roof and was started by roofers heating tar.