While Bard was distracted with talking to Bain, Smaug swooped down and smashed through the roof of the belltower with his talons and his tail. It collapsed on top of Bard and Bain, knocking the boy off the edge. 

Meanwhile in the boat, Mel and Saeril had to steer carefully through, trying to head for the open water while avoiding all the burning debris that fell from the houses above them.