However, two major factors in the Chicago fire were relatively new. First, a significant fraction of the streets were paved -- the pavement being pine blocks. Second, construction had been with the relatively new balloon-frame technique. That technique was quick and did not require skilled carpenters -- but during a fire, the air space between studs of a balloon frame building creates a "flue" effect for the fire. (From this experience, builders learned the importance of putting fire-stops within the walls.)