The fire's spread was aided by the city's use of wood as the predominant building material in a style called balloon frame. More than two-thirds of the structures in Chicago at the time of the fire were made entirely of wood, with most of the houses and buildings being topped with highly flammable tar or shingle roofs. All of the city's sidewalks and many roads were also made of wood. Compounding this problem, Chicago received only of rain from July 4 to October 9, causing severe drought conditions before the fire, while strong southwest winds helped to carry flying embers toward the heart of the city.