During cast iron's heyday, many architects thought it to be structurally more sound than steel. It was also thought that cast iron would be fire-resistant, and facades were constructed over many interiors built of wood and other flammable materials. When exposed to heat, cast iron buckled, and later cracked under the cold water used to extinguish fire. In 1899, a building code mandating the backing of cast-iron fronts with masonry was passed. Most of the buildings that stand today are constructed in this way. It was the advent of steel as a major construction material that brought an end to the cast iron era.