Elevated storage containers for drinking water are called water towers. Generally constructed in a spherical or cylindrical shape, water towers are built at a height that will allow for sufficient water pressure through gravity flow. In the US, many of the towers that were built during the Industrial Revolution have now been converted to apartments and penthouses. In New York City during the 19th century, building codes required that any structure over six stories high must be equipped with a rooftop water tower. A variety of materials can be used to construct water towers, including steel, concrete, brick, and wood.