reduced. Disadvantages Temperature Steel conducts heat very well; containers used for human occupancy in an environment with extreme temperature variations will normally have to be better insulated than most brick, block, or wood structures. Lack of flexibility Although shipping containers can be combined to create bigger spaces, creating spaces different from their default size (either 20 or 40 foot) is expensive and time-consuming. Containers any longer than 40 feet will be difficult to navigate in some residential areas. Humidity As noted above, single wall steel conducts heat. In temperate climates, moist interior air condenses against the steel, becoming humid. Rust will form, unless the steel is well sealed and insulated. Construction site The size and weight of the containers will, in most cases, require them to be placed by a crane or forklift. Traditional brick, block, and lumber construction materials can often be moved by hand, even to upper stories. Building permits The use of steel for construction, while prevalent in industrial construction, is not widely used for residential structures. Obtaining building permits may be troublesome in some regions, due to municipalities not having seen this application before. However, in the United States, certain shipping container homes have been built in areas outside of the city's zoning code; this meant no building permits were required. Treatment of timber floors To meet Australian government quarantine requirements, most container floors, when manufactured, are treated with insecticides containing copper (23–25%), chromium (38–45%) and arsenic (30–37%). Chromium and arsenic are known carcinogens. Before human habitation, floors should be removed and safely disposed of. Units with steel floors would be preferable, if available. Cargo spillages A container can carry a wide variety of cargo during its working life. Spillages or contamination may have occurred on the inside surfaces, and will have to be cleaned before habitation. Ideally, all internal surfaces must be abrasive blasted to bare metal, and re-painted with a nontoxic paint system. Solvents Solvents released from paint, and sealants used in manufacture, might be harmful to human health. Damage While in service, containers are damaged by friction, handling collisions, and force of heavy loads overhead during ship transits. The companies will inspect containers, and condemn them if there are cracked welds, twisted frames, or pin holes are found, among other faults. Roof weaknesses Although the two ends of a container are extremely strong, the roof is not. In the case of a 20'