Traditionally, houses are built with brick, stone and concrete block work walls, supporting timber suspended floors and a timber roof structure. Mortgage companies tend to prefer traditional forms of construction, as they are well documented and their performance is understood. System built houses were introduced after World War II, to speed construction and produce houses in large numbers. These systems included steel framed houses and pre-cast concrete panel houses. Most were associated with social housing. Often, Lenders are reluctant to provide mortgages on such houses, unless a structural engineer inspects the property and provides the required reassurance of future integrity.