Most British houses are made of brick and cement. In a row of terrace houses (houses joined together), the interconnecting walls are cavity walls: they have a space between them to allow air to circulate. On the interiors, the walls are covered in plaster, and then either painted or decorated with wallpaper. The internal walls of a house fall into two categories: load-bearing walls (those that are structural and support the weight of the floors) andpartition walls (those walls that divide rooms, but can be knocked down.) Floors and roofs are supported bybeams, which are long, heavy pieces of wood or metal.