Rooms within a single-wide are often joined rather than divided by a corridor, as in a railroad apartment or shotgun-style home. If the single-wide includes a corridor, it is usually small to match the narrow orientation of the home. Sometimes these homes are called "one-and-a-half stories" because there is no lower floor except for the basement or ground level. Single-wide trailers are especially common in rural areas where townhouses and other multi-unit dwellings are not feasible due to land costs or planning regulations. They are also used in urban neighborhoods as affordable housing. Because they are so easy to build, if proper care is taken during construction none of these homes will have structural problems when they come off the production line. After the frame is up, the walls and roof are covered with siding or panels and then a metal roof is placed on top. The floor is usually made of wood planks although concrete slabs are becoming more common now. There is only one window per room which allows for cheap manufacturing. Also, there is only one door which can be opened and closed instead of many doors on separate frames that require locking or closing automatically when you leave a room. This makes single-wides easy to burglarize but easy to defend against intruders as well.