Custom-built homes take roughly nine months to build, while tailored production designs take about seven months. Custom builders must estimate construction timelines on an as-needed basis and may find themselves waiting for materials to arrive at their jobsite or for the weather to clear in order to begin work. Townhouses are generally built in less time than single-family homes. It typically takes four to six weeks to build a townhouse, depending on the size. The majority of this time is spent building the interior walls and ceilings. The exterior walls can be constructed in one to two months, depending on the type used. Frame houses are built with wood frames that are then covered with sheathing and siding to create an insulated wall. Concrete block is poured into forms and stacked to create the exterior walls of a townhouse made from this material. Metal buildings come in several different shapes and sizes; however, they all include some form of sheet metal on which the roof is built. Attached garage townhouses are built in much the same way as main dwellings. However, because there is no basement, the structure must be framed above ground level. Openings such as windows and doors are cut into each floor section, which are then lifted into place by a crane.