The load is transported from a slab to the beams, then to the columns, then to lower columns, and eventually to the foundation, which transmits it to the earth in an RCC-framed building. The walls of such constructions are built after the frame is complete. The frame is first covered with sheathing and then with panels or boards. The floor may be made of wood planks or tiles. In steel frame construction, the structural members of the frame are often exposed during construction of the building interior. They may be used as internal support for floors, ceilings, or walls. They may also be left exposed as part of the intended design appearance. In either case, these members must be well-supported by other components of the building envelope if they are to serve their purpose effectively. In concrete frame construction, the exterior wall is usually the primary source of support for the roof. The roof is built on top of the wall, and so provides an additional load-bearing surface. However, because the roof is not tied directly into the floor system, care must be taken to provide sufficient support at the perimeter of the building to prevent sagging. In timber frame construction, the main method of supporting the roof is through the use of purlins attached to the front of the house under which rafters protrude. The ends of the rafters are then joined to the side walls using long horizontal trusses called girts.