The basic element in a roof is a thin, waterproof covering, called roofing (Sec. 12). Because it is thin, it is usually supported on sheathing, a thin layer, or roof deck, a thick layer, which in turn, is carried on structural members, such as beams or trusses. The roof or space below should contain thermal insulation (Fig. 1.6c and d). Exterior walls enclose a building below the roof. The basis element in the walls is a strong, durable, water-resistant facing. For added strength or lateral stability, this facing may be supplemented on the inner side by a backing or sheathing (Fig. 1.5b). For esthetic purposes, an interior facing usually is placed on the inner side of the backing. A layer of insulation should be incorporated in walls to resist passage of heat. Generally, walls may be built of unit masonry, panels, framing, or a combination of these materials. Unit masonry consists of small units, such as clay brick, concrete block, glass block, or clay tile, held together by a cement such as mortar. Figure 1.5a shows a wall built of concrete blocks. Panel walls consist of units much larger than unit masonry. Made of metal, concrete, glass, plastics, or preassembled bricks, a panel may extend from foun- dation to roof in single-story buildings, or from floor to floor or from window header in one story to window sill of floor above in multistory buildings. Large panels may incorporate one or more windows. Figure 1.5c shows a concrete panel

The disadvantages are: It is dependent on estimates from management of percentage of completion and cost to complete. Retained percentages and accounts receivable not yet billed are taken as income although they may not be collected until a much later date. It does not permit the deferral of income taxes, a procedure that may be available with the cash or completed-contract method. Responsibility for job safety rests initially with the superintendent. Various safety manuals are available giving recommended practice for all conceivable types of construction situations: for example, see Manual of Accident Prevention in Construction, Associated General Contractors of America, Inc., 1957 E St., NW,Washington, Because of wide diversity in state safety laws, the Federal government in 1970 passed the Occupational Safety and Health Act (OSHA) (Title 29Labor Code of Federal Regulations, Chapter XVII, Part 1926, U.S. Government Printing Office). Compared with state safety laws of the past. the Federal law had much stricter requirements. For example, in the past, a state agency had to take the contractor to court for illegal practices. In contrast, the Occupational Safety and Health Administration can impose fines on the spot for violations, despite the fact that inspectors ask the employers to correct their deficiencies. OSHA enforcement, however, may eventually be taken over by the states if they develop state regulations as strict as

Another method of lifting wall panels employs a vacuum mata large steel mat with a rubber gasket at its edges to contact the slab. When the air between mat and panel is pumped out, the mat adheres to the panel, because of the resulting vacuum, and can be used to raise the panel. The method has the advantage of spreading pickup forces over the mat area. Panels, when erected, must be temporarily braced until other construction is in place to provide required permanent bracing. (Tilt-Up Concrete Structures, ACI 551.R, American Concrete Institute.) Joints. Joint sealants for panel installations may be mastics or elastomeric materials. These are extensible and can accommodate the movement of panels. Recommended maximum joint widths and minimum expansions for the common sealants are listed in Table 9.27. The joint sealant manufacturer should be asked to advise on backup material for use with a sealant and which shape factor should be considered. A good backup material is a rod of sponge material with a minimum compression of 30%, such as foamed polyethylene, polystyrene, polyurethane, polyvinyl chloride, or synthetic