5/8-inch-thick sheets perform better because of their greater thickness and strength. Standard siding sheets are 4 by 8 feet; larger sizes are available. They must be applied vertically with intermediate  and  perimeter  nailing  to  provide  the desired  rigidity.  Most  other  methods  of  applying  sheet materials  require  some  type  of  sheathing  beneath. Where  horizontal  joints  are  necessary,  they  should  be protected by simple flashing. An  exterior-grade  plywood  should  always  be  used for siding and can be obtained in grooved, brushed, and saw-textured  surfaces.  These  surfaces  are  usually finished with stain. If shiplap or matched edges are not provided, the joints should be waterproofed. Water- proofing often consists of caulking and a batten at each joint and a batten at each stud if closer spacing is desired for  appearance.  An  edge  treatment  of  water-repellent preservative  will  also  aid  in  reducing  moisture penetration.  When  plywood  is  being  installed  in  sheet form, allow a 1/16-inch edge and end spacing. Exterior-grade  particle  board  might  also  be considered  for  panel  siding.  Normally,  a  5/8-inch thickness  is  required  for  16-inch  stud  spacing  and 3/4-inch thickness for 24-inch stud spacing. The finish must  be  an  approved  paint,  and  the  stud  wall  behind must  have  corner  bracing. Medium-density fiberboards might also be used in some  areas  as  exterior  coverings  over  certain  types  of sheathing. Many of these sheet materials resist the passage of water vapor. Hence, when they are used, it is important that a good vapor barrier, well insulated, be used on the warm side of the insulated walls. NONWOOD SIDING Nonwood  materials  are  used  in  some  types  of architectural design. Stucco or a cement-plaster finish, preferably over a wire mesh base, is common in the Southwest and the West Coast areas. Masonry veneers can be used effectively with wood siding in various finishes to enhance the beauty of both materials. Some  structures  require  an  exterior  covering  with minimum  maintenance.  Although  nonwood  materials are  often  chosen  for  this  reason,  the  paint  industry  is providing  comparable  long-life  coatings  for  wood-base materials. Plastic films on wood siding and plywood are also  promising  because  little  or  no  refinishing  is necessary for the life of the building. Installation Siding can be installed only after the window and doorframes are installed. In order to present a uniform appearance, the siding must line up properly with the drip caps and the bottom of the window and door sills. At the same time, it must lineup at the corners. Siding must be properly lapped to increase wind resistance and watertightness. In addition, it must be installed with the proper nails and in the correct nailing sequence. Fasteners One of the most important factors in the successful performance  of  various  siding  materials  is  the  type  of fasteners used. Nails are the most common, and it is poor economy  to  use  them  sparingly.  Galvanized,  aluminum, and stainless steel corrosive-resistant nails may cost more,  but  their  use  will  ensure  spot-free  siding  under adverse conditions. Ordinary steel-wire nails should not be used to attach siding since they tend to rust in a short time and stain the face of the siding. In some cases, the small-head  rails  will  show  rust  spots  through  the  putty and paint. Noncorrosive nails that will not cause rust are readily  available. Two types of nails are commonly used with siding: the small-head finishing nail and the moderate-size flathead siding roil. The small-head finishing nail is set (driven with a nail set) about 1/16 inch below the face of the siding, The hole is filled with putty after the prime coat of paint has been applied. The more commonly used flathead siding nail is nailed flush with the face of the siding and the head later covered with paint. If  the  siding  is  to  be  natural  finished  with  a water-repellent  preservative  or  stain,  it  should  be fastened with stainless steel or aluminum nails. In some types  of  prefinished  sidings,  nails  with  color-matched heads  are  supplied. Nails with modified shanks are available. These include the annularly (ring) threaded shank nail and the spirally  threaded  shank  nail.  Both  have  greater withdrawal resistance than the smooth-shank nail, and, for this reason, a shorter nail is often used. In siding, exposed nails should be driven flush with the surface of the wood Overdriving may not only show the hammer mark, but may also cause objectionable splitting and crushing of the wood. In sidings with prefinished surfaces or overlays, the nails should driven so as not to damage the finished surface. be 4-3