Types MR (moisture resistant) or WR (water resistant) board  is  also  called  greenboard  and  blueboard.  Being water resist ant, this board is appropriate for bathrooms, laundries,  and  similar  areas  with  high  moisture.  It  also provides a suitable base for embedding tiles in mastic. MR or WR board is commonly 1/2 inch thick. Sound-deadening board is a sublayer used with other layers of drywall (usually type X); this board is often 1/4 inch thick Backing board has a gray paper lining on both sides. It is used as a base sheet on multilayer applications. Backing board is not suited for finishing and decorating. Foil-backed  board  serves  as  a  vapor  barrier  on exterior  walls.  This  board  is  available  in  various thicknesses. Vinyl-surfaced board is available in a variety of colors. It is attached with special drywall finish nails and is  left  exposed  with  no  joint  treatment. Plasterboard or gypsum lath is used for plaster base. It is available in thickness starting at 3/8 inch, widths 16 and 24 inches, and length is usually 48 inches. Because it comes in manageable sizes, it’s widely used as a plaster base instead of metal or wood lath for both new construction  and  renovation.  This  material  is  not compatible  with  portland  cement  plaster. The varying lengths of drywall allow you to lay out sheets so that the number of seams is kept to a minimum, End points can be a problem, however, since the ends of the sheets aren’t shaped (only the sides are). As sheet length  increases,  so  does  weight,  unwieldiness,  and  the need for helpers. Standard lengths are 8, 9, 10, 12, and 14  feet.  Sixteen-foot  lengths  are  also  available.  Use  the thickness that is right for the job. One-half-inch drywall is the dimension most commonly used. That thickness, which is more than adequate for studs 16 inches on center (OC), is also considered adequate where studs are 24 inches OC. Where ceiling joists are 16 inches OC, use  1/2-inch  drywall,  whether  it  runs  parallel  or perpendicular  to  joists.  Where  ceiling  joists  are  24 inches OC, though, use 1/2-inch drywall only if the sheets are perpendicular to joists. Drywall of 1/4- and 3/8-inch thicknesses is used effectively  in  renovation  to  cover  existing  finish  walls with minor irregularities. Neither is adequate as a single layer  for  walls  or  ceiling,  however.  Two  1/4-inch-thick plies are also used to wrap curving walls. Drywall of 5/8-inch thickness is favored for quality single-layer  walls,  especially  where  studs  are  24  inches OC. Use 5/8-inch drywall for ceiling joists 24 inches OC, where sheets run parallel to joists. This thickness is widely  used  in  multiple,  fire-resistant  combinations. There are several types of edging in common use. Tapered allows joint tape to be bedded and built up to a flat  surface.  This  is  the  most  common  edge  used. Tapered round is a variation on the first type. Tapered round edges allow better joints. These edges are more easily damaged, however. Square makes an acceptable exposed  edge.  Beveled  has  an  edge  that,  when  left untapped,  gives  a  paneled  look. Tools Commonly   used   tools   in   drywall   application include a tape measure, chalk line, level, utility and drywall knives, straightedge, and a 48-inch T square (drywall  square)  or  framing  square.  Other  basic  tools include a keyhole saw, drywall hammer (or convex head hammer),  screw  gun,  drywall  trowel,  comer  trowel,  and a foot lift. Some of these tools are shown in figure 5-2. The tape measure, chalk line, and level are used for layout  work.  The  utility  and  drywall  knives, straightedge, and squares are used for scoring and breaking drywall. The keyhole saw is used for cutting irregular  shapes  and  openings,  such  as  outlet  box openings. A convex head or drywall, hammer used for drywall nails will “dimple” the material without tearing the paper. The screw gun quickly sinks drywall screws to  the  adjusted  depth  and  then  automatically  disengages. Drywall knives have a variety of uses. The 6-inch knife is used to bed the tape in the first layer of joint compound and for filling nail or screw dimples. The 12-inch finishing knife “feathers out” the second layer of joint compound and is usually adequate for the third or “topping” layer. Knives 16 inches and wider are used for  applying  the  topping  coat.  Clean  and  dry  drywall knives after use. Use only the drywall knives for the purpose  intended-to  finish  drywall. The drywall trowel resembles a concrete finishing trowel  and  is  manufactured  with  a  3/16-inch  concave bow.  This  trowel,  also  referred  to  as  a  “flaring,” “feathering,” or “bow” trowel, is used when applying the finish layer of joint compound. A comer trowel is almost  indispensable  for  making  clean  interior  comers. For sanding dried joint compound smooth, use 220 grit sandpaper. Sandpaper should be wrapped around a sanding block or can be used on an orbital sander. When 5-2