such   as   galvanized   steel   or   copper   tubing. Branches are usually either threaded galvanized steel  pipe  with  drainage  (recessed)  fittings  or copper   tubing. Sizing the Stack The  stack  is  sized  in  the  same  way  as  the building  sewer.  The  maximum  discharge  of  the plumbing  installation  is  calculated  in  drainage fixture  units.  This  figure  is  applied  to  table  7-4 or  table  7-5  to  obtain  the  proper  stack  size. Continuing  our  example,  the  270  drainage  fix- ture units would require a 5-inch stack, if the stack had less than three branch intervals. (No soil or waste  stack  should  be  smaller  than  the  largest horizontal branch connected, except that a 4 x 3 water closet connection should not be considered as  a  reduction  in  pipe  size.) Offsets on Drainage Piping An offset above the highest horizontal branch is an offset in the stack vent and should be con- sidered only as it affects the developed length of the vent. An offset in a vertical stack with a change in direction  of  45  degrees  or  less  from  the  vertical Table 7-4.—Maximum Loads for Soil and Waste Stacks Having Not More Than Three Branch Intervals piping may be sized as a straight vertical stack. In piping where a horizontal branch connects to the stack within 2 feet above or below the offset, a  relief  vent  should  be  installed. A stack with an offset of more than 45 degrees from  the  vertical  should  be  sized  as  follows: 1.  The  portion  of  the  stack  above  the  offset should be sized for a regular stack, based on the total number of drainage fixture units above the offset. 2.  The  offset  should  be  sized  as  for  the building  drain.  See  table  7-3. 3.  The  portion  of  the  stack  below  the  offset should be sized as for the offset, or based on the total  number  of  drainage  fixture  units  of  the entire  stack,  whichever  is  larger.  A  relief  vent should be installed for the offset. Never connect a horizontal branch or fixture to the stack within 2  feet  above  or  below  the  offset. Sizing Individual Waste Lines The  water  closet,  strictly  speaking,  has  no waste.  It  is  usually  connected  directly  into  the stack  on  a  short  as  possible  separate  branch  of its  own  by  the  use  of  a  closet  bend.  The  closet bend is 3 or 4 inches in diameter if made of cast iron  or  steel  and  3  inches  if  made  of  copper. Because lavatories are used for washing hair, loose  hair  is  often  carried  down  into  the  waste pipe,  causing  a  stoppage.  Lavatory  drainage  is improved by using a minimum number of fittings and  by  eliminating  long  horizontal  runs.  The minimum  pipe  size  for  lavatory  wastes  is  1  1/4 inches,  but  1  1/2  inches  is  more  satisfactory. Urinals present a particular problem because cigarette  butts,  cigar  stubs,  chewing  gum, matches, and so on are often discarded in them. These materials can easily cause a stoppage. For this reason, urinals should be equipped with an effective strainer. Size of waste pipe should be at least  1  1/2  inches  for  wall-mounted  urinals  and 3  inches  for  the  pedestal  siphon  jet  urinal. Shower wastes seldom cause trouble since they have a relatively clear water waste flowing through them. The usual diameter of the waste pipe for a single shower is 2 inches if made of cast iron or  steel  and  1  1/2  inches  if  made  of  copper. A domestic kitchen sink requires a 1 1/2-inch cast-iron  or  steel  waste  pipe.  When  a  sink  is equipped with a garbage disposal unit, a minimum of 2 inches is required for the cast-iron or steel drainage  piping. 7-6