Pay  special  attention  to  the  joints  so  roots do  not  grow  into  the  piping.  The  depth  of the  piping  should  be  below  the  frost  line. Also,  you  should  encase  the  piping  with concrete or sleeve it with a metallic material when laying piping under roadways. It  may  be  necessary  to  install  the building sewer and the water-service pipe in the  same  trench  (fig.  7-1).  If  so,  use  the following   precautions: Figure 7-1.—Building sewer and the water-service pipe in the same trench. Ensure  the  bottom  of  the  water  pipe  is  at least   12   inches   above   the   top   of   the building sewer. Place the water pipe on a solid shelf at the side of the trench. Use  hot  lead  when  joining  cast-iron  pipe for  a  building  sewer;  use  a  hot-poured compound in joining bell-and-spigot clay or concrete  sewer  pipe. After  installation,  test  the  building  sewer with a 10-foot head of water, or equivalent test. Grading Install  sanitary  drainage  piping  on  a  uniform slope.  This  slope,  pitch,  grade,  or  drop  per  foot decides the flow velocity of liquid within the pipe. Piping   with   a   diameter   of   3   inches   or   less requires a slope of not less than 1/4 inch per foot. Pipe 4 inches or larger slopes no less than 1/8 inch per  foot.  This  allows  a  velocity  of  not  less  than  2 feet  per  second,  that  provides  the  scouring  action necessary  to  keep  a  pipe  free  from  fouling.  Sewer mains  may  have  slopes  of  less  than  1/8  inch  per foot, as long as there is a cleaning velocity of 2 feet per  second  or  greater.  See  table  7-1  (A)  for  code requirements. Table 7-1 (A).—Discharge Rates and Velocities of Sloping Drains 7-3