98 BULLETIN 414, U. S. DEPARTMENT OF AGRICULTURE. 
The sewer plumbing carries the waste water from the water-using 
fixtures to a point about two feet outside of the building or buildings, 
where it joins the sewer tile. 
The various pipes of the plumbing system are termed, according 
to the function they serve, house drains, soil pipes, waste pipes, and 
vent pipes, which terms are defined as follows: 
House drain: That part of the main horizontal drain and its 
branches inside the walls of a building and extending to and con- 
necting with the sewer tile. 
Soil pipe: A vertical or nearly vertical pipe line extending through 
the roof, receiving the discharge of one or more water-closets with or 
without other fixtures. 
Waste pipe: A pipe extending through the roof receiving the 
discharge from any fixtures except water-closets. 
Vent pipe: A special pipe provided to ventilate the system of 
piping and to prevent trap siphonage and back pressure. 
All the pipes of the system should be of extra heavy cast iron, and 
the diameters of pipes of the various kinds should not be less than as 
follows: 
House drains, 4 inches; soil pipes, 4 inches; waste pipes, 2 inches; vent pipes, 2 
inches. 
All pipes should be water-tight and air-tight and all joints should 
be tightly calked with oakum and molten lead, the amount of lead 
required for a joint being about 12 ounces for each inch in the diam- 
eter of the pipe. 
All changes in direction of both house-plumbing pipes and sewers 
should be made with one-eighth and one-sixteenth bends, not with 
quarter bends; and all connections of two pipes should be made 
with Y branches, never with T's. 
As previously intimated, all soil and waste pipes should be carried 
vertically upward, beyond the highest fixture discharging into them 
to an open end above the roof in order to provide free outlet for all 
gases to the outside air. 
To prevent foul gases from entering the buildings through the 
fixtures, every fixture should be separately provided with a water- 
sealing trap, placed as close to the fixture outlet as possible. The 
action of the trap is explained by reference to the diagrammatic 
representation of a U-trap shown in figure 5(a). The pipe A receives 
the liquid and solid wastes of a sink, basin, or water-closet, while 
the lower end, B, connects with the sewer. Foul gases rise in the 
pipe B, but are prevented from passing to the fixture by the water 
which stands in the trap. 
In addition to its function as an outlet for the gases the vent pipe 
serves to prevent the destruction of the action of the trap by siphon- 
