Sewer piping differs from water piping, in that sewer
pipes are not under pressure. The function that takes
place inside sewer pipes is at ambient atmospheric
pressure, and the influence of gravity.
Sewer pipes drain because they are installed on a slope
and fluids flow downhill. The slope of sewer lines is a
consideration because most effluent floats, and slow
movement of the drain allows solids to flow unobstructed.
But if the flow is too rapid, larger solids tend to tumble,
and become a potential for clogging a drain pipe. Ideally, a
horizontal sewer drain is never more than half full.
Drain lines that must traverse excessive height in a
short lateral distance are installed with vertical drops
rather than excessive slope.
Drain pipes are installed so the slope is consistent from
the location of the fixture that is farthest from the
entrance into a public drain, or to some suitable disposal.
For this reason, the slope of drain pipes is critical to
their efficiency. If a drain from a fixture in an intermediate
area in a system is installed lower than the consistent point
in the drain system, effluent will collect in the low areas,
and the system will not drain well.
In addition to consistent slope, it is a
requirement of the plumbing code for
all turns in direction of piping be done
with 45 degree fittings in the direction
of flow, rather than 90.
This facilitates the movements of slow
moving solids through a drainage
A vent provides a path to
CROSS SECTION Expel toxic sewer gas to
THROUGH SINK The atmosphere
DRAIN AND VENT
Without a vent for
supply of air to
replace fluid in
the drain, siphon Non-integral
action would pull Non-integral
the fluid out of
WATER CLOSETS - Types of Discharge
Blowout – Public - Loud Siphon Jet – Residential - Quiet
The “seal” area in the diagrams indicate the integral trap. Because of
the water that remains in the bowl, sewer gas is blocked from coming
into the room .
The sizing of pipe for drainage is far less complex than
that for water piping, as there is no pressure involved, no
rate of flow of water, and distance makes a difference only
in that enough slope must be available to accommodate the
Three charts are pertinent to drainage.
The first chart lists the fixture unit value for fixtures,
which are not quite the same as for water supply fixture
The next chart shows the size of non-integral traps. A
non-integral trap is a drain from a fixture to a building drain
line, such as the drain mechanism beneath the lavatory in
the bath room or beneath the sink in the kitchen. Non-
integral means that the drain is not part of the fixture.
An example of an integral trap is a water closet or a
urinal, which have their own traps, because of the way
they are made.
The third chart is the one related to determining the
collective size of drain pipes in a system. The fixture unit
values of each unit determines the size of each segment of
pipe in the system that serves the fixtures.
The logical sequence for determining pipe size is first
to draw a plan of the building, showing all the fixtures that
are to be installed. A line that represents the drain pipe is
drawn to connect the fixtures, which extends out of the
building to an available drain deposit, such as a municipal
Beginning at the “high” end of the system, the fixture
unit value for each segment of pipe is shown. Then from
the following chart, the size of drain pipe is found.
The far left column indicates drain pipe diameter.
Notice the top of the chart shows various degrees of slope
for drain pipe. For this course, use the column that allows
for a slope of ¼” per foot. The numbers in the body of the
chart are total fixture units allowed by the pipe sizes for
the slope given.
Remember that a Water Closet requires a 3” drain, and
you cannot install more than two Water Closets on a 3”
drain. A drain line that has more than two water closets
must be a minimum of 4” diameter, regardless of how many
fixture units it supports.
The next slide shows the drainage plan for the
residence that was used to calculate water supply. Realize
that the drainage plan is to determine pipe sizes for what is
called “rough plumbing,” which is that part of drainage that
is installed below the floor and in walls. The smallest pipe
generally used in rough plumbing is 2” diameter.
Finish plumbing, or “trim” is the parts of the plumbing
system one can see, beneath cabinets, under fixtures, etc.,
and is made of materials of a better finish for appearance
and ease of cleaning.
DRAINAGE FIXTURE UNITS
Water Closet 4
20 Kitchen Sink 3
Bath Tub 2
7 Auto Washer 2
Water heater 1
The next sessions will include plumbing waste for
fixtures that are stacked more than two story, and the
method for determining vents and size of vents.