Figure 3-39.Soil pipe branches.
condensation freezes when exposed to low
One way to prevent freezing is to increase the pipe
to a size or two larger than the vertical vent passing
through the roof; or, install high-lead flashing to
provide an insulating pocket of air between the
flashing and the end of the main soil and waste vent
above the roof. Being open to the heat of the building,
the air pocket allows an intermediate warming area for
gases leaving the main soil and waste vent.
Materials used in vent piping ordinarily include
galvanized pipe, cast-iron soil pipe, and at times, brass,
copper, and plastic pipe. Asbestos-cement pipe can
also be used for venting both soil and waste pipe. A
single length of this pipe is often sufficient for venting
a stack. For such an installation, pipe is available with a
machined end. This end is placed in the bell of the soil
or waste pipe, and the connection is made by yarning,
leading, and caulking.
In all phases of the venting system, do your best to
use proper-sized piping. Remember, the diameter of
the vent stack or main vent must not be less than 2
inches. The actual diameter depends on the developed
length of the vent stack and on the number of fixture
units installed on the soil or waste stack. The diameter
of a stack vent should be at least as large as that of the
soil or waste stack.
Both soil and waste pipe BRANCHES are
horizontal branch takeoffs that connect various
fixtures and the vertical stack (fig. 3-39). One method
of installing a branch takeoff from the vertical stack is
to use a Y-branch with a 1/8 bend caulked into it.
Another method is to use a sanitary tee, which is an
extra short pattern 90-degree Y-branch. Of these two
methods, the sanitary tee is better because you
eliminate one fitting and an extra caulked joint; both
are required for the 1/8 bend takeoff. Some local codes
allow you to connect more fixture units to a given size
of stack when a combination Y and 1/8 bend is used.
The combination Y and 1/8 bend may be more
desirable than the sanitary tee. Once either fitting is
caulked into place, the horizontal branch can be
extended as necessary with lengths of soil pipe. They,
too, are joined by caulking.
Pipes carrying waste should be graded downward
to ensure complete drainage. Horizontal vents should
be slightly pitched to facilitate drainage of
In this chapter, only the basic types of vents and the
locations where they are used have been covered;
however, there are many forms of ventilation that can
be applied to a plumbing installation. The types of
vents used on a project are determined largely by the
manner in which the plumbing fixtures are to be
installed and where they are located.
As plumbers know, the subject of vents is the
science of plumbing. Anyone can try to install piping
for a plumbing system; however, if the system cannot
carry waste away, the would-be plumber and the
system are useless. For more information on venting,
consult the National Standard Plumbing Code-
Q17. Acid-resistant cast-iron pipe is an alloy
composed of what two elements?
Q18. What is a trap seal?
Ql9. To prevent siphonage of seal traps in a plumbing
system, what plumbing method should you use?