such as galvanized steel or copper tubing.
Branches are usually either threaded galvanized
steel pipe with drainage (recessed) fittings or
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
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
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