Sizing Stacks and Branches

The term stack is used for the vertical line of soil or waste piping into which the soil or waste branches carry the discharge from fixtures to the house drain. A waste stack carries liquid wastes that do not contain human excrement; a soil stack carries liquid wastes that do.

Most buildings do not have separate soil and waste stacks. A single stack known as the soil and waste stack, or simply the soil stack, serves to carry both soil and waste material. Soil stacks are usually made of cast-iron pipe with caulked joints. They may, however, be made of other materials

Table 7-3.-Maximum Loads for Horizontal Drains

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

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 fixture 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 considered only as it affects the developed length of the vent.

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 offset.

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.

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