Recently, there was a discussion within ASPE Connects Open Forum regarding “Venting Vertical Offsets in Sanitary Drainage Piping.” The author indicated that such vertical offsets have been perplexing to him for many years, and that he had done much research and reading on this subject without finding an acceptable answer to the question: “Do you need to vent vertical offsets in sanitary drainage piping?”

This, in my judgment, gets into a basic understanding of flow within the piping system and the reasoning for providing vents around these offsets.

First, let us get a better understanding of flow within the sanitary system. Based on “Engineered Plumbing Design II,” Chapter 4 “Soil and Waste Stacks,” the flow, according to Dr. Roy Hunter, will form a swirling sheet of fluid around the inner piping wall with the remainder of the piping core filled with vent air. While this condition will change at each horizontal branch waste entry, it will return to the wall flow/core air condition downstream of each branch disturbance. This condition allows for vent air to always have the ability to rise up the stack, while waste flow drops down the stack. This arrangement of flow conditions allows for maintaining the pressure disturbance within the system to remain within the 2” of water column required by code to maintain the trap seal.

However, when there is a change in direction, this wall flow/core air condition changes and can “block” the air core condition. When the change in flow direction goes from vertical to horizontal, a condition known as “Hydraulic Jump” will occur. This condition will “block” the air core as fluid flow encounters greater frictional forces in the horizontal run; causing the fluid to “fill” the pipe and compress the air trying to flow upward to the stack. This condition results in a severe pressure disturbance within that discrete section of piping that needs to be relieved. A similar condition occurs when the flow is offset by 45°; however, the flow does not see the same frictional condition in a semi-vertical condition as it does in a full horizontal condition. Hence, while there is a disturbance to the flow, airflow is not “totally” blocked, resulting in less pressure disturbance.

So, what do the 3-model codes, UPC (Uniform Plumbing Code), IPC (International Plumbing Code) and NSPC (National Standard Plumbing Code), have to say on the subject? Remember, each of these codes should be based on sound engineering practices that utilize the laws of science and physics to assure accuracy.

The UPC has nothing within its code that covers the subject of how to size a horizontal stack offset. Any horizontal offset is sized as a horizontal sanitary pipe with the stack above sized as if it is a separate stack. The stack below the offset is sized as a separate stack, but based on the “total” load of the combined stacks. The “vent” stack associated with the system requires the upper vent stack to be sized for the load on the upper stack. While the vent stack below the offset is sized with the load of both stacks. If the lower vent stack combines with the upper vent stack, it is sized with the load and length of both stacks.

The IPC discusses sanitary stack offsets as follows:

710.1.1 Horizontal Stack Offsets: Horizontal stack offsets shall be sized as required for building drains in accordance with Table 710.1 (1), except as required in Section 711.3.

710.1.2 Vertical Stack Offsets: Vertical stack offsets shall be sized as required for straight stacks in accordance with Table 710.1 (2), except where required to be sized as a building drain in accordance with Section 711.1.1.

IPC Section 711 Offsets in drainage piping in buildings of five stories or more provides further direction.

711.1 Horizontal Branch Connections Above or Below Vertical Stack Offset: If a horizontal branch connects to the stack within 2 feet above or below a vertical stack offset, and the offset is located more than four branch intervals below the top of the stack, the offset shall be vented in accordance with Section 907.

711.1.1 Omission of Vents for Vertical Offsets: Vents for vertical offsets required by Section 711.1 shall not be required where the stack and its offset are sized as a building drain (See Table 710.1 (1)).

711.2 Horizontal Stack Offsets: A stack with horizontal offset located more than four branch intervals below the top of the stack shall be vented in accordance with Section 907 and sized as follows:

1. The portion of the stack above the offset shall be sized as for a vertical stack based on the total number of drainage fixture units above the offset;
2. The offset shall be sized in accordance with Section 710.1.1; and
3. The portion of the stack below the offset shall be sized for the offset or based on the total number of drainage fixture units on the entire stack, whichever is larger (see Table 710.1 (2), Column 5).

711.2 Omission of Vents for Horizontal Stack Offsets: Vents for horizontal stack offsets required by Section 711.2 shall not be required where the stack and its offset are one pipe size larger than that required for a building drain (see Table 7.10 (1)) and the entire stack and offset are not less in cross-sectional area than that required for a straight stack plus the area of an offset vent as provided in Section 907.

711.3 Offsets Below Lowest Branch: Where a vertical offset occurs in a soil or waste stack below the lowest horizontal branch, a change in diameter of the stack, because of the offset, shall not be required. If a horizontal offset occurs in a soil or waste stack below the lowest horizontal branch, the required diameter of the offset and the stack below it shall be determined as a building drain in accordance with Table 710.1 (1).

904.5 Vent Headers: Stack vents and vent stacks connected into a common vent header at the top of the stacks and extending to the open air at one point shall be sized in accordance with the requirements of Section 906.1. The number of fixture units shall be the sum of all fixture units on all stacks connected thereto, and the developed length shall be the longest vent length from the intersection at the base of the most distant stack to the vent terminal in the open air, as a direct extension of one stack.

906.1 Size of Stack Vents and Vent Stacks: The minimum required diameter of stack vents and vent stacks shall be determined from the developed length and the total of drainage fixture units connected thereto in accordance with Table 906.1, but in no case shall the diameter be less.

For vents of horizontal stack offsets, the IPC offers the following:

907.1 Vents for Horizontal Offset of Drainage Stack: Horizontal offsets of drainage stacks shall be vented where five or more branch intervals are located above the offset. The offset shall be vented by venting the upper section of a drainage stack and the lower section of the drainage stack.

907.2 Upper Section: The upper section of the drainage stack shall be vented as a separate stack with a vent stack connection installed in accordance with Section 904.4. The offset shall be considered to be the base of the stack.

907.3 Lower Section: The lower section of the drainage stack shall be vented by a yoke vent connecting between the offset and the next lower horizontal branch. The yoke vent connection shall be permitted to be a vertical extension of the drainage stack. The size of the yoke vent and the connection shall be not less than the size required for the vent stack and drainage stack.

The NSPC Sanitary Stack Offset is as follows:

11.6.1 Vertical Offsets: An offset in a drain stack that is 45 degrees or more from the horizontal shall be sized as a straight vertical stack in accordance with Table 11.5.1B. Section 11.6.2 Horizontal Offsets: Horizontal offsets in the sanitary drain stacks shall be sized according to Table 11.5.1A for building drains and sewers. Drain piping downstream from the horizontal offset shall not be less that the pipe size of the offset.

11.6.3 Sanitary Drain Stacks with Horizontal Offsets:

• a. Where a sanitary drain stack includes a horizontal offset, the upper and below portions of the drain stack shall be sized and vented based on their individual number of branch intervals.
• b. Where a vertical portion of a drain stack has less than five branch intervals, it shall be sized according to Section 11.5.2.1 and vented by stack vent in accordance with Section 12.3.1.
• c. Where a vertical portion of a drain stack has five or more branch intervals, it shall be sized according to Section 11.5.2.2 and vented by stack vent and vent stack in accordance with Section 12.3.1.
• d. Where a drain stack with a horizontal offset is designed as a single drain stack and any vertical portion requires a vent stack, the size of all drain stack piping, including the horizontal offsets, shall be not less that the pipe size at the base of the drain stack. The drain stack shall be vented according to Section 12.3.

Section 11.10 Branch Connections to Offsets in Drain Stacks:

• a. Branch drains shall be permuted to connect to a horizontal stack offset provided that the connection is not within 10 pipe diameters downstream from the upper portion of the stack.
• b. When stacks have five or more branch intervals above the horizontal offset, there shall be no branch connection to the stack within 2 feet above or below the offset.
• c. Where stacks having five or more branch intervals above a vertical offset have a branch connection to the stack within 2 feet above or below the offset, the offset shall be vented as required for a horizontal offset.

12.3.3 Horizontal Offsets: Horizontal offsets in stacks having five or more branch intervals discharging above the offset shall be vented either:

1. By considering the stack as two separate stacks, one above and one below the offset and venting each separately.
2. By providing a yoke vent from the drain stack below the offset to the vent stack required by Section 12.3.1 not less than 3 feet above the offset. This relief vent may be a stack vent for the lower portion of the drain stack.

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As you can see, each of the model codes has a differing view of how to handle these horizontal and vertical offsets within the drainage and vent systems. But they all have a common thread; changes in direction will alter the flow pattern and potentially cause pressure disturbances within the impacted section of the system. And while we, as design professionals, need to follow the minimum requirements of the applicable code, it is more important to follow the intent of the code to assure our design minimizes disturbances to the fluid flow as well as the air flow within the drainage system.

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12.3.4 Vertical Offsets: Where vertical offsets in drain stacks, having five or more branch intervals above the offset have branch connections within 2 feet above or below the offset, a relief vent shall be provided for the lower portion of the stack below the offset.

As you can see, each of the model codes has a differing view of how to handle these horizontal and vertical offsets within the drainage and vent systems. But they all have a common thread; changes in direction will alter the flow pattern and potentially cause pressure disturbances within the impacted section of the system. And while we, as design professionals, need to follow the minimum requirements of the applicable code, it is more important to follow the intent of the code to assure our design minimizes disturbances to the fluid flow as well as the air flow within the drainage system. So, when designing your system and offsets become necessary, consider how you can appropriately vent the system to reasonably maintain a uniform flow of both the waste and the vent air.