Plumbing: Sanitary Vent Systems

Horizontal Offset of Sanitary Vents Below Flood Rim ElevationDavid Olson, PE

I did my first building code plan review in 1991, a grocery store in Brighton, Colorado. I worked as a sub-consultant to an architect who had a good relationship with the local building official. I took on the project with a little anxiety, never anticipating that 24 years later I would still be doing code reviews. At that time, most building departments in the State of Colorado were enforcing the Uniform Code’s – Building, Mechanical and Plumbing. Today’s International Code’s are quite similar in content and organization, with some noticeable variations. For example, Chapter 9 of the UPC was entitled Vents, and Chapter 9 of the IPC is called…Vents. Go figure. The International Code Council also saw fit to break fuel gas piping into its own code document, and has supplemented the Uniform codes with numerous specialized code books such as the Private Sewage Disposal Code, the Residential Code, the Existing Building Code and the Property Management Code to name just a few.


Within plumbing systems there are a few necessary elements – sanitary waste piping, sanitary vent piping and domestic hot and cold water piping. These piping systems are supplemented by medical gas systems, fuel gas systems, compressed air systems, domestic hot water circulation systems. Each system has special rules and regulations. I have often thought and said that all you need to perform plumbing design is a currently enforced code book. The rules for pipe sizing and installation are all readily available in the code document. An old plumber once said to me a wise proverb… “Shit goes down, farts go up (and pay day is on Friday)”. That’s a little over simplified, but is certainly quite descriptive.

In the design of sanitary waste and vent systems air control is most important. Plumbing fixtures are either direct connected, or indirect connected. In a direct connected fixture, the waste piping is connected to the building sewer system directly…without any sort of air gap. Obviously, the inside of sewer systems are often not very pleasant, particularly when considering the odors that are contained. For this reason, it is essential to incorporate a sanitary trap within the waste piping system for each plumbing fixture. This is the piping trap, or P-trap, that you will see when you look under your bathroom or kitchen sink. There is a “U” shaped bend in the waste piping that will trap some of the drainage from the fixture – using it to form a blockade against backflow by the offensive odor within the sanitary waste piping system. We call this the trap seal. Did you ever go into a public restroom and smell really raunchy air? That is probably sewer gas caused by the floor drain trap drying out (yes, the same floor drain that really doesn’t stay primed by daily mopping of the floor…). Once the trap seal evaporates, voila…sewer gas invades the room. Yuk!

Indirectly drained fixtures have air gaps. With a few exceptions, these fixtures are not trapped. The waste outlet from the fixture flows down to a floor sink or other approved indirect waste receptor. So long as the indirect waste receptor has a primed trap, there is no chance of backflow of sewer gas, or sewage for that matter, breaching the gap between the receptor and the fixtures indirect drain line. It is most common to see indirectly drained fixtures within commercial kitchens, with drainage systems for food preparation sinks and scullery sinks.

Sanitary vent systems provide a key function for the maintenance of the above described trap seals. The sanitary vent breaks the vacuum created by quickly draining fixtures and prevents the removal of the trap seal. Think back to grade school science experiments. Remember watching a bowl of water quickly draining out a hole in the bottom? There is a vortex created, like an inverted tornado in the remaining volume of water in the bowl. When the last of the liquid drains from the bowl, you hear a sucking sound as the drain pipe searches for air to replenish the fluid rapidly exiting. I recall being enthralled by the drainage tornado at the conclusion of my bath’s as a young boy. Perhaps the bathtub in our old house wasn’t vented so well. By design, the sanitary vent replenishes this airflow just downstream of the P-trap, thus breaking the vacuum caused by the drainage cascading down the sanitary waste line. As such, the trap seal is maintained, and does not become additional flow down the sewer line within the building. So, sanitary vents and the short extensions above the building’s roof are actually not venting farts, as the old plumber suggested, but rather they serve as an inlet of airflow into the sanitary waste and vent system – much to the relief of our noses!

Section 905.3 of the Uniform Plumbing Code included the following text;

Unless prohibited by structural conditions, each vent shall rise vertically to a point not less than six (6) inches (152 mm) above the flood level rim of the fixture served before offsetting horizontally, and whenever two or more vent pipes converge, each such vent pipe shall rise to a point at least six (6) inches (152 mm) in height above the flood level rim of the plumbing fixture it serves before being connected to any other fixture. Vents less than six (6) inches (152 mm) above the flood level rim of the fixture shall be installed with approved drainage fittings, material and grade to the drain.

The structural condition that was normally encountered was when a plumbing fixture was too far from a wall or acceptable place to rise a sanitary vent vertically. Table 10-1 of the same code provided maximum distances that a p-trap could be located from the vent connection. The larger the waste line diameter, the further between the P-trap and vent riser that was allowable. If there was no way to reach an acceptable vent riser location, plumbing designers and installers would simply roll the sanitary vent vertically off the trap arm, below the flood rim elevation (usually the floor level for floor drains and sinks), and offset this line horizontally to a place where the vent line could rise vertically. The code allowed this provided that the piping below the flood rim elevation was installed as waste piping. Most inspectors I knew, and within the plan reviews that I conducted, interpreted this as also requiring sanitary cleanouts within the vent risers. This allowed the vent piping that was below the flood rim elevation to be cleaned out when it eventually clogged up and ceased to function as a sanitary vent.

Drainage is very quick and stupid. The flood rim elevation is the point of a plumbing fixture where drainage will overflow if too quickly filled, or if the drain is clogged. If a floor sink, basically a floor drain with a larger sump to collect and store excessive short term flow, fills to nearly the flood rim elevation during use, the pressure head developed by the fluid in the drain body will cause drainage backup into a vent line that is installed below the floor level. Usually, drainage that is carried away by a floor sink or floor drain is not really clean. It has normally got dirt, grease or some other contaminant within its make-up of drainage effluent. When this drainage backs up into the sanitary vent, over time, the vent line will be blocked by deposited material.

The International Codes came to be in the waning years of the last century (way back in the late 1990’s). They included a similar restriction with respect to horizontal offsetting of sanitary vent lines below flood rim elevations. Section 905.4 is entitled Vertical rise of vent, and it reads as follows:

Every dry vent shall rise vertically to a minimum of 6 inches (152 mm) above the flood level rim of the highest trap or trapped fixture before being vented.  

In addition to the standard code books, the International Code Council also publishes documents known as Code and Commentary’s. Within these books, there are included explanations for most of the requirements included within the standard codes. If I ever have a question about the meaning of a code section, or its purpose, I refer to these commentary volumes. The IPC Code and Commentary for section 905.4 says,

This code provision is very important for the proper design, longevity and maintenance-free operation of a plumbing system. Surprisingly, this is also one of the most frequently violated code provisions.

The intent of this section is to prohibit horizontal vent piping from occurring where it will be subjected to waste flow. A vertical rise of the vent pipe substantially reduces the possibility of it having a blockage or stoppage. Most pipe stoppages occur in horizontal pipes. The vertical rise of the vent to a point above the flood level rim is necessary to protect the vent from a backup of waste in the event of blockage in the drainage system. A blockage causes the drainage to rise in the vent pipe and in the plumbing fixture. Any drainage system backup will flow over the fixture’s flood level rim, thereby preventing it from entering the horizontal pipe.

A stoppage in the drain may be easily cleared by rodding through a cleanout; however, this is not the case with a vent blockage, which is seldom, if ever, accessible.

I frequently see this code provision violated in submitted construction drawings that I review for code approval. When I speak with the plumbing designers, it is always their desire to minimize the amount of piping required to complete the waste and vent piping system. I tell them that the code approved way to pipe fixtures is to offset the trap arm (the pipe between the P-trap and the vertical waste piping in the wall) towards the wall where the vent line will rise up. Yes, sometimes the waste line is a few feet longer in overall length, and subsequently slightly lower in invert. However, it is rarely a game changer in the finished waste and vent system.

The code does allow a number of other venting methods that when employed, may even eliminate the need for a vent line for a particular fixture. I suggest that a review of the latest edition of the International Plumbing Code would be in order for finding an acceptable venting solution. It is important to remember – the code does not allow for horizontal venting below the flood rim elevation. Don’t do this – your nose will know!