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This page defines plumbing vent system terms, distances, and functions, including how plumbing vents work on buildings, why plumbing vent piping is needed, and what happens to the building drains when the vent piping is not working. We define the soil stack, waste stack, wet vents and dry vents, and we summarize the distances permitted between plumbing fixtures and their vent piping.
Our page top sketch of a plumbing stack vent and other sketches included below are provided courtesy of Carson Dunlop.Also see Plumbing Vent Defects for a guide to inspecting plumbing drain waste vent piping for defects.
Citation of this article by reference to this website and brief quotation for the sole purpose of review are permitted. Use of this information at other websites, in books or pamphlets for sale is reserved to the author. Technical review by industry experts has been performed and is ongoing - reviewers welcomed and are listed at "References." © Copyright 2009 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.
Plumbing Vent System Terms, Definitions, & Types of Vents
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How & Why Building Plumbing Vent Piping WorksThe plumbing vent system is part of the building plumbing drain system and serves two basic functions: 1. Allow building drains to flow freely by allowing air into the drain system, avoiding the vacuum and slow drainage that would otherwise occur at fixtures. Now perform the same experiment, but punch a hole in the bottom of your soda bottle just before you turn it upside down. Because air can easily enter the bottle the soda flows nicely out of the bottle mouth. |
2. Allow sewer gases to be vented safely outdoors. Because sewer gases may flow back up into the building drain piping from a public sewer or private septic system, and because some sewer gases are included in building waste flowing through the piping, the plumbing vent system needs to carry these gases outside, usually above the building roof, where they are disposed-of safely and without leaving unpleasant, or possibly dangerous smells and gases inside the building.
The basic plumbing vent terms are shown in the sketch at left. In these notes, the plumbing stack vents and other sketches included below are provided courtesy of Carson Dunlop.
What is the Plumbing Stack Vent?
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Here we show the main building vent pipe, the plumbing stack vent, connecting inside plumbing drains to a vent pipe that extends above the building roof in order to let needed air into the drains and to vent sewer gases harmlessly above the building. Below we explain how building plumbing vent systems work and why plumbing vents are needed for drain functrion and plumbing drain safety. |
What is the Difference Between a Soil Stack & a Waste Stack?
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The soil stack is the large-diameter main vertical waste pipe or building drain, or vertical portion of the "main drain" in the building. The soil stack pipe is normally extended outdoors above the building roof, as shown in the page top sketch. Other main building drain piping sections that slope closer to horizontal are connected to the soil stack but move waste horizontally where needed in a building. |
The waste stack pipe shown in the sketch refers to any other vertical drain piping in the building that does not carry soil (sewage) from a sanitary fixture (toilet). Typically waste stack piping carries drainage away from sinks, tubs, and showers.
How far can the plumbing vent piping be located from a plumbing fixture?
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As we discuss at DRAIN NOISES, if the horizontal distance between a plumbing fixture and the vertical vent piping is too great, the fixture may not drain properly, producing slow drainage or gurgling noises. Poor drainage is not just an annoyance, it can be unsafe since there is also the risk that the poorly-vented plumbing fixture will lose the water from its plumbing trap, then permitting sewer gases into the building. As we show in Carson Dunlop's sketch, the distance allowed between a plumbing fixture (actually the fixture plumbing trap) and the vertical vent piping varies between a minimum and maximum as a function of the pipe diameter. |
What is Direct Venting of Plumbing Fixtures?
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But if a plumbing fixture is close enough (five feet or less) to the main waste stack pipe (vent), the fixture does not usually require its own plumbing vent piping, and it is considered a direct-vented plumbing fixture. Of course this rule presumes that the drain piping between the fixture trap and the waste stack is properly installed and properly sloped. The usual slope on the fixture drain piping is 1/4" of slope per 12" (foot) of horizontal distance or "run" of piping. |
What is a Wet Vented Plumbing Fixture?
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When you flush a toilet it sends a sudden large volume of waste and wastewater into the building drain waste vent (DWV) piping. This surge of wastewater can certainly create a vacuum problem in the waste line if the vent piping system is inadequate, blocked, or missing entirely. It is exactly this condition that produces the gurgling or even siphonage out and loss of water in nearby sink or tub traps when you flush a toilet in a building where the vent piping is inadequate. See Plumbing Drain Noises. A toilet that is located too far from the soil stack can be wet vented as shown in Carson Dunlop's sketch. The drain piping for a sink (basin) or other fixture located closer to the soil stack than is the toilet can provide a pathway to let air into the horizontal waste piping used by the toilet to carry waste to the soil stack. |
But a wet vented fixture requires a larger drain pipe diameter in its wet portion as we show in the sketch. This diameter increase helps assure adequate air flow into the drain system in the event that the sink basin (in this example) happens to be draining at the same moment that the toilet is flushed.
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MVOCs & MOLDY MUSTY ODORS
ODORS & SMELLS DIAGNOSIS & CURE
PLUMBING TOPICS
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SEPTIC SYSTEMS HOME
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First Steps for Sewer Gas Odors
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Building Drain & Sewer Line Odors
Cure Odors in Septic Systems
Other Causes of Odors from a Septic System
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Plumbing Vent Definitions, Types
Plumbing Vent Defects
Septic System or Sewer Piping
Site, Weather, or Failing Neighbors
Tests for Indoor Sewer Gas
Tracking Odors to Source
SEWER GAS ODORS in COLD WEATHER
Toxic Gas Test Procedures
- Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
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More Information on Building Diagnostic Inspections and Repairs
- Cold Weather Septic Odors or Sewage Odor Diagnosis for additional odor tracing and cure advice focused on sewer gas odor tracking in cold or wet weather.
- Drain Noises: may indicate defective or clogged plumbing: how to diagnose and cure drain sounds
- The Septic System Information Website home page for this topic
- Septic Systems Inspection, Testing, & Maintenance- online textbook. Detailed how to inspect, maintain, repair information
- The Home Buyer's Guide to Septic Systems
- Septic Tank Pumping Guide: When, Why, How to pump the septic tank
- Home & Outdoor Living Water Requirements
- Septic Tank Capacity vs Usage in Daily Gallons of Wastewater Flow, calculating required septic tank size, calculating septic tank volume from size measurements
- How Big Should the Leach Field Be? - table of soil percolation rate vs. field size
- Septic System Drainfield Absorption System Biomat Formation - what leads to drain field clogging and expensive drainfield repairs
- Table of Required Septic & Well Clearances: Distances Between Septic System & Wells, Streams, Trees, etc.
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What is a Septic System An Engineer's View - Types of treatment tanks, adsorption systems, pumps, and other special equipment are discussed in some further detail in this text
Components of a Septic System- the Basic Parts of a Conventional Septic Tank and Leachfield, a chapter in the Home Buyers Guide to Septic Systems
A Toxic Gas Testing Sampling Plan for Residential Indoor Air Investigations - Pennsylvania State Wastewater Treatment Fact Sheet SW-161, Septic System Failure: Diagnosis and Treatment
- Pennsylvania State Wastewater Treatment Fact Sheet SW-162, The Soil Media and the Percolation Test
- Pennsylvania State Wastewater Treatment Fact Sheet SW-l64, Mound Systems for Wastewater Treatment
- Pennsylvania State Wastewater Treatment Fact Sheet SW-165, Septic Tank-Soil Absorption Systems
- Document Sources used for this web page include but are not limited to: Agricultural Fact Sheet #SW-161 "Septic Tank Pumping," by Paul D. Robillard and Kelli S. Martin. Penn State College of Agriculture - Cooperative Extension, edited and annotated by Dan Friedman (Thanks: to Bob Mackey for proofreading the original source material.)
Pennsylvania State Fact Sheets relating to domestic wastewater treatment systems include
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