FHB Logo Facebook LinkedIn Email Pinterest Twitter Instagram Plus Icon Close Icon Navigation Search Icon Navigation Search Icon Arrow Down Icon Video Guide Icon Article Guide Icon Modal Close Icon Guide Search Icon Skip to content
Subscribe
Log In
  • How-To
  • Design
  • Tools & Materials
  • Videos
  • Blogs
  • Forum
  • Magazine
  • Members
  • FHB House
  • Podcast
Log In

Fine Homebuilding Project Guides

Plumbing

Guide Home
Chapter
  • Plumbing Basics
  • Bathrooms
  • Kitchens
  • Roughing In
  • Water Heaters, Washers, and More
  • Maintenance and Repair
How it Works

Drain-Waste-Vent Systems

Although hidden behind walls and in floors and ceilings, the pipes that make up a house’s drain-waste-vent system are the balance of the plumbing equation.

By Justin Fink Issue 254 - Oct/Nov 2015

Synopsis: The pipes that make up a house’s drain-waste-vent (DWV) system are the exit for the water provided through the supply lines. They also carry waste from the toilet, and they connect to outside air for venting sewer gas and relieving pressure. Project House editor Justin Fink explains how these systems evolved and the rules and best practices for installing them to ensure that they do their jobs properly.

Although hidden behind walls and in floors and ceilings, the pipes that make up a house’s drain-waste vent (DWV) system are the balance of the plumbing equation. They are the exit for the water provided through the supply lines, they carry waste from the toilet, and they connect to outside air for venting sewer gas and relieving pressure. These relatively large diameter pipes rely solely on gravity, but they aren’t easily routed through the structure of a house.

With three chapters in the IRC devoted to DWV systems, not to mention the tables and examples illustrated in the appendix, there is a lot to know about the layout and installation of such a system. Pipe sizes that vary based on usage; routes through framing bays that may already be packed with other mechanicals; pitch tolerances of plus or minus just 1/4 in. per ft.; fittings with subtle differences in inlets, outlets, and applicability; and an array of minimums and maximums to consider when it comes to tying everything together are just a handful of the many situations, rules, and restrictions plumbers must understand.

The stakes are high, because an improper DWV setup can lead to slow drains, gurgling fixtures, sewer gas entering the living space, recurring clogs, and potentially hazardous overflows. When installed well, though, a DWV system will go unnoticed, with solids, liquids, and gases flowing smoothly. Here’s how it works.

Evolution of the modern drain

Unprotected- If your sink, bathtub, and other fixtures were connected to drain lines that followed a straight, downward-sloping path to a main sewer outlet, water or waste would drain from each fixture with the help of gravity, air from the room would mix in and relieve the vacuum created by the draining water, and everything would flow without trouble. The problem with this setup is that air being allowed to flow into the drain system means that air also is allowed to flow out and into the room, bringing sewer gas with it.

Trap- To prevent the entry of sewer gas, every fixture is required to have a U-shaped section of piping, known as a trap. Located downstream of the fixture (or in the case of a toilet, as part of the fixture itself), the trap holds water, blocking sewer gas from coming up through the fixture’s drain. The problem with a trap is that as water flows through a drain line, it creates a vacuum behind it (negative pressure) and pushes air in front of it (positive pressure). The flow of this “water slug” can cause water to be sucked out of a trap that it’s passing near, and it may also cause air to push through and gurgle in nearby traps downstream of its path. To relieve this pressure differential, code requires vents.

Trap and vent- Nothing more than a connection to exterior air, a vent neutralizes air pressure within the drain lines. This neutral pressure can be achieved with lots of different configurations: an empty (dry) pipe that connects to the top of a trap arm like a snorkel, a waste (wet) pipe that is oversize in order to provide enough room for both water and airflow, and variations on these setups.

drain-waste vent systemFrom Fine Homebuilding #254.

To view the entire article, please click the View PDF button below:

 

Sign up for eletters today and get the latest how-to from Fine Homebuilding, plus special offers.

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters
View PDF
Previous: Sensible Plumbing Next: Vent Options for Plumbing Drains

Guide

Plumbing

Chapter

Plumbing Basics

View Comments

  1. User avater
    Blue_Oak | Aug 23, 2017 11:00am | #1

    Umm, where exactly is the video?

    All I see is an advertising video.

  2. Paul_V | Nov 24, 2017 01:14pm | #2

    There is no video. There is a pdf you can view.

Log in or create an account to post a comment.

Sign up Log in

Become a member and get full access to FineHomebuilding.com

Plumbing

Plumbing

Trusted, comprehensive guidance from the pros for tackling any plumbing project

View Project Guide

View All Project Guides »

Become a member and get unlimited site access, including the Plumbing Project Guide.

Start Free Trial

Plumbing Basics
  • Plumbing Systems
  • Materials
  • Plumbing Tools
Bathrooms
  • Sinks
  • Showers & Tubs
  • Toilets
Kitchens
  • Kitchen Sinks
  • Water Filtration
  • Appliances
Roughing In
  • Layout
  • Installation
  • Copper Pipe
  • PEX Tubing
  • PVC Drainpipe
Water Heaters, Washers, and More
  • Water Heaters
  • Whole-House Filters
  • Sump Pumps
  • Washers
  • Sprinkler Systems
Maintenance and Repair
  • Sinks
  • Toilets
  • Showers & Tubs
  • Systems

Fine Homebuilding

Follow

  • twitter
  • facebook
  • instagram
  • pinterest

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Membership & Magazine

  • Online Archive
  • Start Free Trial
  • Magazine Subscription
  • Magazine Renewal
  • Gift a Subscription
  • Customer Support
  • Privacy Preferences

Taunton Network

  • Green Building Advisor
  • Fine Woodworking
  • Fine Gardening
  • Threads
  • About
  • Contact
  • Advertise
  • Careers
  • Copyright
  • Terms of Use
  • Site Map
  • Do not sell or share my information
  • Privacy Policy
  • Accessibility
  • California Privacy Rights

© 2023 The Taunton Press, Inc. All rights reserved.

X
X
This is a dialog window which overlays the main content of the page. The modal window is a 'site map' of the most critical areas of the site. Pressing the Escape (ESC) button will close the modal and bring you back to where you were on the page.

Main Menu

  • How-To
  • Design
  • Tools & Materials
  • Video
  • Blogs
  • Forum
  • Reader Projects
  • Magazine
  • Members
  • FHB House

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Popular Topics

  • Kitchens
  • Business
  • Bedrooms
  • Roofs
  • Architecture and Design
  • Green Building
  • Decks
  • Framing
  • Safety
  • Remodeling
  • Bathrooms
  • Windows
  • Tilework
  • Ceilings
  • HVAC

Magazine

  • Current Issue
  • Past Issues
  • Magazine Index
  • Subscribe
  • Online Archive
  • Author Guidelines

All Access

  • Member Home
  • Start Free Trial
  • Gift Membership

Shop the Store

  • Books
  • DVDs
  • Taunton Workshops

More

  • FHB Ambassadors
  • Reader Projects
  • Podcast
  • FHB House
  • Customer Support

Account

  • Log In
  • Join

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Follow

  • twitter
  • facebook
  • instagram
  • pinterest

Join All Access

Become a member and get instant access to thousands of videos, how-tos, tool reviews, and design features.

Start Your Free Trial

Subscribe

FHB Magazine

Start your subscription today and save up to 70%

Subscribe

We hope you’ve enjoyed your free articles. To keep reading, become a member today.

Get complete site access to expert advice, how-to videos, Code Check, and more, plus the print magazine.

Start your FREE trial

Already a member? Log in