Tips for Plumbing Vent Piping

I’ve got a remodeling project coming up and I intend to do the plumbing work. I’m good to go on the water lines, but the drain lines and plumbing vents are my weak spot. Can you give me the executive summary on plumbing vents? Is there a fail safe method of venting so that everything will work fine down the road? What are the biggest things to avoid?

You’ve asked for the impossible. I’ve been a master plumber for over 25 years and I don’t know if it’s possible to do a Vulcan mind meld between you and I, but I’ll give it my best shot. Plumbing vent pipes often confuse many, and even apprentice plumbers who are somewhat familiar with the trade often make serious mistakes when it comes to plumbing vent systems.

The first thing I want to mention is to make sure you check to see if you’re allowed to install the piping. Some states and towns only allow licensed plumbers to do this work.

The reasoning is based on public health. If you make mistakes when you plumb, you can get some people seriously ill or even cause death.

Plumbing Vent PipingTo help you understand the need for plumbing vents, let’s talk about what happens in drain piping when water travels down through the system. In a properly designed plumbing drain and vent system, there is air in the pipes before water is poured down a drain or a toilet is flushed.

As soon as you introduce water, and lots of it quickly, into a plumbing drain, the dynamics of the air changes. The water surging into the system displaces the air often pushing it down the drain in front of the rushing water. This air needs to be replaced so a vacuum doesn’t form in the system.

Vacuums in plumbing drain lines are bad, very bad. You’ve possibly heard a vacuum getting satisfied if you’ve been in a bathroom when a tub or sink drain gurgles when you flush the toilet. At a friend’s house, this would happen every time his washing machine would drain.

When the washing machine pump came on, his kitchen sink would gurgle and the water in the trap under the sink would be sucked dry. This allowed sewer gas to enter his house and vermin that are crawling around in the sewer lines. Yuck!!

To prevent traps in downstream fixtures from being sucked dry like my friend’s kitchen sink, you install a vent pipe, usually within 3 feet, close to the fixture trap. This vent pipe rises vertically towards the roof where it opens to the atmosphere to get the needed replacement air.

Usually a pipe that’s 1.5 inches in diameter is sufficient to vent any residential fixture. But understand that some plumbing codes have very specific sizing requirements. If you start to collect vent pipes from other fixtures as you head to the roof vent, the pipes will have to get bigger, just as plumbing drain lines and building drain pipes get bigger the more water that enters them.

Vent pipes can be tiny and work. I’ll never forget visiting a farm owned by another master plumber friend of mine. For fun he vented all of the fixtures in a large bathroom with 1/2-inch copper water lines! But you know what, enough air was able to pass through that tiny pipe to satisfy each of the fixtures. It was just a simple experiment he did as he knew it would never pass an inspection. Lot’s of air can pass quickly through a small unobstructed pipe.

To be safe, extend a vent pipe from every fixture. Certain fixtures can be wet-vented, this means two fixtures share a common vent, but since I can’t be at your house to mentor you on this complex technique, just install separate vents for each fixture. Be sure any vent line that has to run horizontal actually has a tilt to it so any condensate water that forms in the pipe drains down to the sewer or septic tank.

There are many things to avoid when installing plumbing vent pipes. I’m not a huge fan of the mechanical vents that you might install under an island sink or in some other location where running a traditional atmospheric vent is next to impossible. Every mechanical vent I’ve installed has failed  over time.

Mandy, you live in a coastal area, so it doesn’t get bitterly cold for too long. But if you live in an area that gets frigid for long periods of time, you have to make sure the vent pipe both above and below the roof is a large pipe, say 4 inches in diameter, so that it doesn’t get choked off with frost buildup. I’ve seen this happen and it’s almost unbelievable to think that ice could form in a vent pipe.

Avoid the temptation to forgo a full-sized vent in your plumbing system. Some new plumbing codes are moving away from full-sized plumbing vents. I’m not a fan of this. A full-sized vent is a primary vent where the drain line transitions at some point and becomes the vent pipe that exits the roof.

In many an older home, this drain pipe is perhaps 4 inches in diameter and stays that size all the way through the roof. Other vent pipes that are smaller may connect to this full-sized vent, and that’s perfectly fine.

Article Courtesy of Tim Carter, Ask The Builder (

Categories: Home Improvement

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