Q&A / 

Plumbing Vent Piping Tips

DEAR TIM: 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 failsafe method of venting so that everything will work fine down the road? What are the biggest things to avoid? Mandy P., Portland, ME

DEAR MANDY: 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.

The center vertical pipe is both a drain and vent pipe. The vertical pipe to the right is a vent pipe from below. They all connect to the larger pipe in the ceiling before it exits the roof. PHOTO CREDIT:  Tim Carter

The center vertical pipe is both a drain and vent pipe. The vertical pipe to the right is a vent pipe from below. They all connect to the larger pipe in the ceiling before it exits the roof. PHOTO CREDIT: Tim Carter

To 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.

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17 Responses to Plumbing Vent Piping Tips

  1. Tim, we are having our bathroom remodeled. The plumber put in the tub and shower pipes but did not allow for enough distance for the fixtures to go on after tile was put on the walls.
    Does this mean a tear out of the tile or wall behind the tub wall? Is there a possible fix without this extreme measure?

    • your question requires lots of typing, plus I have some questions for you so I can give you the correct answer(s). I only do pithy answers here in the comment section. If you want to protect the investment you have in your house and not waste time or money *hoping* you make the right decision, you should talk to me on the phone for just 15 minutes. It'll be the best investment you've ever made in your home!

  2. I am making a flat roof (deck patio) over an area where there is a black plumbing vent pipe. I woud prefer to cut it to size below the roof deck, put a 45 degree elbow on it and run the pipe out of the side of the building. Are there any issues with that?

  3. A Wet Vent System is allowed per code in GA, but do you think a properly installed wet vent system is acceptable or good? Thanks.

    • You can run a vent pipe out a side wall. But you should have asked me if "I, Tim Carter" would do it. The answer is I'd NEVER personally run a vent pipe out of a wall. Always out of a roof. But you can do what you want.

  4. Hi Tim, I am a electrician by trade ,but a okunber I am not, I however am tackling my "whole" home remodel. I have a 4 inch septic drain from my basement wall the old plumbing was just a simple 3" vent from the main 4" drain as all fixtures drained here only one vent was used. I have upgraded to two full baths from one and redone my roof . I moved the 3 " stack farther up the roof line and some 4 feet over ,now preventing a straight run from septic exit to roof. My question is ,can my 3 inch vent pipe have any vertical runs. I would have one under the floor joices and another below roof to accomplish a connection from septic exit to roof exir , and all fixtures connecting along the way? Please help

    • I think you mean can the vent pipe be "horizontal", right? If that's what you mean, the answer is Yes. But, all horizontal runs MUST HAVE a pitch so condensation water in the vent pipe DRAINS to the sewer or septic. Water can't collect in a vent pipe as it could eventually choke it off from the atmospheric air at the roof.

  5. I have a question my bathroom stall is.not.going down quickly im not sure why. It seriosuly ramndomly happened yesterday nd now my bathtub isnt going down quick enough either. any tips? Someone told me use peroxide but I have no.idea how to even use that

    • Maybe there's a hair clog. That's the most common reason a shower or tub drains slowly. Don't pour peroxide down the drain, that's a waste of money. Just get the hair out.

  6. Tim

    I have just had a home built in the late 60's gutted and remodeled. Against my better judgement I allowed the plumber to connect the new abs plumbing system (drain and venting) to the existing copper vent stack(s) to avoid working on the new roof that had been installed just prior to me puchasing the home. Now that the work is complete and the walls are all closed up I have an intermittent howling noise that have narrowed down to be originating from the area of one of the vent stacks in the house. All of the online info I can find describes a problem that sounds like mine however the cite the cause to be a clogged or backed up drain. As I mentioned all the plumbing is new so that is not the case. Is there anything else that could be causing the noise?

  7. I have a vent pipe that suddenly had water start coming out of it and when this happened one of my toilets will not flush. Can you help me with this?

  8. Hi I've got a problem with my drain pipe in the wash room when the water spins out of the washing machine and go down the drain it gets full and come back up. Well I tried using Drano liquid 3 bottles still nothing happened, a snake line was used to see was any blockage nothing happened, I even used a pressure hose but nothing happened the water still came back up. Do you have any helpful tips to use?

  9. Hi, I'm actually curious like Boysie. I'm having the same issue. I thought maybe it was the septic, but no other issues reported in the house (its been about a week). Coincidentally, this happened on one of the coldest mornings of the year (in IL). I thought maybe the vent pipe had ice formed, but could not see anything. Other vent pipes a few days later, I had noticed had ice formed on the cap early in the AM, but were not longer visible later in the day - no problems with showers, toilets, etc, only the washing machine.

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