# Why does a pipe vent need to go out of roof?

coodyApril 20, 2011

A pipe vent goes out of my house roof.

Apparently, the pipe vent collects rain and dew water. One of the pipe connector is loose, that cause leaking on my ceiling. Can you explain why the pipe vent should go out of the roof to collect the rain and dew water? What will it happen if there is no pipe vent out of the roof?

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jolsongoude

Your plumbing will not work without the vent. It's not there to collect water - that's just a bonus feature! Fix the connection and your problem should go away.

April 20, 2011 at 4:58PM
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bevangel_i_h8_h0uzz

Without a pipe vent (stack vent), as wastewater flows down your waste pipe, a partial vacuum would develop at the top of the pipe which would stop the wastewater from flowing the rest of the way down.

Think about what happens when you stick a straw into a glass of water, place your finger over the top of the straw, and then lift the straw out of the glass of water. A tiny bit of water dribbles out of the straw and then water stops flowing even though there may still be a couple of inches of water at the bottom of the straw. What is happening is that, as the first little bit of water flows out, the air at the top of the straw has to expand to fill the space. This means the air molecules in the top part of the straw are more spread out than air molecules in the air that surrounds the straw. So you have a "pressure differential" between the air at the top of the straw and the surrounding air. Or, another way to say this is to say that there is a "partial vacuum" in the straw. The water that stays in the straw is held in place by the partial vacuum. As you know, once you release your finger from the top of the straw, gravity pulls the rest of the water in the straw out.

It doesn't take much of a pressure differential to stop water from flowing down a pipe.

Without a vent pipe, flushing your toilet would be like trying to get all the water out of a straw with your finger held tightly to the top of the straw.

It is frustrating to have leaks around the vent stacks BUT the leaking in your ceiling are not caused by rainwater flowing thru the stack vent pipe itself. The leaks are due to improper flashing around the stack vent pipe.

Get the flashing fixed and the leaks will stop.

April 20, 2011 at 5:03PM
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renovator8

When a stack vent is blocked or capped it would not stop water from flowing in the waste system because there is plenty of air available through the fixtures.

The purpose of venting is to prevent water in the fixture traps from being siphoned away thereby allowing gasses from the sewer to enter the house.

So, without the vent at the roof, the plumbing would work fine but you would hear a lot of gurgling and then you would notice an unpleasant smell.

It is possible to install a mechanical air valve within the house in place of a thru-the-roof vent pipe but many jurisdictions do not allow them.

Here is a link that might be useful: air valve

April 20, 2011 at 7:00PM
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rollie

Understand the vent, but why thru the roof? Why not just terminate it at the sidewall, below the soffit. Odors?

April 20, 2011 at 10:29PM
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renovator8

Yes, in fact on windless summer days it is best if the vent is high on the roof.

April 21, 2011 at 6:45AM
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brickeyee

I have seen more and more vents showing up in the front roof of houses instead of being hidden in the back.

It saves a few feet of pipe on jobs when cost is king.

The while system is called DWV, Drain-Waste-Vent.

Vents are used to prevent trap siphoning and the loss of seal that would occur.

Sewer gas is flammable, can be poisonous, and stinks.

Vents need to be high to prevent the sewer gas from being pulled into the windows or attic.

April 21, 2011 at 9:42AM
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coody

I saw the pipe goes down from the roof and connects to the house drainpipe, sharing with the washing machine drain pipe. I still do not understand why one end of the pipe needs to go out of the roof. It is just like leaving one end of the house drainpipe on the roof and the other end of the drainpipe connects to the sewerage. I saw a lot of houses actually having no pipe going out of the roof. Is it because it is for unclogging the house pipes from the other end on the roof? Without that thinking, I really can not understand why one end of the house drainpipe needs to go out of the roof.

April 21, 2011 at 1:05PM
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renovator8

It's pretty simple. One end of a sewer system is connected to your plumbing fixtures and the other end is connected to the city sewer pipe in the street (or your septic system).

There is sewer gas in the pipe in the street and it will rise up into your house unless there is an air seal. That seal is provided by water filled "traps" at each fixture that are usually a U or S shaped bend in the waste pipe immediately behind and below the fixture. In order to keep the draining waste water from pulling (ie siphoning) the water out of the trap there must be a "vent" pipe that supplies air at a location in the system before the waste water has traveled far enough to empty the trap.

Those vent pipes all come together above the highest fixture and because they must have a source of air and they also contain sewer gases, they must continue to the outside of the house. Because sewer gasses rise out of these pipes they cannot be placed near a window so the roof is the best place for them.

April 21, 2011 at 3:10PM
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brickeyee

"I saw a lot of houses actually having no pipe going out of the roof."

You missed them then.
They are probably on th eback roof and not visible from the street side.

"Is it because it is for unclogging the house pipes from the other end on the roof?"

No, it is the vent line for sewer gas and to prevent trap siphoning.

Do a search on DWV and you will see diagrams and explanations of the system.

All the plumbing codes require at least one main vent.

April 21, 2011 at 4:01PM
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coody

Ok, I got your idea. One end of pipe going out of the roof is to keep the draining waste water from pulling. However, we can see lots of houses, especially old houses, do not have a pipe going out of the roof. The house drainage is still good. Is there much difference between those two different houses, a pipe going to the roof and no pipe on the roof? If the roof was not slope, I would really want to climb onto the roof and cover the pipe to see what would affect the house drainage. Can you continue to throw some light on it? Thank you.

April 21, 2011 at 4:05PM
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brickeyee

In very old houses the vent may have been buried in a chimney, but it is there.

It is very common in Europe to do this.

The vent has to be there for the DWV system to maintain trap seals.

You would have traps siphoning out dry (like the 'glub glub' you hear at the end of the bowl emptying in a toilet as it siphons itself.
We allow it in toilets because as the tank refills some of the water is sent to the bowl to refill the trap.

S traps (like the one in a toilet) without a trap primer are NOT allowed to prevent the trap seal from being compromised every time the drain is used.

"I would really want to climb onto the roof and cover the pipe to see what would affect the house drainage."

"Those who cannot remember the past are condemned to repeat it."
George Santayana, Life of Reason, Reason in Common Sense,
Scribner's, 1905, page 284

April 21, 2011 at 4:55PM
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coody

I want to confirm one thing. If the pipe should go out of the roof, the rain will go into the pipe. Is it correct? Should we prevent the rain and dew water from into the pipe on the roof or just leave the pipe with open to the air?

April 21, 2011 at 6:09PM
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worthy

Santayana

Sigh.

****
The bathroom in a rental house I had always had an odour that I chalked up to the five-seven tenants sharing the single bath. When they moved out and I renoed the house for resale, the plumber found the vent ended inside the wall. Everything drained nicely. And no ugly pipe on the roof.

April 21, 2011 at 6:14PM
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robin0919

coody, that black thing around the pipe is called a 'boot' and it has built-in flashing. If you think you have a leak in that area, you might want to call a roofer to inspect to make sure the boot is installed correctly.

April 21, 2011 at 7:05PM
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renovator8

Coody, why would you be concerned about water entering a pipe system designed to drain large amounts of water from your plumbing fixtures to the sewer in the street?

The leak is not originating from inside the pipe; the flashing boot is old and is leaking. If you went up there you would probably find a tear in it.

You can't pretend you don't know what would happen if you blocked the vent pipe; it has been made perfectly clear to you in many responses.

Here is what Wikipedia has to say about a blocked roof vent:

"A blocked vent is a relatively common problem caused by anything from leaves, to dead squirrels, to ice dams in very cold weather. Symptoms range from bubbles in the toilet bowl when it is flushed, to slow drainage, and all the way to siphoned (empty) traps and sewer gases entering the building."

The houses you are seeing without vents either have them where you did not notice them (they are usually black instead of white) or they have mechanical vents (air admittance valves) inside the house. If that's what you want you should call the local building department and ask if that type of vent is allowed in your area.

I suspect it would be cheaper to hire a roofer to replace the existing flashing boot.

April 21, 2011 at 8:56PM
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galore2112

It does not have to go through the roof.

From the plumbing code (I have the 2003 edition), P3103:

It must not be less than 4 feet directly beneath a door, openable window or other air intake of the building or adjacent building.
It must not be within 10 feet horizontally from such an opening unless it is more than 2 feet above the top of the opening.

If you terminate through a wall and not through the roof, it also must be a minimum of 10 feet from the lot line and 10 feet above the highest grade within 10 feet horizontally. They must not terminate under a soffit that has vents.

In a nutshell, terminating the vent through the roof is the cheapest, easiest method and therefore common.

April 21, 2011 at 9:54PM
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coody

Thanks for all your replies. I am learning. I took another to show two stuffs on my roof. The left one seems for the air flow. It does not connect any pipe. The right one is a pipe that I am talking about. It goes down and connects to my house drainpipe. Yes, I did see the built-in flashing. But the leak does not happen in that part, the black thing around the pipe. Rather, it happens in the lower part of the pipe where it has a . The U connector is loosing. So water leaks. Thus, I am thinking where the water flows from through the pipe.

Since the pipe goes down to the sewer under the ground and the water does not flow upwards, I then check the pipe from the other end that is through the roof. I am waiting for the rain coming because I do not know whether the rain will go into the pipe opening on the roof.

I guess the rain probably goes into the pipe opening on the roof. So the leak happens on the loosing U connector. Though I can seal the U connector, I think why I can't add a cover to cove the pipe opening on the roof if that is the root cause? If no water flows down through the pipe opening on the roof, the leak will never happen even the U connector is loosing again. Is it correct or I misunderstand something? Again, I am learning.

April 22, 2011 at 12:16AM
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renovator8

The water could be running along the outside of the PVC pipe from the roof flashing boot until it hits the elbow fitting.

If you are sure the elbow fitting is leaking, cut it out and replace it. From the photo it appears that the elbow might not have been glued.

It would be foolish to go out on the roof to install a cover when you could make the repair from inside for less cost, time and risk.

Fix what's broken and stay off of roofs.

April 22, 2011 at 7:49AM
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brickeyee

"From the photo it appears that the elbow might not have been glued."

Look very carefully at the elbow.
At the very least it appears no purple primer was used, and possibly it is not even cemented.

While PVC joints can be pretty tight without cement, they are NOT water tight for any length of time without cement.

If you can pull the pipes out of the elbow it would mean they are not cemented.

April 22, 2011 at 9:38AM
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klabio

coody,

After reading your thread I am getting the impression you don't have a lot of Do It Yourself skills. That is not meant to put you down but in your case it might be a good idea to call a plumber and tell him you are getting water on your ceiling from a problem with your plumbing stack. It could be as easy as gluing the joint but it could be the boot.

From the pictures you look to have a very steep roof. It will be very hard to get up there safely and if the flashing is leaking around the pipe your idea of capping the pipe won't stop the leak and it will cause smells in your house. The stack is there for a reason and a plumber will be able to stop your problem. It will cost you more than fixing it yourself but I don't believe that you have the experience to pull it off.

The members on this board have put it in fairly simple terms but if you don't have the basics your best bet is to bring in a professional. Especially if you ever plan to sell that house.

April 22, 2011 at 11:11AM
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renovator8

This project is rapidly turning into a candidate for the site linked below.

Here is a link that might be useful: awards

April 22, 2011 at 12:01PM
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coody

There was thunderstorm last night. I did not see water flowed down through the pipe. So, the pipe opening on the roof is actually water proof. I still have no clue why the elbow ever leaked. Where did the water come from? I am not going to investigate it but will seal the loosing elbow joint and patch the back side of the ceiling board.
I have pulled the pipe out of the elbow. See the picture. The back side of the ceiling board paper under the elbow joint has been black because of the leak. I am going to the Home Depot this weekend and buy the necessary material to repair the loosing elbow joint and patch the back side of the ceiling board. I hope the elbow joint will be leak proof but not permanently sealed in case I need to pull the pipe out of the joint without cut the pipe. The pipe right end goes to the roof and the left end goes down to share the washing machine drainpipe on the first floor. Could you experts tell me what material I should buy to tight the loosing elbow joint and patch the back side of the ceiling board? See the elbow joint and back side of the . Can you see what type of the ceiling board? The white stuff is insulation material covered on the back side of the ceiling board in the attic. Thank you for your help.

April 22, 2011 at 5:12PM
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robin0919

The guys at HD should be able to tell you what you need to do. Take the pics with you.

April 22, 2011 at 6:59PM
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sue36

If the look of the white pipe bothers you paint it the color of the roof. We did that and ours basically disappeared (ours is in the back, but a white pipe on a dark roof does stand out).

April 22, 2011 at 7:29PM
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coody

I am sorry I should say the right end of the pipe goes down to the first floor and the left end of the pipe goes to the roof.

April 22, 2011 at 7:45PM
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bus_driver

All those pipe fittings should have been properly cemented and permanently joined at time of installation. Some plumbing codes require the protection of PVC from ultraviolet degrade. Painting with latex paint is an approved method. Paint (with latex) the exterior exposed pipe any color you choose.

April 22, 2011 at 7:46PM
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renovator8

It would be silly to seal the pipe in the fitting with a temporary sealant. After the fitting has been properly sealed with primer and cement there should be no reason to ever disturb the pipe again.

How you repair the gypsum board damage depends on the degree to which the board is damaged on the bottom side.

Don't make simple problems complicated.

Here is a link that might be useful: primer and cement for PVC pipe

April 22, 2011 at 9:54PM
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renovator8

The drywall damage is beginning to look more like it was caused by mice rather than water.

April 23, 2011 at 4:48AM
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brickeyee

"I still have no clue why the elbow ever leaked. Where did the water come from?"

It leaked because it was NOT cemented.
If it had been cemented you never could have gotten it apart, the pipe has to be cut apart.
The water very likely may have come from rain or condensation inside the pipe.

"I am not going to investigate it but will seal the loosing elbow joint and patch the back side of the ceiling board."

You seal the pipe by correctly cementing the joint.
Any hardware store should have the required primer and cement.
Watch out, the purple primer permanently stains anything you drip it on.

The back of the ceiling is the least of your worries, the show side is what needs to look nice.

April 23, 2011 at 9:09AM
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coody

I probably have found where the water came from. The leak happens when the first floor washing machine running. You are right. The pipe goes down to the first floor and shares the washing machine drainpipe and bath room pipes. The leak is due to the water evaporation and condensation passing through the inside pipe from the first floor. The rain does not go into the pipe on the roof. My question is why there should have a pipe on the roof to evaporate the water inside the pipe into the air? It seems unnecessary to evaporate the water inside the pipe into the air but just drain out all water into the sewer, right?

April 24, 2011 at 5:06PM
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bus_driver

Explanations about the pipe through the roof have proved fruitless thus far. Will it resolve matters if we agree that the LAW says the pipe must go through the roof? Now the things remaining are to make the pipe itself leakproof by permanently cementing it with approved (by the LAW) materials and insure that rain is not entering around the EXTERIOR of the pipe.

April 24, 2011 at 7:04PM
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renovator8

It is certainly possible that moisture laden air rising in the pipe condenses when it reaches the colder pipe in the attic and that the condensate water leaks out of the pipe as it runs downward past the loose joint. But that would only happen in the winter.

Obviously, the rain does go into the pipe on the roof when it rains but just like condensate water it simply runs down into the sewer.

As you should know from the repeated explanations on this forum, the pipe on the roof serves only to avoid siphoning of fixture traps and to vent sewer gasses to the open air and has nothing to do with evaporation. You are either not reading the explanations carefully or you are a leg puller.

You should glue the pipe joint and move on.

April 24, 2011 at 9:13PM
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brickeyee

:It seems unnecessary to evaporate the water inside the pipe into the air but just drain out all water into the sewer, right?"

The pipe has nothing to do with evaporating water.

It is designed to deal with sewer gas and trap siphoning.

Try to actually read and understand the answers that you have been given.

April 25, 2011 at 9:20AM
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aidan_m

Could you post a picture of yourself?

April 28, 2011 at 4:47PM
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billkinca

"If the roof was not slope, I would really want to climb onto the roof and cover the pipe to see what would affect the house drainage."

That might be a fun experiment to try, but you will want to invest in a mop and a gas mask first. No sense in not being prepared.

July 9, 2011 at 3:39PM
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renovator8

July 9, 2011 at 9:23PM
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billkinca

"The drywall damage is beginning to look more like it was caused by mice rather than water."

Might be cheaper to invest in a cat...

July 10, 2011 at 1:30AM
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davidro1

1/.
I understand that this thread died "naturally" earlier this year.

I sense that a new generation of legpullers has been invented or created.
This year I've seen a number of other similar patterns in other forums.

So, beware! Professional exasperators have been released to the world. It's worse than Pandora's box. It might get so bad that one day it ends up killing all forum spirit.

2/.
Sewer gas is flammable, can be poisonous, and usually stinks.
It can also be both odorless and lethal.

August 3, 2011 at 10:11AM
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dgmarie

I do not think English is Coody's first language.

August 5, 2011 at 10:40PM
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