Plumbing vent

Does anyone know if there is a minimum distance that a plumbing vent should be from a washroom fan vent or roof vent?

I did a recent roof inspection and noticed that the plumbing vent is approx. 1’ away from a roof vent. Should I call this out?

No I don’t. Personally I don’t think I would make a big deal out of it. The only possibility of sewer gas entering the attic vent is probably nothiing to worry about, particularly if there are adequate soffit and roof vents.

Ray is correct. Which leads to the question of the purpose of the plumbing vent.

As I understand it, the purpose of a plumbing vent is to equalize pressure in the drain lines so that vacuum situations do not occur, allowing the waste water to drain properly, not to vent sewer gas.

Traps in the drain lines prevent sewer gas from entering the drain lines.

Hope this helps



They must vent something otherwise you would not see frost develop on them in the winter or see moisture vapour being emitted sometimes on really cold days.

I always thought they allowed air in and out?


I would suggest that any open pipe would let air in and out. I belive the primary purpose of the plumbing vent is as I noted earlier, but I could be wrong


Drain, Waste, and Plumbing Vent Systems Explained

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by: askafriend
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Drain waste and vent piping are the two most important components in plumbing. These are the systems that take water away from your home. Let us examine how do they work. Drain Waste-Vent System and Gravity Earlier galvanized caste iron was used in the piping system, but now-a-days PVC piping is generally used. PVC piping is much better than the galvanized caste iron, as the later one clogs much easier. This distribution PVC piping system brings the water to the toilet bowl and the drain waste-vent system takes all the used water and waste away from the home to the sewer or septic system. The functions of most sewer systems are based on gravity. That is the reason why everything goes down. In this process, the wastewater runs down the drain, enters the waste line, and finally leaves the bowl. Supply lines in the drainage system are fitted at forty or ninety degree angles. This drainage specification goes with the flow and helps the gravity of the sewer system to move the wastewater. The Venting System Venting system is a piping system and one of the most important components of drainage. The venting system consists of pipes. You must have noticed several pipes sticking up through the roofs of the houses in your neighborhood. These pipes are the venting system. This system is also an essential requirement by the building code, as it prevents the water to draw out of the traps that are under sinks. Moreover, the other main function of this system is to allow gases to escape. It also helps wastewater to drain properly by relieving air pressure. House Trap and Lateral Line When the plumber ties the waste water pipes into the sewer line, he/she also installs house trap and lateral line. The main function of the house trap is to keep the waste and odors flowing out of the home, while the lateral line ties the house to the municipal sewer line. This lateral line first goes into a sewer system down the street, and then it collects through a main distribution line, and finally goes to a sewer processing plant. Air Admittance Vents Air admittance vents are fitted under the sink. It helps sewer gases from coming out while drawing a bit of air in from under the sink. This way, these vents save money for you by reducing the amount of waste line vent stacks that you run. Moreover, they also help in reducing the possible number of penetrations through the roof. Other Important Things ?- The sewage lines coming into the house must be higher than where they go to connect to the city sewer line. This is an essential requirement, as we know that the sewage moves by gravity. ?- There must be at least 2%-3% fall in your sewer system. However, it depends on the elevation of your home as well as the elevation of the city, which is the location of the connection. ?- The fall the drain waste-vent system that carries fluids through the sewage system must be at least 0.25 inch per foot. ?- There must be two-degree fall into the main line for the lateral. Hence, here we see what are the components of a drain waste-vent system, and how do they work.


Looks like Doug and I are both right. :slight_smile:

You asked!!

(1) Yes, the venting does equalize pressures and prevents traps from possibly being sucked dry.

(2)Following on #1 of equalizing pressures: By allowing air in front of the waste/water to vent, the possiblity of trap seals being “blown out” (with gas release) by positive air pressure is prevented.

(3) Venting allows drains to flow more freely by (a) relieving air pressure in front of a slug of water/waste as it move down the pipe and (b) by allowing air into piping behind the slug of waste to remove the vacuum created by the draining waste itself (imagine emptying a pop bottle by turning it upside down…it’ll drain quicker if there was another hole to allow air entry in the now upside down bottom of the bottle).

(4) Yes, sewer gases and other stinky/noxious/explosive vapours (from crap that people should not be throwing down their drains ) are released to the atmosphere. These generated gases would create a positive pressure in the system and blow up through the trap seals, if not released.

Not that well written!!

The main function of the house trap is to keep the waste and odors flowing out of the home, (most sytems do not have a house trap today. This is incorrect anyways)

A trap does not function to keep waste flowing out, that is the function of the slope and gravity.

I would not put a house trap on a septic system, septic gases have to be released and thats the function of the venting system at the roof.

It wouldn’t make any differance if you did or not as far as releasing septic gases.
The vent stack is not the only part of the septic system that is open to atmosphere.:wink:

Where is the other open area to atmosphere? Sure you get off gassiing over the leach bed, but there still has to be a release of pressure inside the tank as waste enters and exits.

Unless the the discharge line is plugged between the tank and the leech field, any gas will vent through the leech field vents.:slight_smile:

Nice link BTW.

Leech field vents?

Someone really liked my quick critique of the barely passable explanation of DWV systems with a red demerit and signed it, too! They must have written it! WEAK! WEAK! WEAK!

“What a butthead critique - Mr.X”

… and the moral of the story is turn off your rating system, like I did. No one can rate me unless they post it on the open forum.

I guess Mr. X is the real butt head.

Your point?

I have never seen a leech field without vents in this area unless they had been damaged and buried.

The septic tank is vented via the roof vent.

I have yet to see a leach field vent system in my area. The fact is in my area the roof vent is the venting system for the septic tank.