How to Repair Common Plumbing Drain and Vent Problems

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How to diagnose and correct common drain and venting problems in your plumbing system.

Plumbing problems can be very frustrating to repair, even if you know what the problem is and can access it easily. When it comes to drain lines and vents, this is not always the case. Lines can be clogged where access is limited or inside walls, vent pipes can be separated or never installed. Hopefully this article will help you diagnose some of the more common problems with plumbing drains and vents before you call a plumber and spend hundreds of dollars on repairs.

Drainage Systems

The basic components of the drainage system, commonly referred to as soil or waste lines, are the drain lines that carry water and waste to either a septic tank or public sewer system and the vent lines which allow air into the system while the water is draining and vents sewer gases that are created inside the pipes. The vents play a vital role in equalizing the pressure in the waste lines and prevent air from being sucked through other fixtures in the home. Sometimes this system is referred to as drain-waste-vent, or DWV.


The water in traps in the drain lines prevent sewer gases from entering the home, but when water flows through the pipe, the air between the trap and the main sewer line cannot escape and is pushed ahead of the water flowing down the drain line. This creates a vacuum which is why there is a vent line.

Vents must be sized correctly to work properly and in most cases they are the same size as the drain line they are venting. Typically sinks and tubs have a 2 inch vent and toilets have a 3 or 4 inch vent. Problems arise when additional fixtures are added too far away from the existing vent to allow air into the drain line. In some cases, the only path for the air is through a trap of another fixture. This is when you hear bubbling or gurgling sounds in your sink or toilet.

Types of vent piping

Main vent: This is the line of pipe running vertically beside the soil stack. Branch and fixture vents axe connected with it, and through it each trap is supplied with air from the outside atmosphere. The main vent is connected to the outside either by being extended through the roof separately or by being run back into the soil or waste stack at a point above the highest fixture connection.

Branch vent: Connected to the main vent, this pipe serves two or more fixture vents. It and the fixture vent should be run to a height above the overflow level of the fixture being served. This prevents sewage from draining through the vent line if the fixture waste branch gets clogged.

When properly connected, vent lines will remain free and any stoppages in the drain piping will be indicated by the backing up of waste into the fixture.

Fixture vent: Each fixture should be vented and trapped separately so that trouble with any one fixture will not affect any other part of the system. Fixture vents are used to prevent siphoning, to relieve pressure on trap seals, and to draw off back pressure from gases generated by decomposing sewage. It is usually run as a continuous vent, that is, only one fitting is used to connect vent pipe, waste pipe, and fixture trap or branch. This is most often accomplished by the use of a T or Y fitting.

Individual trap vent: A trap may be supplied with air near its crown by the installation of a vent at the trap itself. In this way, the possibility of a vacuum forming at this point is eliminated.

Dual connection: Though separate venting and trapping of fixtures is advised, there are certain exceptions to this rule. In the case of a combination unit, such as a laundry tub and sink, or when two such separate units are set back to back, a single trap and vent will suffice if the dual connection is made above the level of the trap crown. The water closet has its own trap built into the bowl and is vented separately to prevent back pressure, but not siphoning.

Dry Venting: This is a vent line that only carries air into the system.

Wet Venting: This is a section of vent pipe that acts as a vent for one fixture and a drain for another fixture; as in the case of a sink draining into the main and also tying into the vent. Wet vents must be one size larger than the drain line coming from the fixture.

Wet Venting

Air Admittance Valves

Air admittance valves (AAVs) are pressure-activated, one-way mechanical vents, used in a plumbing system to eliminate the need for traditional venting and roof penetrations. They perform the same function as a check valve does for water as in the case of sump pump discharge lines. A discharge of wastewater causes the AAV to open, releasing the vacuum and allowing air to enter plumbing vent pipe for proper drainage. When not in use the valve remains closed, preventing the escape of sewer gas and maintaining the trap seal. Using AAVs can significantly reduce the amount of venting materials needed in a plumbing system, allow greater flexibility in the layout of plumbing fixtures, and reduce long-term maintenance problems associated with conventional vent stack roofing penetrations.

AAVs can last for over 20 years and most US manufacturers offer a minimum of a 20 year warranty.

They should not be used as a replacement to a normal venting system as should be used sparingly. Some local building codes severely restrict their installation or forbid their use entirely.

Common Vent Problems

A few of the more common problems associated with vent lines are clogging, broken, or missing.

If the vent line is missing, you should have a plumbing come in and install a vent in the correct location of the proper size. Permits should be obtained and there will most likely be work to open up a wall to run the vent line and minor roof repairs to seal the roof penetration.

Clogging is fairly common, even though now water enters the vent lines. Birds sometimes build nests on the top of the vent which sometime fall into the pipe. Leaves and other tree debris can also fall into the vent. To keep this from happening, you can install a screen which fits into the end of the pipe or a mushroom cap which has a gap all the way around for air to enter the pipe and is attached to the vent pipe by 3 or 4 screws.

To unclog a vent you need to get up on your roof and run a snake down through the pipe. Use caution if you have a steep roof. Some people have used garden hoses to flush the vent line out, but if the clog becomes trapped below a fixture, you can fill up the drain line and backup a sink, tub, or toilet. Another option if you have an attic where the vent runs through is to carefully cut the vent line and run the snake through the line there. Reattach the vent line with either a rubber coupling or glue it together with a PVC coupling. Older homes may have either cast iron or copper vents, so it would be best to clear these from the roof.

A broken line is also common as sometime vents are run horizontally. Some jurisdictions allow horizontal venting for small distances as long as the vent pipe is pitched towards the fixture it is serving. This allows condensation to drain back down into the drain line. Some locales require vents to be installed at a 45 or 60 degree angle.

Double “S” traps are sometimes found under sinks and are prone to clogging as the water has to travel uphill and then downhill into the lowest trap.


If the horizontal distance between a plumbing fixture and the vertical vent piping is too great, the fixture may not drain properly, producing slow drainage or gurgling noises.

Poor drainage can be unsafe since there is the risk that a poorly vented plumbing fixture will lose the water from its plumbing trap permitting sewer gases into the home.

Other Common Causes

Plumbing fixtures that are installed more than 5 feet from the vertical plumbing vent that is supposed to serve them.

Island sink vents require special drain vent piping details to assure that such these fixtures will drain properly.

Sometimes a washer and dryer are moved to a more convenient location, such as room on the first floor instead of the basement, it is fairly easy to add the drain and water lines to serve the washing machine, but the vent is usually too far away and omitted.

Plumbing fixtures whose vent diameter is too small in the building or above the roof line can result in frost clogging in freezing climates.

Heavy snow can cover short vents and prevent air from entering them or gases from escaping.

Identifying the Problem

Identify which fixtures are producing the sound: if it is it all fixtures in the home then it is a system drain problem or that the building has only a single inadequate vent system or no venting at all. If the sound occurs only at a specific fixture, there is a blockage or vent problem to a specific bath, kitchen, or laundry area and its vent or drain piping.

By flushing the toilets you and then running a sink you will see if the drains are slow, and you may hear gurgling at a nearby sink when the toilet is flushed, take a look underneath the sink. Check the trap under the sink, if is an “S” trap it is probably not vented and the flushing toilet is trying to draw air into the drain line from the nearby sink when the toilet is flushed. If the trap is shaped like a "P" over on its side, with its horizontal outlet running into the wall, we can't see if the sink is really vented or not, but the style of plumbing is more modern and it might be vented. Look in the attic for a plumbing drain line that passes vertically up from the floors below and out through the roof. If you can't find one, the building may not have proper plumbing vents.

Look outside for plumbing vent pipes terminated up through the roof in one or more areas. If you see a plumbing vent far from where plumbing fixtures are located they may have been built without proper plumbing venting.

Check out the septic system: for signs of backup, blockage, or odors outside or muddy or soggy areas, smelly areas, may indicate that the septic system, or part of it, are failing.


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