Sewer Gasses and Water Traps


Modern western living uses a lot of water to wash our bodies, clothing, dishes, hands, and of course flush our toilets. As we don't want all the waste water in our homes, we connect our drains to waste and sewer piping to carry it away.  Unfortunately the various organic materials, household solvents, and bacteria can combine in the sewer to create bad smelling, unhealthy, and even possibly explosive gas risks to occupants in the home.

Sewer Gasses

Everything you or your neighbours drain down the sink will contribute to sewer gases and this can include nasty products like paint thinners, gasoline, bleaches, and varnishes. In addition to direct toxic chemicals poured down drains, the breakdown of organic products from human waste and dirty dishes will produce gasses like:

  • Methane - Methane is a byproduct of the organic breakdown of waste materials. In high enough concentrations, methane can be explosive.
  • Hydrogen Sulphide - This rotten egg smelling gas can be detected by human smell in very small levels and can cause irritation of the eyes, coughing, headaches, dizziness, and in extremely rare household circumstances it can cause loss of consciousness and death 
  • Carbon Dioxide - This gas exists as part of our normal breathing air however long term exposure to higher concentrations can lead to breathing issues and heart rate problems
  • Ammonia & Others - There are many other gasses that can exist in the sewers depending on what has been recently drained. Health effects vary with each but these would be less rare than the gases above.

Water Traps

In order to prevent sewer gases from following waste pipes back to our drains, a simple low-tech solution called a water trap is used. A water trap, located as near each drain as possible, is used to trap some of the most recently drained water creating a blockage that prevents low pressure sewer gasses from entering the home. The most common configuration for these traps should be visible in sink cabinets and is called a 'P' trap based on the shape.

Common 'P' Trap. This example is chromed but they can be commonly plastic, brass, or copper as well.

'P' Traps

The 'P' trap uses gravity to push water from above the drain level through a half loop and down the discharge pipe.  When the drain above empties, pressure on the drain stops and a small amount of water is 'trapped' in the bottom of the loop creating the air trap. Water flowing in the discharge pipe past the trap can create suction which would draw the water out of the trap which is why discharge pipes need to have an air 'vent' connection to break the suction and allow water to flow only with gravity, leaving the water in the trap.

'S' Traps

Incorrect 'S' Trap will siphon water out of the loop.

'S' traps are the most common incorrect trap arrangement found in home inspections as they are commonly done by amateur plumbers who are not aware of the suction effect in the drain line.

An 'S' trap is created whenever there no air vent in the drainage line before another change in direction for the water drain. In some cases this will make an obvious 'S' shape, other times it will look correct under the sink but a closer examination may reveal the drain pipe directs water down at the wall with no vent before hand.

Double Trap with first trap from sink disposal on right re-trapped after connection to left sink side.

Double Traps

Another common trap problem happens under double kitchen sinks or under double bathroom vanity sinks which is double trapping. Only one trap is needed where there is one drain line to the air vent. Double trapping a sink waste line will either create an 'S' trap situation or it will encourage blockages in the drain because of poor water flow.

Toilet Traps

All drains in the home including showers, sinks, and floor drains need to have correct 'p' traps. The only water drain in the home that does not need a trap are toilets as the design of the porcelain system has a built in trap. There are a few different ways these traps can be done in toilet systems but as these are engineered, we only need to know there is a correct one in place. 

Trap Primers

One risk of water traps is that if water is allowed to evaporate out with time, there won't be any water left in the trap to prevent gases from entering the home. This is not a very common problem with frequently used sinks, laundry, and showers but it can be a problem for infrequently used fixtures and particularly floor drains which may never see water unless it is an emergency.

In order to keep water in the trap of floor drains, a trap primer can be installed on a water supply pipe to direct a very small trickle of water to a floor drain. Other trap primers will direct a small amount of water from a frequently used tap, like a laundry tub, to the floor drain. Alternatively to a trap primer you can pour a small amount of vegetable oil or mineral oil into a trap to prevent water from evaporating but this will only last until water is run down the drain.

Final Thoughts

Sewer gases are one of the concerns that home inspectors are looking at when it comes to plumbing systems. Incorrect drain traps are often a sign of amateur plumbing work which should also be a red-flag for home buyers that not all the work in the home is done by professionals. As always, having a home inspection before buying a home is a home buyers best protection that their home will be safe and solid.

By James Bell - Owner/Operator of Solid State Inspections Inc.