Have you noticed any foul odors in your basement that you just can’t get to the bottom of?  It might be sewer gases coming in to your home.  Every plumbing fixture needs to be equipped with a trap, which is basically a dip in a pipe that water fills up.  This water sitting in the trap is what prevents sewer gases from coming in to your home.  The photo below shows a “P-trap” – this is the type of trap you’ll find below sinks, showers, and bathtubs.

A trap is one of the most important items in the plumbing system.

Question – What are (plumbing) Traps all about ?

Answer -They keep the sewer gases out of the home. Not only are these gases unpleasant, they’re unhealthy. All plumbing fixtures have a trap (or they should have one).

Question – How does a trap work?

Answer – The good news is they’re basically very simple.  As you can see in the drawing the water in the trap blocks the “bad” air from the sewer from entering the home, and goes up and out to the exterior through the vent. When water goes down the drain the water level in the trap rises and fills the trap keeping the “bad” air from the inside of your home. As more water fills the trap the water level rises over the “weir” and goes down the drain.

Water flowing down the drain

Trap full of water keeping Mr. Yuck out

There are many different types of traps. The most common and correct is the “P” trap. A “P” trap can be a solvent trap, where all the components of the trap are glued together. These are typically used where there is no access to the trap; e.g. buried in a floor or wall for a shower or clothes washing machine. And there are removable traps for cleaning; these are secured with compression fitting one at the tailpiece and one at the trap arm. Traps get filled with lots of nasties such as hair, soap, grease…etc. Simply remove the trap, flush it out, and reinstall it. When a solvent trap gets clogged it must be cleaned out with a “snake” tool. As this is more difficult and can scratch the finishes of the fixture a removable trap should be used whenever practical.


“S” traps are no longer installed “or shouldn’t be”. When a sink full of water is drained into an “S” trap a syphon is created and will suck all the water out of the trap rendering it useless. Should this happen simply run a little water into the drain to fill the trap. As doing this can get old fast, you may want to have a plumber remedy this.

The shape of the trap is important so as not to continually fill with nasties. A trap should be half round and smooth as in the drawing.

Fixtures that are not in use will also dry out, The water just evaporates! This will happen in 2-4 weeks of non-usage. Capping or plugging them is the best way to seal them off – or just run some water down the drain periodically.



Bottle traps are decorative and are used when the trap will be exposed.

There are many rules in installing a plumbing system. It is far more complicated than water goes down.

Your Home Inspector will identify any defects. Here are some defects – None of these will work!





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