Many older homes, in which the plumbing has not been updated, have a flaw in their plumbing. The flaw resides in the type of trap that is used with plumbing fixtures such as sinks, showers, toilets, and bathtubs. The two types of traps you will most likely encounter are an S-trap and a P-trap. The purpose of any trap, whether it is an S-trap or P-trap, is to keep out harmful sewage gases from entering your home as well as preventing large items from passing through and harming the deep interior plumbing. (If you have ever dropped some jewelery down the drain it is the trap you can thank for saving it!) Older homes used an S-trap compared to the newer design of the P-trap.
However, there is a flaw with the S-trap that causes it to not work properly. The S-trap has the potential to water siphon, meaning that the trap loses the water which creates the seal that prevents harmful gases from entering your home. The flaw can lead to a potentially harmful backup of gases into your home. If there is a foul odor coming from your sink or toilet it is most likely caused from the flaw in the s-trap. Subsequently, this flaw has led both the Us and Canada to banning the use of s-traps. Older homes still have s-traps and they are still available for purchase, however if your home is being inspected it may not pass inspection. To ensure the safety of everyone in your home, as well as bring it up to date with building codes, it would be a good idea to replace all S-traps with P-traps.
The P-trap garners its name from its actual shape, which resembles the letter "P". P-traps commonly come in PVC or ABS piping, but are also available in a variety of finishes for when the pipe is exposed and in clear view. The newer design of the P-trap fixes the problem with water siphoning and ultimately keeps your home from developing a backup of harmful sewage gases. The P-trap will always hold the water seal that blocks out the gases. P-traps can be used for plumbing that exits through both the wall and the floor, making them usable for any plumbing fixture.
There may be some difficulty in replacing your old S-trap to a P-trap so it would be advisable to seek professional help before taking on the replacement yourself. Do not risk having foul odors and harmful gases in your home and update to the P-trap.