However, when comparing a toilet P-trap and S-trap, P-traps feature a modern design, reduce siphoning risks, and meet updated plumbing codes. S-traps are less expensive and fit older plumbing systems, but they're more likely to allow toilet gases to escape and may not meet modern plumbing codes.
P-traps keep their water seal better, so they're less likely to let odours into your bathroom. A skew trap connects through a side wall instead of the floor or back wall. It's perfect for tight spaces or bathrooms with unique plumbing needs and is usually easier to install.
The P-trap is now the standard since the S-trap was deemed ineffective and prohibited in new construction under the Universal Plumbing Code. The S-trap was found to be prone to siphoning and drying out, which leaves the apparatus ineffective.
S-traps were once a common sight in older plumbing systems, but they have fallen out of favour due to their tendency to siphon water out of the trap, leading to sewer gas leaks and drainage issues.
Answer: The biggest risk of an S trap is siphoning, which can cause the trap to lose its water seal. This allows dangerous sewer gases to enter your home, creating potential health risks and unpleasant odors.
This is because the “S” trap will siphon or suck water out from the trap which will end up releasing methane (sewer) gases into the home. So, it is a code violation and if you are doing remodeling, you will have to replace it.
The S-Trap's Major Flaw
The “s” design could cause a water siphon, removing the water built up in the curve, opening the seal, and allowing sewer gas to pass through. This flaw is whys-traps are no longer allowed in plumbing code and should be replaced if you have one.
What is an S-trap? S-traps are a type of plumbing trap that blocks sewer gases in sinks and other plumbing fixtures. The S-shaped pipe configuration traps wastewater before it drains to the sewer line, creating a water seal that prevents gases from flowing up through the drain.
If a P-trap has become empty of water or dried out for any reason, it will allow the sewer gas air inside the waste pipes to come pouring into where we live and work. Every P-trap on every drain needs to have a vent that protects it; that's what all those pipes are that we see sticking out of our roofs.
Code Compliance
S traps are prohibited throughout the United States by the International Plumbing Code. They have been banned since 2006 because of the siphonage hazard. It could cause the water seal to escape the conduit.
A toilet trap sounds like something you'd want to steer clear of, but it's actually a key component of a toilet drain that helps keep sewer gases out of your home.
The S-trap and P-trap are both fixtures in a plumbing system, but each function is different. The P-trap is designed to seal off the drain pipe when it meets the floor. On the other hand, the S-trap is used to seal off any water that might escape from a sink or other fixture downstream from the trap.
They offer a significant design advantage over S traps. P traps have several advantages over S traps: They are less likely to lose their water seal due to siphoning. They are easier to clean and maintain thanks to their more accessible shape.
Most toilets have larger trapways than the minimum standard for better performance. The most common sizes for toilet trapways are between 1.75 and 2 inches, although some models can be as large as 3 inches in diameter.
Your plumbing system, however, is a two-way street. A specialized fitting called a p-trap creates a powerful barrier that stops sewer gas and odor from entering your home. The trap is employed in any plumbing system that uses a drain and accepts sanitary waste from tubs, showers and sinks.
S-Trap: Best for floor-mounted drainage systems. P-Trap: Used when the drainage pipe runs through the wall.
Determine your pan type
The S-trap has a waste pipe that's connected to the floor, while a P-trap pipe connects to the wall.
They frequently siphon their trap seals, thereby releasing sewer gas into your home. That's why all U.S. plumbing codes outlaw S-Traps (IPC 1002.3, UPC 1004.1).
For this reason, S-traps was officially banned under the Uniform Plumbing Code, and P-traps became the new standard. The shape of the P-trap is just one feature that helps stop this from happening.
S traps can be avoided by venting the vertical drain that the p trap drains into and ensuring that the top of the horizontal drain does not fall below the trap weir. In retrofitting older systems, an air admittance valve can be used to relieve vacuum pressure, thus preventing siphonage of the trap seal.
In all of these cases, water has to pass through two separate traps, called a double trap. But don't confuse this with two separate traps, which is fine. Of course, those corrugated things in the photo above aren't proper, but the configuration is correct. Plumbing codes expressly prohibit the use of double 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.
Every sink, bathroom fixture, and drain line in a home is equipped with a P-trap, making them a universal and essential component of the modern plumbing infrastructure.
Full “S” traps, bell traps, drum traps, crown-vented traps, and running traps are prohibited. Fixtures shall not be double- trapped.