S-Traps vs P-Traps – October 27, 2014

S-Traps vs P-Traps – October 27, 2014

S-traps vs p-traps were common Years ago however most plumbing codes have banned S-trap configurations in favor of P-trap style fixtures. S-traps are no longer used in modern plumbing because on rare occasions with just the right conditions the water can be sucked out of the trap allowing sewer gas to enter your home.


The risk from sewer gas can be much worse than just that terrible smell. Sewer gases can be poisonous or even explosive and are not to be treated lightly.

S-trap failure

S-traps failure requires a large amount of water to flow quickly past the trap. This is

s-traps are longer approved for use in plumbing

usually caused by allowing a sink full of water to drain rapidly. The easy fix is to always refill the trap with water after using the sink.

Plumbing codes, however, do not allow for common sense fixes, they are designed to make things work as predictably as possible. Years ago most plumbing codes banned S-trap configurations in favor of P-trap style fixtures. S-trap fixtures are most often found in older homes and homes remodeled by do-it-yourselfers.

On the other hand, if your house does have S-traps it is not a major concern, most people will never experience a sewer gas smell. The perfect solution would be to tear out the wall and install a vent stack that would go through the roof.

Luckily there are three other things you can do:
• It is probably OK to do nothing as long as you do not have a problem
• You can install an Auto Air Vent (AAV)
• You can modify an S-trap to work similarly to a P-trap

Here are two possible solutions for s-traps

The AAV is usually frowned on by most plumbing codes but may be the only answer short of tearing out the walls and ceilings to run a vent through the roof.

Many different styles of AAV are available starting at $10 and S-trap to P-trap conversion kits are available from hardware and home improvement stores for $30 TO $40. Please note, proper stack venting is always preferred to alternate methods when possible.

Another solution is to change the S-trap

to a modified p-trap by extending the trap arm to a maximum of 2 1/2 times the diameter of the drain line (3 1/4″  for 1 1/4 and 3 3/4″ for 1 1/2 drain lines).  Neither fix is allowed by most plumbing codes for new construction.  The alternative however is much more palatable than tearing out the wall and busting a hole in the roof for a proper vent.

a modified P-trap is a better solution

Properly maintained s-traps do not necessarily need to be replaced, however, there are acceptable solutions should you experience sewer gas odors.

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