S-trap vs P-trap Explained in Easy to Understand Terms

The S-trap is no longer allowed in new construction

Years ago most plumbing codes 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 requires a large amount of water to flow quickly past the trap. This is 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 makes 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 similar to a P-trap

Here are two possible S-Trap solutions

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.

AAV valves may work but are not the real solution for s-trap ptoblems not usually code approved
an AAV valve is one solution

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 time 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

using a modified p-trap is a better solution
a modified P-trap is a better solution to replace a s-trap

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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Some information came from the following sites
http://en.wikipedia.org/wiki/Trap_(plumbing)
http://www.doityourself.com/stry/kitchen-sink-drain-ptrap-or-strap
http://www.structuretech1.com/2012/06/a-primer-on-s-traps/

http://www.theplumbinginfo.com/plumbing-traps/
http://www.plbg.com/forum/read.php?1,483467

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