Your Home Inspector just called out an S-trap. What is it?
A trap is simply a configuration of the drain pipe underneath a sink or any other plumbing fixture. Years ago, a trap that looked like the letter "S" on its side could be seen beneath the sink of many homes throughout Northern New Jersey. And sometimes a do-it-yourself-er, when faced with a sink and a drain pipe that don't line up, avails themselves of the endless range of solutions at Home Depot and chooses to install an S-trap or a Universal trap. I can't believe they're actually allowed to sell them without a warning label.
In more recent years, most plumbing codes banned S-trap configurations in favor of P-trap style fixtures. S-traps are no longer used in modern plumbing because the water can be sucked completely 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, as sewer gases can be poisonous or explosive.
How else can an S-trap fail?
It 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. One fix is to simply refill the trap with water after using the sink.
Why is the Home Inspector calling it out?
An S-trap is an easy find and an easy fix, but usually not a big deal. If your house has an S-trap it is not a major concern; most people will never experience a sewer gas smell. The perfect solution is to tear out the wall and install a vent stack that goes through the roof. Luckily, there are three other things you can do:
It's probably OK to do nothing as long as you don't have a problem
You can install an Auto Air Vent (AAV)
You can modify an S-trap to work similar to a P-trap
The AAV is usually frowned on by most plumbing codes (and Home Inspectors always call them out) but they 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.
Another solution is to change the S-trap to a modified P-trap by extending the trap arm. A modified P-trap is a better solution. A plumber can usually find a way to make a P-trap work and correct the deficiency. S-trap to P-trap conversion kits are available from hardware and home improvement stores for $30 to $40.
Neither fix is allowed by most plumbing codes for new construction, but they are probably the best solutions for older homes. These alternatives are much more palatable than opening a wall and making a hole in the roof for a correct, modern vent.
However, please note that proper stack venting is always preferred to alternate methods when possible.