How To Look At A House
McGarry and Madsen's home inspection blog for buyers of
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What is an S-Trap? Why is it a problem?
Sunday, October 14, 2018
An S-trap looks like the photo above: kind of a sideways S-loop configuration before it heads directly downward. This type of drain has been banned by the building codes for decades, because it is considered an “unvented” drain. Venting is necessary for a sink to drain properly. In essence, sufficient air has to get into the pipe in order to displace the water and allow it to exit easily. A simple experiment can demonstrate the problem with an unvented drain. Place your thumb over a straw that is partially immersed in a glass of water. As you lift the straw out of the glass, the water level in straw stays intact, rising about the surrounding water in the glass. When you release your thumb, the water in the straw drains to the level of the rest of the water in the glass.
Although an S-trap is able to gulp some air for displacing the draining water, so it’s not exactly like the straw experiment, the air is not sufficient for good drain flow. Plus, S-traps tend to suck out the water seal in the trap (necessary to keep sewer gas from rising up into through the sink into the home) as they finish draining.
A properly installed P-trap, like in the diagram above, will always keeps it’s water seal. If you have S-trap drain, and notice sewer-type odors in the room, you can run the water slowly down the drain for a few seconds to replenish the trap-seal as a temporary fix. But, of course, the best solution is having a licensed plumber bring the drain piping up to modern standards.
Here’s an example of an S-trap that might not be obvious at first glance. Both sinks have a pipe configuration that creates an S-trap, with the potential for siphoning.
Another requirement of an efficient plumbing drain system is at least one vent pipe that extends above the roof, to allow sewer gas that rises into the drain piping behind the sink traps to exit into the atmosphere. The vent pipe also provides atmospheric pressure to the drains.
Occasionally, we see vent pipes that terminate in the attic or on an exterior wall near a window. Both of these installations allow the possibility of sewer gas migrating back into the home.
Although TV home-improvement shows sometimes make installing your own plumbing look easy, doing it right requires both training and years of experience. Plumbing repairs should always be performed by a licensed plumber. The plumbing inside your home is so critical that every building department in the United States requires plumbers to be licensed.
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