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Originally published Friday, December 14, 2012 at 8:00 PM

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Plumbing the function of S-trap drain

The industry standard for new and remodeled kitchens and bathrooms is a P-trap drain that, when properly installed, will maintain a water seal in the lowest part of the trap.

Scripps Howard News Service

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Homefix

Q: Last week, my home was inspected for the buyer. When my agent gave me the buyer’s request for repairs, one thing took me by surprise.

I have lived in this home for over 20 years and never had a problem with the plumbing, yet the inspector said I needed to replace the traps and vents for the bathroom. Something about an S-trap not working and there was no vent.

Sometimes the sink and the shower are slow to drain, but I just snake them and they work fine. Is this something I can do myself, or do I have to call a contractor?

A: S-shaped drains, also known as S-traps, were used for many years before industry-standard changes.

First, we need to talk about the plumbing vent. Without a vent system, the drains will drain very slowly and can become easily clogged again and again. That’s why you had to snake the drains so often to make them work.

A vent does two things:

• It allows sewer gases to escape to the outside through a vent pipe, which extends above the roof.

• The vent provides air to the drains. Without air behind the moving water, the drains will create a negative pressure between the fixture opening and the water, and the water will drain very slowly.

If you have ever put your thumb over a drinking straw and pulled cola from a cup, you have created a negative pressure inside the straw and it holds the cola against gravity. It’s only when you take your thumb off the straw that the cola drains back to the cup because the straw can now vent.

This brings us to the role of the S-trap.

The purpose of a drain trap is to hold a small amount of water to prevent sewer gases from escaping through the sink or shower drain. The toilet has a built-in trap that keeps the bowl partially filled at all times.

Without a vent, an S-trap can allow the water in the fixture to drain so quickly that it often siphons the trap dry and exposes the room to sewer gases.

The industry standard for new and remodeled kitchens and bathrooms is a P-trap drain that, when properly installed, will maintain a water seal in the lowest part of the trap.

There are strict codes for installing a P-trap and vent system, and my recommendation would be that you contact a licensed plumber. The health of the new buyer is at risk if the plumbing is not installed correctly.

Dwight Barnett is a certified master inspector. Contact him at C. Dwight Barnett, Courier & Press, P.O. Box 268, Evansville, IN 47702, or visit www.barnettassociatesinc.com. Sorry no personal replies. Always consult local contractors and codes.


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