Plumbing S-Traps

By Brandon Lyczewski, Pro-Tech Home Inspection Services

You may have purchased or considering the purchase of a home that was not built within recent years. If this is the case for you, you may find that some or all of the plumbing fixture drains in the home have S-traps installed. If you had a home inspection prior to your purchase, the inspector may have recommended you replace S-traps on fixture drains and install proper P-traps, but why? To understand why it is recommended to replace these traps, we should understand the purpose of traps on drains and how they work.

First, a word on venting. A residential DWV (drain, waste, vent) system is designed to maintain neutral air pressure inside the drain lines. When a column, or “slug” of water passes through the waste pipe, it compresses the air in front of it while creating a vacuum behind. Both of these conditions need to be mitigated in order for the drain to work properly. If the air ahead has nowhere to go, it can impede flow or create positive pressure elsewhere in the system. If the air behind isn’t replaced, it may be drawn through the drain and siphon the trap dry. Proper venting is a critical component of waste drain design to regulate these pressures, and most modern homes have multiple vent stacks that extend above the roof line. When plumbing is retrofitted on an older home, it’s not always as easy to ensure new fixtures are vented properly. Some old homes may have just a single vent stack.

A trap works by a bend in the drain pipe which traps a certain amount of fluid, referred to as a trap seal, in the drain. Thus, the name “trap”. This fluid retained in the drain provides a barrier that sewer gases, odors, and mice or other pests cannot pass through to enter the dwelling.

S-traps are easily identified in homes because they are bent in the shape of an S. The lack of a horizontal run means the column of water passing through the drain is particularly susceptible to siphoning. This siphoning action may be accompanied by an audible “gurgling” sound from the sink drain. If the trap seal is siphoned dry, we’ve opened a passageway for sewer gasses, odors and pests/rodents to gain entry into the living space. For this reason, most jurisdictions do not permit S-traps as part of the waste drainage system. Here’s an illustration, courtesy of Carson-Dunlop, to help you understand how the S-trap can be susceptible to a siphon.

The trap design permitted by current plumbing codes is called the P-trap, which is easily identified by its “P” shaped bend. P-traps help prevent siphoning by utilizing a horizontal run that helps to slow the flow of water and facilitate venting. Here’s a simple illustration courtesy of InterNACHI to help show the difference between the trap designs.

Even with P-traps installed, fixtures that are not commonly used may dry out through the process of evaporation. If there are fixtures in your home that are rarely used (including floor drains that do not service condensation lines), it would be a good housekeeping tip to run water through them regularly. If you ever suspect sewer or septic gas odors, this is a good first step to take.

Practically speaking, many S traps perform just fine but the configuration is not proper or “by the book,” for good reason. Replacing S-traps in your home is a good proactive measure to take to upgrade your plumbing. There are a few cost-effective ways to replace S-traps that are not vented properly, such as replacing S-traps with P-traps and installing a Studor AAV (air admittance valve). This application is not generally a code compliant method, but is sometimes used when installing a new vent stack or connecting to an existing one is impractical (such as in additions, kitchen islands, etc). It is always the best recommendation to consult with a licensed plumber to obtain advice on the best method for improvements or fixes.