What is an S-Trap and why should I care?

An S-Trap is an arrangement which traps water in a return bend, much like a P-Trap, but has a second return bend into a vertical drain line instead of a J-Bend into a horizontal drain line. This could also include a standard P-Trap which connects to a horizontal drain line which turns down into a vertical drain line without proper venting. In either case the configuration can cause loss of trap seal through siphonage.

Pipes are filled with static air. When a flow of water or waste enters the pipe it displaces the air – pushing it forward down the pipe. As the water travels down the pipe a vacuum is created behind the flow drawing in new air, preferably through a vent in the piping system. In the case of an S-Trap the suction of the water moving down the vertical pipe pulls the water out of the trap until air can be drawn in through the drain.

Water closets are designed to take advantage of this action and are a perfect example of an S-Trap.

Water passing through the trap and down the pipe pulls all of the water and waste out of the bowl until the vacuum is broken by air. Then the tank valve slowly refills the trap and the bowl so that the seal is maintained. An S-Trap can work – as in the case of a water closet – if there is a mechanism to replace the water in the trap, otherwise the trap flushes and the seal is broken.

Today we rarely encounter S-Traps except in flushing fixtures. They are forbidden under the plumbing code. The most common mistake in a plumbing rough is making the drain arm – from trap to waste stack and vent – too long. If the top of the waste pipe at the point where it meets its vent is below the water level (Trap Weir) in the trap, that is technically an S-Trap – and acts as one causing the trap to siphon.

Since the waste line requires a minimum grade, the longer the horizontal pipe is the greater the difference in elevation. Eventually the top of the pipe at one end is the same elevation as the bottom of the pipe at the other. This is known as being out of the vent. That is because the top half of the pipe is helping air pressure balance between the vent and the trap. When the down side of the pipe gets too low you have created an S-Trap.

There are instances where S-Traps are virtually unavoidable. Most often this occurs on floor drains in a large area such as a commercial kitchen or warehouse. In these instances long combined waste and vent mains are used with the top half of the horizontal main acting as a vent. But as the line gets longer the distance from trap to drain becomes longer due to grade on the main piping.

Long vertical drops between drains and traps can cause another problem. The falling water gains momentum and at a certain point it will have enough momentum to pass straight through the trap leaving it dry. For that reason the code restricts the vertical drop to 24”. But what happens when your main is more than 24” below your drains?

This is one of those gray areas adjudicated by the local official. If you keep your trap within the trap weir then the riser to the drain is too long. If you raise your trap above the main waste/vent then you have created an S-Trap. Most inspectors prefer the S-Trap. In most instances the floor drain will be protected with a primer or other means which will insure the trap seal, so possible siphonage is not an issue, but a long drop in the vertical pipe will risk even the water from the trap primer overrunning the trap and leaving it dry.

We often find situations where piping cannot be installed by the strict letter of the code. That is why the code allows for the discretion of the inspector. This is also why it is important, not simply to memorize the code, but understand the intent of the code.

In our example the intent is to maintain the trap seal. Using an S-Trap, though forbidden by code, and a trap primer to insure sealing the trap is better than a long standpipe, also forbidden by code. By understanding the reasoning behind the code we can find efficient and effective installation techniques that solve the problems, or perhaps – as in the case of the flushable water closet – use the very problems that caused the S-Trap to be forbidden to design a fixture that revolutionized sanitary plumbing design.

The plumbing code is written as a guide to minimum acceptable standards. We should be working toward best possible practices and standards. To do that we need to understand why the code is what it is. We must ask ourselves why something is required or forbidden and then we will know not only what is required as a minimum, but how we can provide a better system. If you are chasing best practices instead of minimum standards you'll never worry about a failed inpection.

One thing to remember, however, if you are studying for the exam – the exam is based on the letter of the code and minimum standards. If a vent can be 1 1/2” by code and you size it 2” you are counted wrong. In practice there would of course be no problem oversizing a vent, and the inspector wouldn't say a word, but the test is a test of your knowledge of the code.