Plumbing trap schematic (C) Carson Dunlop Associates Plumbing Trap Leaks
Cause & repair interceptor leaks

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Plumbing drain trap leaks:

This article describes the causes of leaks in plumbing traps (now called plumbing interceptors). We illustrate the locations of leaks and the different types of trap leaks that occur varying by the plumbing trap material: leaks due to loose connections, cracks, or corrosion.

This plumbing traps (interceptors) article series describes plumbing traps (interceptors) and how to diagnose, find, and cure odors in buildings including septic or sewage or sewer gas smells or "gas odors" in buildings with a focus on homes with a private onsite septic tank but including tips for owners whose home is connected to a sewer system as well.

We describe the common problems that occur at plumbing traps: odors, leaks, noises, and we discuss plumbing trap types, requirements, locations, connections, installation, repair and replacement. This article distinguishes between P-traps and older S-traps and explains trap siphonage and the dangers that can result from dry plumbing traps.

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Leaky Plumbing Trap or Interceptor & Other Defects

Photograph of a car radiator hose used as a plumbing drain trap

Here are some plumbing trap (interceptor) defects to check. Any of these can produce drainage problems and in turn may leak or cause sewer gas or septic odors to be produced in or even outside of a building.

Watch out: Improperly installed, damaged, leaky, or missing plumbing drain traps can release smelly and even dangerous sewer gas odors in the building.

[Click to enlarge any image]

Leaky, Missing, or Improperly Mounted/Located Plumbing Traps / Interceptors: auto radiator hoses & paint cans as plumbing traps?

If you are tracking down building sewer gases or smells don't forget to inspect the building plumbing traps for leaks, defects or improper or missing traps.

The photo at above left shows a plumbing "trap" that is guaranteed to produce odors: it's not a plumbing trap at all, but rather this plumbing drain uses a car radiator hose. This was a great idea for an emergency "Sunday night" plumbing repair, but not something to keep in a home.

Note that there is no bend in the trap to hold a water seal - sewer gases will pass readily out of this sink drain into the building. This home made drain pipe also leaks, as you can see by the black mold on paper located below the sink.

Below we see a rusty paint can enjoying a second career as a plumbing trap. Perhaps this midnight-repair was installed as a stop-gap measure to address a leaky trap. Like topsy this repair just grew and grew with blobs of sealant, lagging, plaster or whatever the repair person found at hand.

Our photo of the paint can plumbing trap was provided by Steve Smallman, a professional home inspector in Raleigh NC.

I suspect that originally there was a galvanized iron plumbing trap at this location - since we see that the surrounding drain piping is galvanized pipe.

Someone got so frustrated with leaks at the plumbing trap that he simply put a paint can around it and filled the can with cement, thinking "OK Buster! Let's see if you leak after I'm done with you!". Of course this repair makes future replacement of the trap or even cleaning it a bit of a chore.

Paint can used for home made plumbing trap (C) Steve Smallman

The good news is that often when plumbing traps are leaky from a loose connection the repair may be as simple as gently tightening the coupling nut.

But less good news is that when a plumbing interceptor is cracked or corroded it's time to replace it.

With a bucket under the trap to catch water that will surely spill, remove the trap parts, wipe them off, and carry the lot over to your plumbing supplier to buy new, properly-sized parts to match. That will make reassembly easy.

At PLUMBING TRAP LEAK POINTS we catalog the common leak points at plumbing traps you'll find additional details about tightening the trap connections without breaking them or over-compressing their washers.

Checking the PVC sink trap connections for leaks after a repair (C) Daniel Friedman

Drain Trap Leak Repair Tip

Above I'm checking the white PVC sink trap parts for leaks after repairs to the sink drain.

Run water for 5-10 minutes if you can spare it, then with dry fingers, feel around every sink trap and drain connector nut for water.

That way you may catch and fix a leak before it ruins your kitchen cabinet or bath vanity base.

Where the Plumbing Trap Leaks Occur: Pros and Cons of Plumbing Trap Material Choices

The following OPINION is based on decades of field observations made while inspecting building plumbing systems for leaks, on conversations with plumbers, and by a review of plumbing texts and manufacturer's installation instructions for plumbing traps.

Cracked plumbing interceptor trap nut causes leaks (C) Daniel Friedman

Watch out: one of the most-common causes of leaks at plumbing traps is an over-tightened trap compression nut or "lock nut" that cracks. The manufacturers of ABS and PVC drain interceptors or traps specifically warn against over-tightening for this reason. My photo shows that it's possible to crack even a metal plumbing trap connector.

Leak at the slip joint coupling  nut on an ABS plastic drain trap (C) Daniel Friedman

Above we see leaks at the coupling nut around the connection of the ABS plastic drain trap slip joint connector where it inserts into the ABS building drain.

We don't know if the internal washer was omitted, turned with its beveled edge in the wrong direction, or if the trap coupling nut is too loose, or if it was over-tightened and cracked. When we disassemble this plumbing trap to fix the leak, we will easily discover which of those snafus caused the leak.

Leak at the coupling nut on a chrome plated brass plumbing trap slip joint connector (C) Daniel Friedman

Above: it's clear that this chrome-plated brass plumbing interceptor trap is leaking at the coupling nut for the slip joint connector that connects the trap to the building drain.

Over-tightening the trap coupling nut can also squeeze the washer (don't leave that out) located inside of the coupling nut such that it deforms, also causing a leak.

Chrome plated brass plumbing trap, badly corroded, leaky (C) Daniel Friedman

Above: a corroded leaky S-trap.

Chrome-plated brass plumbing traps / interceptors like the plumbing trap shown above, are exposed to corrosion and leakage, like the corroded, leaky chrome-plated, thin-walled S-trap that we show above, found at a kitchen sink in a New York home.

Just how quickly the trap will corrode through to the point of leakage depends on several variables including the corrosivity index of the water being drained, corrosivity of wastes dumped down the drains, and the thickness of the original material.

Brass plumbing trap (C) Daniel Friedman

Brass plumbing traps have the same corrosion risks as chrome-plated plumbing traps & interceptors. Shown above, a corroded, leaky brass plumbing trap I examined in 2005.

Notice that the trap extension, or plumbing trap arm, also properly called the plumbing trap slip joint coupling is simply slip-fit into the horizontal copper drain pipe where that connection was leaking as well.

Chrome plated plumbing trap completely disintegrating from corrosion and leaking like mad (C) Daniel Friedman

The chrome-plated brass plumbing trap shown above has corroded so badly that it has fallen apart. All of the sink drainage simply falls into the cabinet below.

Notice the additional clogged material at the upper right bend that connects the trap to the building drain.

I think that leaving a plumbing trap or plumbing interceptor for years with a partial clog is a guarantee of a trap corrosion and leak problem even if the building occupants didn't mind waiting for the sink above to drain.

Trap leak points on PVC plumbing traps (C) Daniel Friedman

PVC as well as ABS or for that matter just about any plumbing trap is at risk of leaking at the trap connections to other building drain components like a sink drain basket tailpiece extension insertion into the trap's upper coupling nut (B), or at other trap threaded coupling connectors connectors (C).

But don't mistake a leak at the sink strainer basket (A) for defects in the trap itself. In our photo that stain running down the vertical sink tailpiece extension tells us that there is a leak from above. Not all of the leaks dripping into the plastic bucket under this sink are to be blamed on the plumbing trap.

Plumbing trap home made out of PVC drain piping and a cleanout tee (C) Daniel Friedman

What about a PVC trap that is made-up out of pieces of 2" PVC drain piping?

Above: this home made sink trap uses PVC piping and a black ABS pipe tee as a "cleanout" for the trap. This home made trap might "work" acceptably but I doubt it.

The engineers who design the shape, dimensions, proportions of a plumbing trap take into consideration the tendency of water to siphon out of a trap and design accordingly. This home made trap is un-vented and looks to me like an S-trap that is at risk of losing its water seal.

More about S-traps is at PLUMBING S-TRAP CODES & HAZARDS


Continue reading at PLUMBING TRAP MATERIALS & PROPERTIES or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

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