How To Fix a Leaking PVC P-Trap or Drain Pipe Under Your Kitchen Sink, Wash-Hand Basin or Bathtub

How To Fix a PVC Drain Pipe Leaking at Joint.

14 Reasons Why Your P-Trap or Drain Pipe is Leaking

What is a P-Trap?

A P-Trap is a type of waste water trap used and fitted under kitchen sinks, laundry sinks and wash-hand basins. This trap is also often seen behind a toilet (WC) using a much larger 100mm diameter waste pipe, and it can be installed under bathtubs and showers.

The P-Trap derives its name from its shape featuring a horizontal tubular end and U-shaped tube to form a P. Otherwise, there are other types of waste water traps in use today, for example the bottle trap and S-trap.

PVC P-Trap Cross Section Details_2

Who Invented the Plumbing P-Trap?

The P-Trap is also known as the Thomas Crapper trap after its inventor and designer. In 1880, Thomas Crapper redesigned the S-Bend into a much better U-Bend which would later on be called a P-Trap in this modern era. The S-Bend was invented by Alexander Cummings in 1775 as a solution for a foul-proof anti-smell flush toilet drain. The household toilet invented by Sir John Harrington in 1596 was the first mechanically operated flush toilet making use of a gravity water tank, valve and bowl, rather than relying on a diverted manmade stream to flush away the waste. Despite its usefulness, the Harrington flush toilet had one problem – the foul smell from the waste pipe would find its way into the room and entire house. Alexander Cummings came up with the S-Trap, an idea that would prevent foul air from escaping the drain pipe into the house by creating an air-tight water seal in the U-bend section of the trap.

PVC S-Trap Cross Section Details_2

How Does a P-Trap Work? And Why You Have Foul Air from the Sewer in the House

A P-Trap works in a simple way. The U-section of the P-Trap is what makes it useful. It traps water when the tape water stops running or when the sink is completely drained. The trapped water provides an airtight liquid seal that stops sewer gases from passing through to the sink and into the building. Sewer gases have an obnoxious smell, but they can also pose a health hazard due to their toxic elements as well as a fire hazard when ignited. Sewer gas contains highly flammable compounds like methane and hydrogen sulphide. That’s the reason why you need a P-Trap to shut out these potentially dangerous gases. But a P-Trap only works if there is stagnant water in the U-Bend. Without this water seal, sewer gases will pass through. Conditions which lead to the P-Trap drying up include not using or running water in your sinks for a long time, for example when you go for a months-long vacation. However, it’s not just a dry P-Trap which causes sewer gases to flow into your house. Even if there is a water seal inside the trap, a change in the air pressure on both sides of the drain pipe may cause sewer gases to pass through. When venting (ventilation pipes or stacks) is not installed on your waste water outlets, air pressure will build up in the sewer pipes, forcing the foul air through the water-filled traps and into the building. In this case, aeration will cause bubbles and gurgling which can be heard in the trap.

Whereas high pressure from sewer pipes will force sewer gas through the traps, lower pressure will suck water from P-Traps out to the outlet, leading to a dry trap.

Why The P-Trap Holds Water

When a kitchen sink is drained by removing the plug or when tape water is running, water will pass through the U-Bend until pressure on both sides of the U-Bend reaches an equilibrium. When all the water in the sink is drained or when the tape stops running, the equilibrium pressure is reached. It is this equilibrium pressure as well as gravity that keeps water in the P-Trap.

Why is My Old or New P-Trap Leaking?

If the P-Trap under your kitchen sink or any other sanitary fittings is leaking, there might be one or more reasons causing the leak on the PVC joint or drain pipe. The following are possible causes of a leaking PVC P-Trap:

Broken rubber seal ring or gasket
Dislocated rubber seal ring or gasket
Loose fitting rubber ring seals or gaskets
Loose fitting inlet and outlet slip joint nuts
Loose fitting slip joint coupling nuts
Dislocated washers
Broken washers
Cracked drain pipe or trap
Misalignment of P-Trap pipes/sections
Misaligned slip joint threading
Improper installation of rubber seal rings and gaskets
Wrong methods of joint sealing and pipe jointing
Overtightening of slip joint nuts leading to deformation and cracking of nuts
Clogged drain pipes

Broken, dislocated or loose fitting rubber seal rings or gaskets

In adjustable PVC traps like the McAlpine inlet tubular 32mm P-Trap and Caroma 40mm P-trap, the inlets and outlets are fitted with rubber ring seals or gaskets to make a watertight connection which prevents water from leaking through the joint. In an old trap, the rubber rings may be broken or split causing leaks. Check where the leak is coming from, disassembly the PVC trap, inspect the rubber seals and replace them if necessary. In both old and new PVC traps, the rubber seals may be dislocated or loosely fitted due to incorrect size or pipe diameter specification.

Loose fitting slip joint nuts at the inlet, outlet and couplings

Slip joints on an adjustable PVC trap are connected with threaded PVC slip nuts which are tightened manually using your hand. Loose fittings may be the cause of leaks. If a leak is coming from a slip joint, make sure that the nut is sufficiently tight but not too tight as overtightening may break or damage the plastic nut.

Broken, dislocated and loose rubber washers

Just like rubber seal rings, rubber washers in PVC traps may be broken, dislocated or loose. Once again locate the leak and inspect the joint for any broken or dislocated rubber washer. If the washers are fine, make sure that the nuts are sufficiently tight and compressing against the washers to keep them in place.

Cracked Drain Pipe or Trap

Inspect the PVC trap as well as the inlet and outlet pipes in the plumbing system for any leaks. The pipes may be cracked, and if this is the case, you should replace the cracked pipes, but first find out if the leak or crack is small enough to be repaired permanently with PVC cement ,Epoxy putty or fibreglass resin cloth. Solvent welding also known as solvent bonding is a process whereby the ends of PVC pipes are joined and fused together to form a watertight joint using a PVC chemical welding cement or glue.

Misalignment of P-Trap Pipe Assembly

Even if there is nothing broken, cracked, dislocated or loose on your PVC Trap assembly, the vertical and horizontal alignment of your assembly in 3D space is critical to preventing leaks and maintaining the correct position of your PVC Trap. Misalignment of the assembly is usually the cause of leaks, especially in adjustable PVC Traps and bent configurations. Make sure that the assembly parts are in their precise vertical and horizontal planes. When you get the alignment correct, leaks will disappear like magic.

Bent P-Trap_Alignment in 3D Space

P-Trap_Bent Configuration_L-Shaped Plan

P-Trap_Bent Configuration_L-Shaped Plan_2

P-Trap_Bent Configuration_L-Shaped Plan_3

Misaligned / Uneven slip joint threading

When turning and tightening the slip joint nuts on a P-Trap assembly, you must make sure that the nuts are threaded properly and evenly with the nut face at 180 degrees to the face (circular cross section) of the threaded joint pipe. In other words, the nut face and the circular face of the joint pipe should meet in a straight line and be tightened linearly. When the faces are connected and tightened unevenly at a slant or slope, with more threads showing on one side of the pipe than the other, the joint will be susceptible to leaking. See the diagram below:

Alignment and Misalignment of PVC Pipe Slip Joint Threading – Male and Female Pipe Fittings

Improper installation of rubber seal rings and gaskets

In one case shown below, the outlet of the PVC Trap was improperly joined to the outlet waste pipe using silicone caulking. When the trap was dissembled, an inspection revealed that there was no rubber seal ring on the outlet waste pipe slip joint. The plumber assumed that the gasket rubber ring inside the PVC trap outlet would prevent leaks, and then proceeded to seal the joint using silicone caulking on the outside. You can clearly see a thick coat of loose silicone caulking on the slip joint nut. Silicone caulking was also unnecessarily used in other slip joints of the P-Trap assembly.

S-Trap with Bent L-shaped Configuration Under Old Kitchen Sink - Slip Nut Joint Sealed with Silicone Caulk

S-Trap Under Old Kitchen Sink – PVC Slip Nut Joint Sealed with Silicone Caulk

S-Trap with Bent L-shaped Configuration Under Old Kitchen Sink – PVC Slip Nut Joint Sealed with Silicone Caulk

S-Trap Under Old Kitchen Sink – PVC Slip Nut Joint Sealed with Silicone Caulk

To fix this leaking kitchen sink P-Trap, the first option was to get a new rubber seal ring for the 40mm outlet pipe. However, since there was no time to visit the plumbing store, the only option left was to remove the internal gasket rubber ring on the PVC Trap outlet and fit it on the waste outlet pipe. The problem is that the internal rubber gasket was small and couldn’t fit on the 40mm waste outlet pipe. However, a tighter (smaller diameter) rubber gasket is more preferable than a slightly bigger rubber ring seal if you want to have a perfect watertight seal. To fit the small rubber gasket ring on the outlet pipe, its diameter had to be expanded by heating , then quickly immersed in water for cooling down to avoid burning your fingers. Rubber expands when heated, but it quickly returns to its original shape and dimensions when cooled. The hot rubber gasket ring was taken out of the water as quickly as it was immersed, and then forced and fitted over the 40mm outlet pipe. The joint was then tightened with a slip nut, and boom, the leak was fixed. To heat the rubber gasket ring, you just need to place the ring over the tip of a metal tablespoon or fork and then suspend it over a hot stove plate.

Will Glue Stop a Water Leak?

As you can see, a professionally installed PVC P-Trap or pipe assembly doesn’t need sealing joints with silicone caulking, PVC cement glue, Epoxy putty or fibreglass resin cloth. The rubber seal rings or gaskets are sufficient on their own if properly installed. As for silicone caulking, it’s the worst kind of sealant that you can use on PVC plumbing. Anyway, most adhesives or glue sealants only work temporarily and are not recommended for long-term solutions. Setting or hardening sealants should not be used on PVC joints.

Wrong methods of joint sealing and pipe jointing

Wrong methods of joint sealing have been discussed above. Basically, a properly installed PVC P-Trap doesn’t make use of sealants, glue or tapes. If you need to make use of sealants and glue, it may be that the rubber ring seals are dislocated, broken, loose or improperly installed. It may also be that the P-Trap assembly is not aligned well with respect to the vertical and horizontal axis. Sealants should not be the priority. A P-Trap with sealants all over the joints is a sign of bad installation. The only place that warrants a sealant like PVC cement glue, Epoxy putty and fibreglass resin cloth is where the pipe is cracked, broken or perforated. All your slip nut joints should make use of a rubber seal ring, not sealants.

Overtightening of slip joint nuts leading to deformation and cracking of nuts

An overly tightened PVC slip joint nut may split, crack or deform due to excessive tensile stress or strain. Avoid using tools like a torque wrench or any powered tools when tightening PVC nuts because they tend to exert an excessive tightening torque. PVC nuts can be safely and sufficiently tightened with your hands.

Clogged drain pipes

When your plumbing system is clogged on any line on the internal and external side of your kitchen wall, running tape water from the sink will exert some pressure on the PVC trap as well as other joints in the plumbing system. Slip joints may only be able to handle a maximum amount of pressure, and if the maximum is exceeded, the joint may leak. A clogged drain blocks and hinders the smooth passage of water, creating an excessive amount of pressure on the other side of the pipe. If the PVC Trap under your kitchen sink was not leaking prior to it being clogged, then unclogging the drains rather than dismantling the trap may be the solution. There are a couple of ways to unclog a kitchen sink and its plumbing without dismantling the trap or pipes. This entails using mild acidic dissolvers like caustic cleaners, vinegar/baking soda, salt/baking soda or using a plunger.

How To Fix a Leaking PVC P-Trap Under Your Kitchen Sink

So you woke up in the morning and found that the plumbing under your kitchen sink is leaking? Instead of calling a plumber on your cellphone or landline, you should do a check on your plumbing system to diagnose the problem. It may be fixed with a simple DIY solution that will cost you nothing except your time. Do the following:

Start by removing every household item under your kitchen floor cupboard and sink. Take out all the cooking utensils, equipment, food containers and so on, and place them in another location.
Locate the leakage. Take a water bucket or dish, and place it under the leaking point. The aim is to collect the leaking water in a container to prevent further moisture damage or wetting of the cupboard. Examine the leaking point to find out what may be causing the leak.
Shut off the sink water tape or faucet. Allow all the water in the sink and pipes to drain away.
Once all the water is drained away, dismantle the P-Trap. You may need to dismantle one part or the whole system if there are several leakage points. Check the condition of the pipes, joints, rubber seal rings, washers and slip nuts. Are they in a state of disrepair, dislocated, broken, cracked or loosely fitted? Were they installed correctly? Are they the correct size? If the plumbing system is/was clogged, you will need to start by unclogging it before taking any corrective measures. The leak may as well be caused by a clogged system that is exerting too much pressure on the PVC joints. Unclog the drain pipes, clean the inside of pipes and re-assemble the trap back in position. Open the water tape/faucet and see if the trap is still leaking. If the leak persists, the problem might have something to do with some of the 13 causes outlined in heading 5 of this article. Go back to this section to find the solution.
Let’s assume the leak is caused by broken, dislocated, improperly installed or loose fitting rubber seal rings or gaskets
The proper way to install a rubber seal ring or gasket is shown below. The rubber seal ring is fitted on the unthreaded pipe which goes inside the threaded male PVC pipe end. The rubber seal ring is fitted on the unthreaded pipe with its sloping side facing the threaded male pipe. Before you fit the rubber seal ring on the unthreaded pipe, you must fit the PVC slip nut. The steep side of the rubber seal ring will hold the slip nut in place, acting as a prop or retaining support. Push the unthreaded pipe inside the other pipe, then turn and tighten the slip nut. As you turn and tighten the nut, the nut will push and press the rubber ring against the mouth of the threaded male pipe, firmly sealing the joint to make it watertight.

How To Install Rubber Seal Rings and Slip Nuts on PVC Pipe Joints (1)

How To Install Rubber Seal Rings and Slip Nuts on PVC Pipe Joints (2)

How To Install Rubber Seal Rings and Slip Nuts on PVC Pipe Joints (3)

How To Install Rubber Seal Rings and Slip Nuts on PVC Pipe Joints (4)

How To Install Rubber Seal Rings and Slip Nuts on PVC Pipe Joints (5)

How Do You Fix a Leaking PVC Joint Without Cutting It?

Ideally, PVC pipes should make use of a slip joint and nut, rather than a solvent weld joint. A slip joint and nut ensures that the pipe fittings can be easily assembled and dismantled without cutting any part. The disadvantage of solvent welding is that the fittings are fused and welded together to form a permanent connection that becomes part of the pipe molecular structure. In this case, the connection cannot be dissembled without cutting the pipe. You would have no option but to cut the pipe or weld the existing joint using a non-hardening sealant (penetrating sealer) that penetrates into microscopic gaps that are not reachable with a hardening solvent like Epoxy putty, resin coated fibreglass cloth, Teflon tape or paste.

Spread the love

How To Fix a Leaking PVC P-Trap or Drain Pipe Under Your Kitchen Sink, Wash-Hand Basin or Bathtub