This is a common question that is asked by installers when assembling PVC tapered pipe threads in a piping system. There is a common misconception that Teflon sealant or tape is the best for ALL threaded joints. With metal fittings these products are acceptable, but when working with PVC fittings the potential for failures is exaggerated. Threaded pipe joints are a necessary evil. Designers and installers strive to reduce the number of threaded joints in a system; yet, they are essential for change of materials and convenience of repair.
I have a slow leak in the joint of a 4' PVC drain pipe in my basement. How do I stop this leak short of cutting the joint apart, buying new fittings and regluing it? I have tried using PVC primer and PVC cement, but they don't seem to work. A friend suggested using an 'epoxy putty', which seems to have stopped the leaking for the time being. Sep 17, 2018 - This highly specialized instant pipe leak repair product can make repairs. Plastic joint leaking at elbow how to fix a copper pipe without soldering post. Without thread sealant, PVC fittings may not tighten completely and can.
This illustration shows how the threaded two parts mate when tightened. Note the voids left where the crest and root do not meet. All tapered pipe threads have a small spiral leak path between the root and crest of the mating threads. This leak path needs to be sealed during the joint assembly to obtain a drip free connection. The purpose of a thread sealant is to “plug” the leak passage.
One 'solution' has been to wrap the male threads in Teflon tape as a means of packing the joint to avoid leakage. This creates two additional problems, rather than solving anything. First, although it is not a sealant, it has been adopted for thread sealing because of its anti-friction lubricity which allows more turns to be taken on a tapered fitting. Second, the additional thickness of the tape adds to the wedging force of the male threads. Then, when the threaded joint is backed off, such as for alignment, the tape which has been permanently compressed, has lost its packing ability. Result: a drip or leak!
Teflon tape can be classified as a 'trapped elastomer.” Like imprecise tapered pipe threads it creates an additional variance in the process of joint sealing; however, tape is often banned in irrigation systems because of shredding and contamination. Teflon tape joints serve well in most metallic connections, yet increases the potential for failures in PVC joints.
During the process of tightening a joint the tape is sheared along the flanks, while lubricating, and gets packed into the voids between the root and crest of each thread. In metallic joints the process of packing increases the stress in the male and female parts yet is tolerated by the strength of the materials.
Most materials can tolerate compression loads much greater than tensile. Within a threaded connection the male component is compressed; while the female threaded part is stretched during tightening. PVC does not have the strength to withstand the combined loads of tightening along with the bulk of the Teflon being “packed” into the root/crest voids. It is easier to split smaller diameter threaded joints than larger ones since the stress and strain loads are greater. Teflon, within a “sealant” or tape, make the threads more lubricated, inviting over-tightening. The joint goes together so easily that it does not “feel” tight to the installer. Because the threads are tapered, once the male and female threads are engaged (finger tight- not even hand tight) additional turns cause the female part to stretch or undergo 'strain.' Recommended good practice is to use a thread sealant (not a thread lubricant) and to assemble the joint to finger tight plus one turn, two turns at the most.
When tapered threaded parts are assembled and the PVC female threaded part splits, the most likely cause is from over-tightening. The failure of the female part with a crack that is parallel to the axis of the fitting pinpoints the cause as over tightening. Many times the crack has progressed through the pipe and other parts, but its origin was from within the female threads. A crack or split is always oriented perpendicular to the causing load. In these failures the male threads induced stress (stretch) to the female threads.
In a threaded joint the greatest stress is generated within the meshed threads; so any cracks or failures will originate from the interior and progress to the exterior. The wall thickness of the female threaded portion, part or assembly has an insignificant effect on preventing failure from cracking.
When assembling threaded PVC fittings, a sealing compound that is non-hardening is the best. Recommended good practice is to use a thread sealant (not a thread lubricant) to assemble the joint to finger tight plus one and one-half (1½) turns, two turns at the most. Teflon tapes and hardening pastes can permit a leak path to develop when a joint is backed off, mechanically flexed or expanded with changing temperatures. Conversely, a non-hardening compound is forced by the internal fluid pressure into potential points of leakage; performing a true sealing role. The sealing compound must be compatible to all materials and media to which the joint will be exposed. Many brands of pipe sealants contain oils, solvents or carriers that can damage PVC. A proper sealant must be approved by the manufacturer to be harmless to the joint materials and not contaminate the fluid in the piping system. Finally, a sealing compound must not lubricate the joint to the point that over-tightening is encouraged.
Note: Additional information about threaded pipe joints is available at:
If you have a shower leaking from the shower arm, you might need just a simple fix or a somewhat major repair, depending on where the leak is and your plumbing setup. To confirm the terminology, a shower arm is the short length of pipe that comes out of the wall and connects to the showerhead. Shower arms typically have a slight bend near their middle, and they almost always have threads on both ends. One end threads into a fitting called a drop-ear elbow inside the wall; the other end receives the showerhead, which simply turns onto the exposed end of the arm. The first step in dealing with a shower leaking from a shower arm is to determine exactly where the leak is coming from.
Where Shower Arms Can Leak
A shower arm can break in the wall or cause the vertical pipe in the wall to break if too much pressure is applied while unscrewing the showerhead or shower arm. Damaging the shower arm while the showerhead is being replaced is a surprisingly common occurrence. The shower arm may also become damaged simply by the repeated pressure of adjusting the showerhead over years of use. Sometimes it is the threaded end attached to the showerhead that gets damaged, but more problematic is when pressure on the shower arm causes damage to the connection inside the wall. It's also possible that either of the threaded joints simply isn't sealing properly.
Leaking at the Showerhead
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If your shower leaking seems to be coming from the base of the showerhead, look for a crack in the showerhead and the shower arm. Many showerheads look like metal but are actually plastic with a chrome finish. Plastic cracks a lot more easily than metal (but metal can crack, too). If there are no visible cracks, try the following fixes:
- Unscrew the showerhead. If necessary, hold the shower arm with pliers to keep it from turning. Tip: Wrap the pliers' jaws with masking tape to prevent scratching the shower arm.
- Clean the threads of the shower arm, removing any old plumber's tape, pipe-joint compound, and mineral deposits.
- Wrap plumber's tape around the threads of the shower arm, wrapping in the same direction that the showerhead will twist back on.
- Reinstall the showerhead, tightening it until it is very snug. You shouldn't need to tighten the showerhead with pliers, but if tightening is difficult, hold the shower arm with one pair of pliers and tighten the showerhead with another pair.
- Test the connection for leaks. If it still leaks, tighten the showerhead a bit more. If that doesn't stop the leak, replace the shower arm (see below).
Leaking Inside the Wall
Now it's time to consider that drop-ear elbow. Just like the showerhead, the threaded connection between the shower arm and the elbow can leak. The remedy for this is similar to the showerhead fix: remove the shower arm, clean the threads, and reinstall the arm (or replace it if it's cracked or corroded) with a new application of plumber's tape. The arm simply twists into the elbow like a bolt into a nut. Just be careful not to cross-thread the connection when you start turning. Also be careful not to tighten the arm too much and risk damaging the elbow or the pipe.
Leaking Beyond the Shower Arm
If reinstalling or replacing the arm doesn't stop your shower leak, you may have a problem with the drop-ear elbow or with the vertical shower pipe. There are several different ways to fix this, but all require gaining access to the shower plumbing. If someone installed an access panel on the other side of the shower wall, you're in luck; if not, you'll probably have to cut a hole in the backside of the wall to access the plumbing.
Once the plumbing is visible, you can see where the leaking occurs and what type of piping you have. If it's galvanized pipe, the drop-ear elbow is probably threaded onto the end of the vertical pipe and you can simply twist off the old one and twist on a new one. If the piping is copper, all of the joints are probably soldered. This means you'll have a decision to make. Soldered copper is still the gold standard of plumbing connections; if you want the same quality, hire a plumber for the repair. He or she might be able to cut the vertical pipe and add a new section, along with a new drop-ear elbow, using a soldered coupling. You might decide that it's a good time to replace the entire shower faucet.
A DIY Alternative
You can splice in a new section of shower pipe and a drop ear elbow using push-in fittings (SharkBite is one brand). There's nothing wrong with this option, provided you install the fittings properly. The easiest repair is to cut the shower pipe above the faucet and splice in a new length of pipe using a push-in coupling, then install a push-in drop-ear elbow to the top end of the pipe. Secure the elbow to the wall framing and install a new shower arm to complete the repair.