Why Pipe Joints Fail: Copper, PEX and Plastic
Read the evidence in a failed joint, tell installation errors from water, heat and chemical attack, and choose a repair that stops the same failure from coming back.
Table of contents
Anyone can cut out a leak. A senior fitter finds out why it leaked. A failure at one joint may be a single slip by a tired apprentice. It may also be the first of hundreds, caused by the water, the pump, the wrong fitting, or a product nobody thought would touch the pipe. The repair for one is a new coupling. The repair for the other is a different plan for the whole system. Learn how each joining method fails, and you can read the evidence, answer the owner's questions, and stop it from happening again.
What You Will Be Able to Do
- Save and read the evidence from a failed joint
- Name the main failure modes for solder, press, PEX, and solvent joints
- Spot failures caused by water speed, temperature, and chemicals
- Tell a one-off failure from a system-wide problem
- Choose a repair that fits the cause
Treat a Failure Like Evidence
Do not throw the failed piece away. Save it, and write down what you know.
- Record the location and take photos before you cut.
- Note the conditions. Is it hot or cold? Recirculating or dead-end? What is the pipe's age? Is this a repeat leak?
- Cut out a whole section, with pipe on both sides of the joint.
- Cut it open along its length and look inside.
- Ask how many others could share the cause. Same crew, same batch, same water, same pump?
| Failure timing | Think about |
|---|---|
| At the test | Joint never made correctly |
| In the first year | Installation error or a stress built in |
| After several years, in many places | Water, chemistry, velocity, or material |
| After a change | A new pump, chemical, or pressure setting |
Copper Solder Joints
A good solder joint fills the whole socket by capillary action. Most bad ones never filled it.
| Failure | What you see | Cause |
|---|---|---|
| Lack of fill | Solder only at the edge of the socket when cut open | Dirty copper, too little heat, or water in the line |
| Overheating | Black, burnt fitting, and flux gone | Too much flame, so solder will not flow |
| Flux corrosion | Green crust and pitting near the joint | Too much flux, or lines not flushed |
| Pull-out | A joint that slid apart | Tube not fully seated, or an unsupported load |
A joint that looks fine outside can have only a thin ring of solder. That is why you cut a suspect joint open. When you take it apart with heat, you lose the evidence.
Erosion-Corrosion
Moving water can wear copper away, especially when it is hot or chemically aggressive. The Copper Development Association says water speed should stay under about 5 to 8 feet per second to avoid it. The lower end suits small tube. Engineering references give lower limits for hot water: about 5 feet per second up to about 140 F, and 2 to 3 feet per second for water that runs hotter than that.
Look for smooth, shiny, undercut pits, often in a horseshoe shape, just after an elbow or tee, or after a burr that was never reamed. Recirculating hot water lines are the classic spot, because the water never stops moving.
Worked example. A hotel's hot water return in 3/4-inch Type L copper springs pinhole leaks after 3 years, just past several elbows. The water runs at about 150 F. A flow reading shows 6 GPM. The inside diameter is 0.785 inches, which squares to 0.616. 0.408 x 6 divided by 0.616 is about 4 feet per second. That is above the 2 to 3 feet per second suggested for water that hot, and it runs day and night. New elbows will fail the same way. The fix is to bring the flow down with a properly set balancing valve, or a smaller pump, sized by whoever designed the system. Then repair the damaged pipe.
Annealing From Brazing
Brazing heats copper hot enough to soften, or anneal, the tube near the joint. Pressure ratings for annealed tube are lower than for hard-drawn tube, so a brazed joint on a high-pressure line must be rated on the annealed value. Check the Copper Tube Handbook or the job's engineering documents.
Mixed Metals
Copper connected straight to galvanized steel in wet conditions sets up galvanic corrosion. The steel side wastes away. Use a transition your code accepts, such as a listed dielectric fitting or a brass connector. Keep copper hangers or isolated hangers on copper too.
Press Joints
Press joints fail in a small number of ways, and most show up at the test.
| Failure | What causes it |
|---|---|
| Leak or pull-out | Tube not inserted to full depth |
| Leak at the seal | O-ring nicked by a burr, or dirt on the tube |
| Weak grip | Wrong jaw, a worn jaw, or a partial press from a dying battery |
| Seal breakdown | Wrong seal for the fluid or temperature |
Check the depth mark. A press joint with the mark far from the fitting's edge was never fully inserted. Pressing it again will not seat the tube, so cut it out and use a new fitting. Jaws wear, and a press tool needs service on the maker's schedule. Standard EPDM seals are for water. They break down in oil and gas service, which need their own listed fittings.
PEX Joints
Most PEX failures trace to one of these.
| Failure | What causes it | How to spot it |
|---|---|---|
| Crimp leak | Crimp not checked, or tool out of adjustment | Fails the go/no-go gauge |
| Mixed parts | Ring, fitting, or tool from different systems | Unmatched markings |
| Slanted cut | Too little tube under the ring | Look at the cut |
| Early expansion leak | Joint pressurized before it shrank in the cold | Tube end not fully seated |
| Tube split | Sunlight damage or a kink | Chalky surface, or a white stress mark |
Chlorine and Sunlight
PEX is rated for how well it stands up to chlorine and sunlight. The print line on the tube gives a material code such as PEX 5306. The first digit is its chlorine resistance, tested under ASTM F2023. The second digit is its UV rating: 0 means no rating, 1 means one month of sun, 2 means three months, and 3 means six months. That rating is a limit on exposure time during construction. It does not mean you can leave the tube out in the sun for good.
Hot, heavily chlorinated water running all the time is hard on PEX. If you are installing a recirculating line, read the tube's rating for that use.
Brass Fittings That Lose Their Zinc
Brass is copper and zinc. In some water, high-zinc brass loses its zinc and leaves a weak, spongy copper behind. This is called dezincification. Forensic reports on failed PEX systems have found this in high-zinc brass fittings, with some failing in only a few years, and it has led to class action lawsuits.
Look for white or crusty buildup on fittings, a drop in flow, and fittings that crumble. If you find this in one place, look in others of the same type. This is a material and water chemistry problem. Your supplier and the fitting maker can tell you what alloy suits your water.
Solvent-Welded Joints
A solvent joint only works if both surfaces soften and fuse. You can read a failed one by cutting it open along its length.
| What you see | Cause |
|---|---|
| Dry, shiny area in the socket | Not enough cement, or cement that dried before assembly |
| Pipe not reaching the bottom | Pushed back, or not fully inserted |
| Thick puddle inside the pipe | Too much cement, which softens the wall |
| Joint split right after testing | Tested before the cure time |
| Clean separation of the pipe and socket | Wrong cement, or gelled cement |
Primer stain alone does not prove a good joint. A joint can show purple and still have no fusion. A failed joint is always cut out. No wrap or patch fixes it.
Heat and Pressure Ratings
PVC loses strength quickly as it warms. One pipe maker lists these factors for pressure rating.
| Water temperature | Share of rated pressure |
|---|---|
| 73 F | 100 percent |
| 90 F | 75 percent |
| 100 F | 62 percent |
| 120 F | 40 percent |
| 140 F | 22 percent |
Worked example. A PVC line rated at 280 psi at 73 F carries 120 F water. 280 x 0.40 gives 112 psi. Your system may run under that, but a pressure surge may not. This is why PVC is not used for hot water supply. Use CPVC, PEX, or copper where heat is expected, and rate them at their own limits.
Threads in Plastic
Threading cuts away pipe wall. Thread only Schedule 80 or heavier plastic pipe, and cut its rated pressure by 50 percent. Threaded plastic fittings can split from overtightening, especially a metal male thread going into a plastic female thread. Tighten as the maker directs, usually by hand plus a set number of turns. Use a sealant the maker approves.
Chemical Attack on CPVC
CPVC can crack when it meets some chemicals while it is under stress. This is called environmental stress cracking. It needs both the chemical and a stress such as pressure, installation strain, or temperature change. The crack can show up weeks or years later, far from where the chemical touched the pipe.
The list of things that can cause it is long. It includes some thread pastes, cutting oils, spray foams, firestops, pesticides, and some cleaning agents. CPVC makers keep compatibility programs, so use their lists. Look for cracks that start at a stress point, like a fitting, with a clean, brittle look.
If a CPVC system starts to crack in many places, get the maker's technical team involved, and collect a sample. Treat it as a chemical problem until you know otherwise. Then look at everything that touched the pipe.
One Joint or the Whole System?
Worked example. An owner reports a third leak on the same floor of a 2-year-old building. Each leak is at a solvent-welded CPVC tee in a ceiling. You cut one open, and the cement looks well done. The crack is at the fitting's shoulder, and it looks brittle. You check the job records and see that a spray foam was added in that ceiling after the rough-in. You call the pipe maker and send a sample. A simple coupling repair would leave the same foam on the rest of the pipe. If it proves incompatible, the fix is to deal with the foam and the pipe it touched.
| Sign | Points to |
|---|---|
| Same fitting type failing in many places | Material, water, or a batch |
| Leaks only on one crew's work | Technique or a bad tool |
| Leaks only on hot lines | Heat, velocity, or chlorine |
| Leaks after a pump or chemical change | That change |
Choosing the Repair
| Cause | The repair that fits |
|---|---|
| One-off installation slip | Replace the joint, and coach the crew |
| Tool problem | Service the tool, and check that crew's other joints |
| Velocity | Fix the flow, then repair the pipe |
| Water chemistry | Change the material or treat the water, with expert help |
| Chemical contact | Remove the source, and assess the pipe it reached |
| Wrong material | Replace with a rated material |
Keep your owner informed of what you know and what you do not. Some failures need a lab, a maker's rep, or an engineer. That is good judgment. Keep your samples, because insurers and makers may ask to see them.
Common Mistakes
| Mistake | What it costs |
|---|---|
| Throwing away the failed joint | Lost evidence, and the same failure later |
| Adding solder to a leaking joint | A joint that still leaks |
| Replacing elbows without fixing velocity | The same pinhole leaks in a few years |
| Patching a failed solvent joint | A repair that will fail again |
| Using an unapproved thread paste on CPVC | Cracks long after the job ends |
| Calling a pattern bad luck | A whole-system problem left to grow |
Check Your Understanding
1. You cut open a leaking solder joint, and solder only reaches about 1/8 inch into the socket. What likely went wrong?
Solder never filled the joint. The likely causes are dirty copper, too little heat, or water in the line. Remake it from clean parts.
2. A building has had three leaks in a year. All three are on hot lines, in work done by different crews. What does that pattern point to, and what does it make less likely?
It points to heat, velocity, or chlorine, because the leaks follow the hot lines. One crew's technique or one bad tool is less likely, because several crews did the work. Cut a failed section open, then measure flow and temperature.
3. A PEX print line reads PEX 5106. How long can this tube sit in the sun during construction, by its code?
The second digit is 1, which means a one-month UV rating. Check the maker's instructions too.
4. A PVC line rated at 200 psi at 73 F will carry 100 F water. What is its pressure rating then?
200 x 0.62 is 124 psi.
5. Your apprentice used a thread paste from the truck on a CPVC adapter. What do you do?
Check whether the paste is listed as compatible with CPVC. If it is not, or you cannot tell, replace the joint and use a maker-approved sealant.
Key Takeaways
- Keep the failed joint. Cut it open and read it.
- Timing and pattern tell you if it is a one-off or the whole system.
- Copper fails from poor fill, flux, velocity, and mixed metals.
- Press joints fail mostly from depth, and PEX mostly from mismatched parts or skipped gauges.
- Plastic loses strength fast with heat, and threading halves its rating.
- CPVC can crack from chemicals under stress, so use the maker's lists.