# What causes pinhole leaks in copper pipes? > What causes pinhole leaks in copper pipes, what the pattern of leaks says, what San Francisco's water does and doesn't explain, and when to repair or repipe. URL: https://hilltoprepipesanfrancisco.com/guide/copper-pinhole-leaks/ Learn There is no single cause. Water chemistry, flow speed, workmanship, sediment and electrical conditions can each thin a copper wall until it opens, and where the leaks appear is the most useful clue to which one is at work. Last Updated: October 30, 2026 Copper pinhole leaks have no single cause. They form where the inside of the wall corrodes in one small spot until it breaks through, and that pitting can be driven by water chemistry, high flow speed, sediment, workmanship at the joints, stray electrical current or thin-walled tube. One pinhole doesn’t prove a system is failing; several on the same run usually point to a cause that will keep working. [Hilltop Repipe San Francisco](https://hilltoprepipesanfrancisco.com/) handles leak repairs, [copper repiping](https://hilltoprepipesanfrancisco.com/copper-repiping/) and repiping in other materials across San Francisco, Daly City and South San Francisco, with the work carried out by licensed and insured contractors. This guide explains how a pinhole forms, how to read the pattern and how to choose between a repair and a repipe. Key takeaways - A pinhole is a pit that has corroded through the pipe wall from the inside. - Water chemistry, velocity, sediment, joint workmanship and electrical conditions can each contribute; none explains every case. - Leaks clustered on the hot side, on one run or near fittings each point to a different mechanism. - San Francisco's water is soft and pH-adjusted for corrosion control, and copper at the tap tests far below the action level. - One pinhole is a repair; repeat pinholes on the same run are a reason to replace that run or more. ## How a pinhole forms Copper protects itself with a thin oxide film on its inside surface. Most of the time that film keeps corrosion slow and even across the whole pipe, and the wall loses metal so gradually it outlasts the building. A pinhole happens when corrosion concentrates in one spot instead. Something breaks the film locally, a pit starts, and the chemistry inside the pit keeps it growing deeper rather than wider. The wall thins at that point until water pushes through a hole a fraction of a millimeter across. Because the damage is inside, copper can look sound from the outside right up to the first drip. A small green or blue-green spot on the outside of a run, away from any joint, is often the first sign of a pit that has reached the surface. A pinhole leak usually starts as a fine spray or a slow drip that wets insulation, drywall or the floor below before anyone sees it. ## What can drive the pitting Several mechanisms are recognized in the trade, and more than one can act on the same pipe. - **Water chemistry.** The balance of pH, alkalinity, dissolved minerals and disinfectant affects how stable the protective film stays. Very soft or aggressive water, and changes in treatment, can all play a part in some systems. - **Flow velocity.** Water moving too fast for the pipe size wears away the film, especially at elbows, tees and just past fittings, where flow turns and churns. Hot water lines and recirculating loops run hotter and move water more often, which is why pinholes in some houses cluster on the hot side. This is often called erosion corrosion. - **Sediment and particles.** Sand, scale or rust loosened from old galvanized pipe upstream can settle in copper and start pits beneath them. - **Workmanship.** Excess flux left inside after soldering, burrs from cutting that were never reamed out, and joints overheated or underfilled can each start corrosion at the joint. - **Electrical conditions.** Where metal water pipe is part of the electrical grounding path, stray current can flow through it. Faulty grounding or bonding is a possible contributor that an electrician checks. - **Thin-walled tube.** Type M copper has a thinner wall than Type L, so a pit of the same depth reaches through sooner. California’s code requires Type L for water distribution, with Type M allowed above ground in or on a building. Homeowners report pinholes in brand-new copper as well as in old systems, which is a reminder that age alone doesn’t explain them. ## What the pattern of leaks tells you The location of leaks is the most useful evidence a homeowner has. A log of where and when each leak occurred, with photos, helps whoever assesses the system. - **Hot side only, or mostly.** Points toward temperature and velocity, including a recirculation pump that runs fast or constantly. - **Near elbows, tees and fittings.** Points toward turbulence and erosion, or toward joint workmanship. - **Scattered along straight horizontal runs.** Points toward water chemistry or particles settling on the bottom of the pipe. - **Concentrated where copper meets galvanized.** Points toward galvanic corrosion at a mixed-metal joint; San Francisco now requires a dielectric fitting or a brass nipple at least 6 inches long there. - **One run, repeatedly.** Points toward a cause specific to that run: its size, flow or how it was installed. None of these proves a cause on its own. They decide what gets checked next. ## What San Francisco’s water does and doesn’t explain SFPUC’s water is soft, with average hardness of 62 ppm, a pH of about 9.3 adjusted for corrosion control, and chloramine as the disinfectant. In SFPUC’s 2024 tap sampling at 50 homes, the 90th-percentile copper level was 72 parts per billion against an action level of 1,300, which shows little copper being released at the tap across the system. No SFPUC source ties pinholes to the city’s water. Chloramine is a reasonable question for rubber seals and some fittings, which makers rate for it, but it is not an established cause of copper pinholes here. [South San Francisco](https://hilltoprepipesanfrancisco.com/service-areas/south-san-francisco/) is a different case. Cal Water blends local groundwater, with average hardness of 384 ppm, into its supply there, so scale and sediment inside pipes are a more real factor than in San Francisco. The [guide to San Francisco water and your pipes](https://hilltoprepipesanfrancisco.com/guide/san-francisco-water-and-your-pipes/) covers the chemistry in more depth. ## What to do when a pinhole leaks Close the nearest stop valve, or the main valve if the leak is in a wall or ceiling. In San Francisco, SFPUC says most main shutoffs are outside the house or just inside, in line with the meter at the sidewalk; owners should not open the meter box. A leak that sprays into a wall or ceiling is an emergency, and [emergency pipe repair](https://hilltoprepipesanfrancisco.com/emergency-pipe-repair/) is available 24/7 for burst and leaking pipe. For houses and small buildings, SFPUC’s leak alert flags 48 hours of continuous use of at least 7.5 gallons an hour, and a concealed leak, one behind a wall or underground, qualifies for a credit of all of the excess use, against half for a visible leak, for up to two billing periods, if the owner applies within 15 days and proves the repair. A plumber’s invoice that describes a concealed leak matters for that claim. The [SFPUC leak alert guide](https://hilltoprepipesanfrancisco.com/guide/sfpuc-leak-alert/) and the [water leak detection](https://hilltoprepipesanfrancisco.com/water-leak-detection/) page cover finding a leak you can hear but not see. ## Repair, partial replacement or repipe A single pinhole on otherwise sound copper is a repair. A listed repair clamp can stop it, and in San Francisco a repair that does not cut into or remove pipe is exempt from a permit. Cutting out the damaged length and joining in new tube is the more lasting fix and needs a permit, with the work inspected before the wall closes. The [pipe repair](https://hilltoprepipesanfrancisco.com/pipe-repair/) page covers those jobs. Repeated pinholes on the same run change the answer. Replacing that run, or every line on the hot side, is a [partial repipe](https://hilltoprepipesanfrancisco.com/partial-repiping/), and its scope should say which old copper stays. When leaks appear on several runs, or the copper is thin Type M throughout, a whole-house repipe often makes more sense than a string of repairs, because each repair opens a wall that has to be patched again. The typical San Francisco-area range for a whole-house PEX repipe is $8,500 to $16,000 for one bath, $12,000 to $22,000 for two and $14,500 to $28,000 for three; copper usually costs more, by a margin sources put anywhere from about 10% to 60%. The [repipe vs repair guide](https://hilltoprepipesanfrancisco.com/guide/repipe-vs-repair/) walks through the decision. Epoxy lining is sometimes offered for pinholes. It coats the inside of the existing copper rather than replacing it; the [pipe lining vs repiping guide](https://hilltoprepipesanfrancisco.com/guide/pipe-lining-vs-repiping/) compares the two. ## Replacing copper with copper, or with PEX If the cause is found and corrected, such as a recirculation pump running too fast, an undersized line or a mixed-metal joint, new copper can be the right material again. If the cause can’t be pinned down, changing material takes copper-specific mechanisms out of the picture, though PEX has its own requirements around heat, daylight and fittings. Neither choice is right for every building. The [PEX vs copper guide](https://hilltoprepipesanfrancisco.com/guide/pex-vs-copper/) sets out the tradeoffs. To have a pinhole history assessed, call or send the form with where and when each leak occurred. ## Questions about this guide ### Why do pinhole leaks show up mostly on hot water lines? Hot water lines run warmer and, where there is a recirculation pump, move water more often and sometimes faster than the pipe size suits. Heat and velocity both wear at the protective film inside copper, especially at elbows and just past fittings. A hot-side pattern points toward flow and temperature rather than the water supply as a whole. ### Does chloramine cause pinhole leaks in copper? No published San Francisco source says so. SFPUC disinfects with chloramine and adjusts its water to a pH around 9.3 for corrosion control, and its 2024 tap sampling found copper far below the action level. Water chemistry is one possible factor in pinholes anywhere, so it is weighed alongside flow speed, workmanship and the pattern of the leaks. ### Is green crust on copper joints a leak? Not always, but it is worth watching. Green or blue-green crust at a joint usually means water has been seeping slowly, often from flux left on the outside or a joint that weeps under pressure. Isolated green spots along a straight run, away from joints, can mark pitting from the inside, which is a stronger warning sign. ## Related service ### Copper repiping New hot and cold supply in rigid copper tube, joined by solder or press fittings, with dielectric transitions where it meets steel. Copper repiping ## More guides - Decision Guide [Repipe vs repair: when is a full repipe worth it?](https://hilltoprepipesanfrancisco.com/guide/repipe-vs-repair/)A repair fixes a defect. A repipe replaces a material that is failing everywhere. The work is telling the two apart, and the evidence is in the pipe, the pattern of leaks and the flow. - Learn [Is San Francisco water hard or soft?](https://hilltoprepipesanfrancisco.com/guide/san-francisco-water-and-your-pipes/)The chemistry of Hetch Hetchy water, read for what it does to pipe: why old steel here clogs with rust rather than scale, what chloramine means for material choice, and why the answer changes a little across the county line. - Decision Guide [PEX vs copper for a repipe: which should you choose?](https://hilltoprepipesanfrancisco.com/guide/pex-vs-copper/)Both materials are code-approved in San Francisco. The real differences are in how each one travels through a building, what it costs, how it is joined and what has to change around it. - Decision Guide [Epoxy pipe lining vs repiping: which one fixes old pipe?](https://hilltoprepipesanfrancisco.com/guide/pipe-lining-vs-repiping/)Lining coats the inside of the pipe you have; repiping replaces it. Lining opens fewer walls, but it keeps the old pipe's layout, valves, threads and size, and in San Francisco those are often the problem. - Learn [What does an SFPUC leak alert mean?](https://hilltoprepipesanfrancisco.com/guide/sfpuc-leak-alert/)SFPUC watches every San Francisco meter hour by hour and writes when water never stops flowing. What sets the alert off, how to check it yourself without touching the meter, what it costs to ignore and how to get part of the bill back.