Q. Last week I experienced a water leak in my finished basement. After cutting through the ceiling I saw dripping water that appeared to be following down from both waste pipe and water lines below the sub floor and which lead up through the living room wall to the second floor master bathroom. The amount of water wasn’t a lot, just enough to to be described as constant. No signs of water anywhere else. Forty eight hours later I had the views of two different plumbers. They recommended exploratory surge ry but could not explain why the dripping slowed and subsequently stopped after two days. Rather than ripping out walls and pipes I told them I’d like to have a reasonable explanation of why the dripping stopped. Both admitted that they didn’t know and had not seen this before. I’m hesitant to do anything now because I had the same experience two years ago in the unfinished side of the basement. Water dripped off and on from a copper line below the joists for several months and then stopped. The loc ation of the leak wasn’t found even after a plumber’s inspection. My two story wood frame home is fourteen years old with copper pipe for public water supply. I have PVC lines for waste to municipal sewer. Your help would be much appreciated.

A. If the leak is a slight drip that stops after a while and can not be attributed to waste line leak then the first thing that comes to my mind is a pinhole leak. They start as very small openings in the wall of copper pipe that shoot out a minute water stream that can evaporate almost as quickly as the water leaves the hole creating a mineral build up that can eventually seal the hole. Or get worse, it’s a roll of the dice.

Pinhole leaks used to occur primarily in pipes on well water and for that the plumbing code requires a heavier copper pipe, Type L, to be used. Copper pipe is not all created equal and there are different grades of copper, the most common are Type M and Type L. Type M is thinner and is used on municipal water supplies that carry treated water that is supposed to be gentler on metal piping. Type L is a thicker, hard copper pipe used on systems fed by well water that can be acidic and chemically aggressi ve to copper. But over the past 25 years pinhole leaks have been showing up with much greater frequency on plumbing systems fed by public water supplies using Type M pipe.

We tend to see them on the hot water side around here and pipe age doesn't seem to matter. Why does one pipe develop a pinhole leak and the pipe next to it not? Or one or two houses in the same neighborhood go pinhole leaky and the others not-- all built by the same builder using the same plumber. Don't expect any real answers soon. Jacksonville, Florida became so frustrated over the problem a few years ago that they banned using copper in new residential construction in favor of cpvc plastic but the st ate plumbing code allows copper and over-rules local laws. The Jacksonville plumbing inspector I spoke with told me they now only permit heavier copper (Type L) or cpvc to be used.

Check with anyone in or around the building trades and you will hear all sorts of notions as to the exact but elusive cause for pinholes but no clear or definitive reason has ever been identified. Everything from lightning to the Ph levels in the water h as been studied but no one has successfully replicated the problem in the lab no matter how hard they try. The scientific method requires being able to independently replicate a causal assertion in the lab as proof. Dr. Marc Edwards, a PhD at Virginia Tech, has been researching this issue since 1990 and he is still stumped. He is currently looking at the potential of unanticipated consequences of the EPA's mandated water quality for municipal water supplies under the 1973 Safe Drinking Water Act and i ts subsequent amendments requiring further removal of those very things which used to coat the inside of copper pipes thereby protecting them.

Complete re-piping can cost in the six to nine thousand dollar range and insurance doesn't cover it. There are some repair methods that coat the inside of the pipe with a plastic lining but I have never seen it done personally so other than noting that i t's done I can't say any more about it.

Pinhole leaks usually show up as green dots on the side of the pipe that tend to be about a quarter of inch across. You may not initially, or ever, see water streaming from one. Don't go scraping one with a knife unless you have a repair method in your other hand. I have seen clamp-like devices sold to patch pinhole leaks but if you catch yourself using one just be aware you are starting down a path that leads to eventual total pipe replacement. I've spent years patching pinhole leaks in one house-- pi pe at a time-- and I'll bet there's not much left of the original piping by now. When you replace a section of pinholed pipe, use Type L copper or cpvc.

My recommendation is the next time such a leak shows up consider that exploratory surgery and prepare yourself for a repiping job. You don’t want someday to discover the hard way a pinhole gone long undiscovered causing wood rot and damage that will ex tend the cost of repairs exponentially.