While it seems that your dripping pipe may have no stress on it, that's not completely true. Plumbing systems are constantly subjected to stresses that, while they may not be obvious, certainly can contribute to the wear and tear factor and lead to the copper pipe leaks you're experiencing. Here are examples of the stresses that can cause leaks.Expansion/contraction: If this is a hot water pipe (or next to one), it is constantly expanding and contracting due to the changing water temperature. Even if it is a cold water pipe, going from room temperature to ground-temperature cold causes the pipe to move. While this is not an issue mid-span in the pipes, the elbows and other joints can find this particularly stressful as a section of the pipe joint moves at a slightly different rate.
Abrasion: While it's hard to think of water as an abrasive, it most certainly can cause erosive effects, including copper pipe leaks. Think of the power of a pressure washer, for example. While extreme, this shows you just how abrasive water can become. At lower flows and lower volumes, water, over time, can erode sections of pipes and joints and cause leaks. Have you ever seen a copper downspout? The next time you do, look at the elbows and you'll see how simple rainwater can erode holes in this soft metal.
Water hammer: Water weighs about eight pounds per gallon. When a faucet is opened, the water rushes forward in the pipe, building up centrifugal force. When the valve is closed, that mass of water screeches to a stop, sometimes shaking the pipes. This is generally referred to as water hammer and, while unusual, can on occasion break a pipe joint free.
Pinhole leaks: Pinhole leaks are believed to be caused by the corrosive chemical reaction between water and some copper pipes, and can also cause leaks.So as you see, there are actually quite a number of things that can put stress on your plumbing system and lead to copper pipe leaks. If you can handle the repair, open the ceiling and make it. Just be darn careful with that blowtorch!