Once that valve is turned off and then turned back on, the old internal seals, packing material, washer, or stem seal may no longer reseal properly. The movement can disturb hardened, dried, corroded, or deteriorated sealing material. That is why a valve that “was not leaking before” can begin dripping after it is operated.
The most common failure point is the packing around the valve stem or the internal washer/seal. Plumbing repair sources explain that shutoff valves can leak when the packing nut becomes loose, when corrosion damages the valve, or when the valve has not been exercised or moved for a long period of time. Balkan Plumbing notes that these leaks occur when the packing nut has loosened over time, corrosion has damaged the valve, or the valve has not been moved in quite a while.
The underlying cause is usually age-related deterioration, not installer abuse. Over time, valve packing can harden, shrink, lose elasticity, or disintegrate. Once the handle is turned, that old packing may no longer compress tightly around the stem, allowing water to seep through. George Brazil Plumbing describes this directly: packing material can harden or disintegrate over time, and when it no longer forms a solid seal, the valve leaks.
There is also a practical maintenance issue: cutoff valves are mechanical parts, and mechanical parts do not love being ignored for ten years and then suddenly asked to perform flawlessly. Roto-Rooter notes that when a valve has been left off or unused for a long period and is finally switched back on, it is common for the valve to begin dripping.
Bottom Line
A flooring installer may be the person who discovers the problem, but the usual cause is pre-existing valve deterioration revealed by normal operation. Turning the cutoff valve does not “break” a healthy valve. It exposes a valve that was already worn, dried out, corroded, or near failure.
In plain job-site language: the valve was already tired. The installer just woke it up.
