A saddle valve is a small clamp-on valve that taps a water line by driving a piercing pin through the pipe wall, often used to feed a fridge ice maker or a filter. It is quick and cheap, but it leaks often and many plumbing codes prohibit it, so plumbers replace it with a proper tee and shutoff.
A saddle valve is a small valve that clamps around an existing water pipe and taps into it by piercing the pipe wall, without cutting the pipe or soldering anything. It is also called a piercing valve or a needle valve. People reach for it because it is the fast, cheap way to grab a little water from a nearby line, usually to feed a refrigerator ice maker, a water dispenser, or an under-sink filter, in a spot that had no water connection before.
Here is how it works. The valve is a clamp with two halves that bolt around a pipe, typically half-inch copper. Inside is a sharp steel pin. When you tighten the valve down and turn the handle closed, that pin is driven through the copper, making a small hole. Open the handle and water flows out through the new hole and into the tubing you attached, feeding the appliance.
The problem is reliability, and it is a serious one. A saddle valve seals that pierced hole with a small rubber gasket pressed against the pipe. That gasket is a weak point. It degrades over time from constant contact with the water, and the tiny hole and needle are prone to slow leaks. Because a saddle valve is usually installed in a hidden spot, behind a fridge, inside a cabinet, or in a crawlspace, a slow leak can drip undetected for months, causing mold and water damage that costs far more than the valve ever saved. For example, a saddle valve behind a refrigerator can weep for a year before anyone notices the warped flooring.
For these reasons, many plumbing codes prohibit saddle valves, or at least do not recognize them as an approved permanent connection, because code favors accessible, listed shutoff valves. The proper fix is to cut the pipe and install a real tee fitting with a quarter-turn ball valve or angle stop, which gives a reliable, code-compliant connection. If you have a saddle valve feeding an appliance, treat it as a leak waiting to happen and plan to replace it.