Copper is the most common material for supply pipe; it resists corrosion, so water runs freely and pipes remain leak-free for many decades. In an older home, you may find galvanized steel pipe, which tends to clog with minerals and rust over time and can develop leaks. Nowadays, many homes have plastic supply pipe, most commonly cross-linked polyethylene (PEX), which installs quickly and is expected to last virtually forever.
Water arrives via a main supply pipe, which is typically 1 inch in diameter or larger. (A pipe is measured by its inside dimension, so a 1-inch copper pipe is about 1 1/8 inches in outside diameter.) In most cases, the pipe runs through at least one main shutoff valve, located outside the house in a “Buffalo box” buried in the yard near the house or just inside the basement or crawlspace. It then usually passes through a water meter, and there is likely another main shutoff after the water meter.
The main supply line usually runs to the water heater, where it divides into cold and hot water pipes. From there, supply pipes almost always travel in pairs, hot and cold. Pipes from the water heater are typically 3/4 inch but may be 1/2 inch. Horizontal pairs run below walls and then vertical pairs, called risers, run up to the various rooms.
In newer homes, there are separate lines running from the water heater to each room, so water use in one area does not affect use in another area. In an older home, a single line may loop throughout the house, meaning, for instance, that if someone flushes a toilet downstairs the cold water supplying a shower upstairs will have lessened pressure, causing the shower water to suddenly become hot.
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