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The most common type of plastic used in residential plumbing is PEX (cross-linked polyethylene). PEX has gained significant popularity in recent years due to its flexibility, ease of installation, and resistance to scale and chlorine. It can expand and contract, has a lower risk of freezing and bursting in cold temperatures, and is available in long lengths, which reduces the number of fittings required. This can lead to cost savings in both material and labor during installation.

Additionally, PEX does not corrode or develop pinhole leaks, which can be a concern with other materials like copper. It is also resistant to chemicals found in water. These features make PEX a preferred choice for many plumbing systems in residential applications, especially for hot and cold water lines.

While other types of plastics like PVC and CPVC are used in plumbing, particularly for drain, waste, and vent systems as well as hot water distribution (in the case of CPVC), PEX's advantages in flexibility and resistance to damage from temperature fluctuations and pressure changes contribute to its status as the most common plastic material in residential plumbing.