PEX (cross-linked polyethylene) is a flexible plastic tubing that has largely replaced copper for supply line rough-in in residential construction. PEX is faster to install, significantly less expensive than copper, freeze-resistant (it expands rather than splits when frozen — critical in Utah’s winters), and immune to the corrosion and pitting that affect copper over time in areas with hard or acidic water. PEX-A, installed with expansion fittings, is the highest-quality PEX type and the one RainFire Builders specifies as standard. Copper remains appropriate for specific applications requiring rigidity, high-heat runs near equipment, or where local code requires it.