Which requirement is essential for installing underground piping according to code?

The requirement that materials must be approved is essential for installing underground piping according to code because it ensures that the materials used meet specific safety and performance standards mandated by regulatory bodies. Approved materials are tested and verified to withstand environmental conditions, resist corrosion, and properly handle the pressures and contents they will transport. This compliance is crucial for the long-term integrity of the plumbing system and helps prevent potential failures that could lead to leaks or contamination of surrounding areas.

While considerations related to cost, flexibility, and toxicity of materials are important in their own contexts, they do not hold the same regulatory significance as the approval of materials under plumbing codes. Only approved materials guarantee that all necessary safety and performance evaluations have taken place, making this requirement the cornerstone of code compliance in underground installations.