Master Plumber Practice Exam 2026 - Free Plumber Practice Questions and Study Guide

Cast-iron soil pipe is typically designed to be utilized in various applications within plumbing systems. However, threading these pipes is not recommended because of the potential for creating weak points that could fail under pressure. The integrity of cast-iron pipes is crucial for withstanding the mechanical stresses and corrosive environments typically found in soil and drainage applications. When pipes are threaded, the material is weakened at the cut points, making them more susceptible to cracks and leaks over time.

In contrast, using cast-iron pipe below ground, painting it, and employing it for drain lines are all acceptable practices. Below ground, the pipe provides durability and resistance to damage from soil. Painting can help prevent rust and prolong the life of the pipe. Additionally, using cast-iron for drain lines is common due to its strength and sound-dampening properties. Thus, the only inappropriate practice among the options given pertains to threading, as it compromises the pipe’s structural integrity.