For other than prefabricated shower bases, a water test at least _____ inch(es) deep at the threshold is required for shower liners.

The correct answer specifies that a water test at least 2 inches deep at the threshold is required for shower liners, particularly when not using prefabricated shower bases.

This standard is important for ensuring that any water that may accumulate during use is contained within the shower area, preventing leaks that could potentially damage the surrounding structures. A depth of 2 inches provides an adequate volume of water to effectively evaluate the integrity of the shower liner, allowing for a thorough assessment of its performance. This requirement is part of plumbing code regulations that aim to safeguard against leaks and enhance the longevity and functionality of shower installations.

In contrast, a requirement for a depth of 1 inch would likely be insufficient, as it would pose a risk of failing to detect leaks or breaches in the liner, while a depth of 3 or 4 inches might be excessive for the testing needed and could introduce unnecessary complications. Utilizing a 2-inch depth strikes a balance between effective testing and practicality.