A stack of less than 3 inches in diameter shall be increased to a minimum of 3 inches in diameter before passing through a roof. If a change in diameter is made, the fitting shall be placed at least ______ foot below the roof.

The requirement that a stack of less than 3 inches in diameter must be increased to a minimum of 3 inches before passing through a roof is crucial for ensuring adequate venting and preventing potential issues with drainage or ventilation in plumbing systems. When there is a change in diameter, placing a fitting at a distance below the roof serves multiple purposes: it allows for proper airflow, reduces the risk of blockage, and minimizes the chance of condensation forming within the vent stack.

The specified distance of at least 1 foot below the roof is significant as it provides enough vertical space for the transition to occur without compromising the efficiency of the venting system. This distance ensures that the fitting is well-positioned, which can help in maintaining the structural integrity of the vent stack and ensuring that it functions optimally.

While other distances may seem plausible, the 1-foot requirement is a standard practice that has been established to enhance system performance and reliability, which is why this option is the best choice.