Massachusetts Plumbers Journeyman Practice Exam

When more than four four-inch stacks are connected, what should be the diameter for the vent header extension through the roof?

1. Four inches in diameter

2. Five inches in diameter

3. Six inches in diameter

4. Eight inches in diameter

The correct answer is: Six inches in diameter

The correct choice of six inches in diameter for the vent header extension through the roof when more than four four-inch stacks are connected is based on plumbing code requirements and best practices. When multiple stacks are interconnected, the increased flow of gases and potential for larger volumes of air to be vented necessitates a larger diameter for the venting system. The six-inch diameter allows for adequate ventilation, reducing the risk of back pressure and ensuring that gases from the plumbing systems are expelled efficiently. This is crucial for maintaining proper air pressure in the drainage system and preventing siphoning or potential blockages. In contrast, smaller diameters, such as four, five, or even eight inches, would not provide the necessary flow capacity or might cause inefficiencies, such as increased resistance or potential for improper venting. Therefore, adhering to the six-inch diameter standard helps ensure compliance with the plumbing code while maintaining optimal function of the stacking system.