What is the minimum size of a vent pipe as stipulated by UPC?

The Uniform Plumbing Code (UPC) specifies that the minimum size for a vent pipe should be 1.5 inches in diameter. This size is necessary to ensure that the venting system can effectively allow for proper air circulation, preventing the formation of negative pressure that can lead to siphoning of traps and other drainage issues. A vent pipe of this size is typically sufficient to accommodate the venting needs for most residential plumbing systems, ensuring that drain lines remain operational and free of obstructions caused by improper venting.

Using a vent pipe smaller than 1.5 inches could impede the venting capacity, potentially leading to poor drainage performance and increased risk of drain odors entering the living space. The requirement for a 1.5-inch minimum size is established to balance functionality and practical application across various system layouts.