Why must stacks be extended full size through the roof?

Stacks must be extended full size through the roof primarily to avoid violent pressure fluctuations. When plumbing systems are in operation, they experience changes in pressure due to the flow of wastewater and the movement of air within the pipes. If a stack is not extended above the roofline, it can create pressure imbalances within the drainage system. These imbalances may lead to issues such as siphoning, where water is unintentionally drawn out of traps, potentially allowing sewer gases to enter the building and creating harmful conditions.

Extending the stack full size also ensures that the venting system is operating properly, allowing air to flow in and out freely. This is essential for maintaining the proper function of the drainage system and preventing any unpleasant or hazardous conditions from arising due to inadequate venting.

While other options may address certain considerations in plumbing design, the fundamental reason for extending stacks full size through the roof is primarily related to managing pressures within the system and ensuring safe, effective drainage.