If a vent is flagged, under what condition is it permitted?

A flagged vent is one that has been reduced in size or is obstructed in such a way that it may not function effectively. The correct condition under which a flagged vent is permitted is when the roof is used for purposes other than just weather protection. This can include situations where the roof serves as a usable space, such as a rooftop garden, terrace, or another area where structural considerations allow for a flagged vent.

In such instances, the code may allow certain flexibilities concerning venting requirements, recognizing that the roof's use necessitates adaptations for structural integrity and utility. This is particularly relevant in modern building practices where the versatility of spaces is often emphasized, and where traditional methods may conflict with the functional design of the building.

Thus, the focus on the roof's use beyond solely protecting against the weather acknowledges the need to balance practical functionality with building safety standards, allowing for some modifications while still maintaining the necessary performance of the plumbing vent system.