In which situation must flexible joints be used according to the UPC?

Flexible joints must be used to accommodate expansion and contraction in plumbing systems. This is crucial because changes in temperature can cause pipes to expand and contract. If rigid joints were used, the stress from this movement could lead to leaks or even ruptures in the piping system. By incorporating flexible joints, the plumbing installation can absorb any movement without compromising the integrity of the system.

In contrast, using flexible joints in areas with no temperature variation is unnecessary, as there would be minimal movement from expansion or contraction. While flexible joints can be helpful when installing fixtures, they are specifically required for managing thermal movement in the piping. Rainwater harvesting systems may have unique requirements, but the key function served by flexible joints is to handle the physical stresses associated with temperature changes.