Non-Permanent Plastic Pipe Sealant or Join Improvement?
It appears to me that since it is on the roof, subject to extremes, that the joint needs to withstand the expansion and contraction of the PVC pipe. That joint you are referring to is just that, it is a slip joint that allows the pipe to grow and shrink in length and presumably still not leak, since is a closed system, or supposed to be. There should be a rubber or neoprene "O" ring that allows it to seal with movement. It should just need replacement and reassembled using a silicone lubricant.