Take a close look at the photo, I had to do a double take. The customer couldn't understand why the pipe was constantly freezing up....... A real mystery !
It's not the droop (she said smiling) its the fact that whatever clown fitted the waste pipe plumbed it down via an elbow to 3 feet of pipe. Then connected the pipe to another elbow to make it go up at a right angle back into the stack Dohh!