It’s not too often that I can’t unblock a downpipe, only two in fact, and they were both cast iron so you are limited in what you can do. Technically speaking the figure is zero as both were flowing better but I digress. This one eventually got unblocked, but it had me puzzled until I finally had to take the whole downpipe off.
On my inspection, all looked ok and it didn’t appear that the downpipe was blocked. It’s only after I started to jet wash the downpipes after clearing the guttering when I realised this downpipe wasn’t working.
After repeated flushes, I had to inspect further just to find that screws had been put in where all the elbow joints where. This is a common practice and you will often find a screw holding a section together. however, there is usually only one and about a third of the width of the pipe. Here someone had put two screws the full width of the pipe next to each other. Pretty much ensuring that a blockage will happen. What made this worse is that these screws were at the back facing the wall, so I could not get access without removing the pipe. The screws also only had a 3 quarter thread so I was unable to unscrew them as there was nothing to bite on so I had to cut a sliver of the downpipe off with the screws in.