DIY & Home Improvement

In their case, their shower emptied into the same downpipe that serviced the common roof gutters and their neighbour's sink. There was a blockage as a result of leaves gathering at the bend where the downpipe entered the drain at ground level, hence the backing up.

We were lucky, as it looks like somebody else had had the same problem at some point and had punched a small hole near the downpipe bend, and we were able to use that to access and unblock it with some wire then flush it through with plenty of water.

I'd have your super check that out as a possibility first - there's always a possibility of passing any problems downstream if it's a severe blockage, so maybe not a DIY job for you.