Edwardian house in Edinburgh - now upstairs and downstairs flats. My
mate lives downstairs. All cast iron drain pipes.

About 6 weeks ago there was a flash rainfall with hail and he
discovered hail being washed up into his small bog room sink - he
asked me to take a look sometime and today was that time. A bit of
detective work had to be undertaken as the downpipe the sink drains
into has been covered by the outside stair to the upstairs flat and
there was an option that the sink flowed into another downpipe -
various ploys plus some measurements showed that wasn't possible so we
are pretty sure that it goes into the currently inaccessible pipe that
also drains part of the roof and has outlets from the upstairs flat.

There must be a reason for the backup - seemingly what was the
bathroom in the upstairs flat and had two discharge pipes into this
downpipe, is now just a utility room with a washing machine.

Here's the question - is it likely that the low volume, possibly fibre
laden, outflow from the washing machine is creating a build up in the
drain pipe under the ground ? There is a downstream manhole nearby -
will power hosing be able to clear an upstream blockage like this?

Rob