Hi All

I have a bathroom with a bath, two sinks and a WC. When I run either
sink for any length of time water starts coming up through the plug
hole in the bath! Happily this does not happen when the WC is flushed
- though the WC is immediately next to the soil downpipe and connects
directly into it. Eventually the water which enters the bath does
drain away.

My conclusion from this is that there is a partial blockage in the
underfloor pipe which leads to the soil downpipe after the place where
the closest (to the downpipe) sink joins that pipe. Thus water runs
out of the sinks, hits the block and backs up into the bath (which is
the path of least resistance as it is lower than the sinks). Having
looked at the posts to this group, it seems that I should put a
quantity of diluted caustic soda down the sink in question with a view
to it disolving the blockage. Does that sound correct?

Many thanks for any help.

Chris