Andy Burns wrote:
Capitol wrote:

David Hearn wrote:

Our 1930's semi currently has separate plastic guttering systems, with
each half having their own single downpipe

The supplier is trying to drum up business. Keep it separate. The risk
of a damp wall is minute IMO. Check that your guttering is undamaged
before and after next door has the work done.

If you have corrugated tiles (rather than flat slates) best if the gap
is in a valley, rather than a ridge, to minimise runoff.

I'm not expert but intuitively, I would have thought the exact opposite?