In message l.net,
Dave Liquorice writes
Doesn't make that much difference when it's 'issing it down and the
water is simply overshooting the gutter, rather than the gutter being
full.

If it did it during normal rain I'd do something about it but otherwise
not.

Here we have a large flat roof with reverse fall butting onto the main
roof. Between the two there is a long narrow "gully". At its end it has
a sharp turn formed to send water down a narrow piece of roof into the
main guttering.

It's OK in normal rain. In storms it spouts water about 3 feet straight
out into the garden.

I've followed Dave's "otherwise" option, as I don't think anything else
would be better, short of redesigning the whole roof.
--
Bill