Just having a new roof (not diy sadly!) and the scaffolders came
yesterday, so I couldn't resist hopping up to take a proper look
(actually, I had to because they had disconnected a downpipe !) Now,
I was planning to exploit the scaffolding to replace the guttering
once the roof is done and have been cogitating on whether to use round
or square. I had sort of decided to use round as, although it doesn't
look as good, the idea that flows are faster than in a flat bottom and
so flush the silt out sounds compelling. My roof has quite a shallow
pitch, so tends to form moss, so that may be more of an issue than
silt.

Anyway.....Once I got up there, my existing round guttering was
absolutely chocka with goo, moss and silt while my neighbour's, which
is a square jobbie, was completely clear. Since they were both last
cleaned out at the same time (couple of years ago) this seems
completely against the theory.

Does anyone else have a similar comparison? I guess her gutter might
have a better fall in it than mine (although there's precious little
space for either) but I wonder if, somehow, moss and silt behave
differently in the two different profiles.