On Aug 5, 4:57*pm, GMM wrote:
Both houses are parallel and face south east at the back and north
west at the front. *THere are no trees overhanging either and we have
similar amounts of moss on each roof. *The roofer tells me that the
moss is the result of having sanded tiles on a shallow pitch and
they're about to be replaced with smooth tiles (not for the moss, but
because they've begun to crumble and leak).
So for all practical purposes, the two houses are identical, the only
differences that I can discern being the profile of the gutter and yet
the outcome seems to be the opposite of what may be expected.
As far as I'm concerned, it's fine, as it indicates that the square
gutter (which suits the house better) is not such a bad thing,
although Sod's Law might come into play once the tiles have been
replaced and the moss is no longer a serious problem.

Is hers set at a steeper angle relative to the ground and thus giving
a faster flow?