The Natural Philosopher wrote:


Yes, you may well have a problem, since there needs to be quite a fall
on a soil pipe to ensure the turds get sped on their way.


As others have noted the fall can be as low as 18mm / metre run, which is
only just noticeable with the eye, the idea behind relatively the low
gradients is that if the pipe is too steep the water can flow quicker than
the turds, hence leaving them high and dry in the pipe.

Jon