Why Does Deepflow Use The Same Size Downpipe As Half Round?

68mm Downpipe is perfectly adequate to be used with Deepflow Guttering.

The size of a downpipe is not really a factor when deciding which gutter system to install, as all downpipes have a much greater capacity than the gutters to which they connect.

This is because the flow rate of the water inside the pipe, aided by the enclosed nature of the pipe and (more importantly) by gravity, is much greater than the flow rate of the gutter itself.