Just about every screen for use at the input of a downspout has a very
fine mesh, perhaps blocking debris as small as 1/8", even though such
small stuff would pass harmlessly through a typical 2x3" downspout. This
causes the screen to clog, preventing the gutters from draining and
causing them to overflow.

It seems a better design would be something like in kitchen sink drains
use: a large cross that blocks only the stuff big enough to potentially
cause a downstream blockage.

Thanks for your comments.

Ray