We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Martin Bonner
saying something like:

You did explicitly say "for the verticals", but just to make su
19mm is /not/ adequate for the actual shelves (unless they are about a
foot long, and only holding one layer of paperbacks.

I find 19mm perfectly adequate for bearing assorted loads of books /pc
gear /camera equipment if supported at 3' intervals - could probably go
wider.