On Sun, 17 Jun 2018 17:29:25 +0100
newshound wrote:

I've never made up a shelf out of planks.

I've found T&G timber floorboards are good for making wider shelves

For a long time now I've
always made shelving units up out of 18mm plywood (usually with 6 mm
for the backs).

A sandwich of ply/OSB/etc. glued and screwed to softwood battens at the
edges (with extra in the middle if it's really deep) is a construction
that works well in lots of applications.