I want to build an 8'x8' root cellar with cinder block. I will place 1/2" rebar vertically every other hole (one per block). Can I lay 3/8" rebar horizontally between courses of cinder block, since I want a 3/8" morter joint anyway?

Welcome to AMHD.

I am not a mason but have done a lot of block work.

If u use same rebar as motar joint, u will weaken the holding power of the mortar because the block will actually be seating on the rebar.

Need a larger joint or smaller rebar.

Chuck