When I try to stack a terracotta half slab on an oak slab, or any other slab, there is a gap.

Same goes the other way, if I put down a half slab, and then a terracotta half slab on top there is a gap.

Wasn't an issue, I'm just new to building detailed things.

Hey @chuck252, sounds like you might be trying to mix slabs of two different types? This isn't supported by default (as I think you realized) but there is a mod called Double Slabs that you could check out that allows this.