Keeping pieces discrete

I’m building a structure from “plywood” and dimensional “lumber” using rectangles and push/pull. I’ve got my base and my uprights and when I try to put a “roof” on it - another rectangle I’d like to push/pull, the rectangle subdivides by the pieces it’s sitting on. How do I keep the pieces discrete from one another?

Groups and Components are what separates geometry in SU.
Generally speaking groups are for single use and components are for multiple use. As in a group is technically an individual but a component can be one of many, so edit one component and you edit them all, edit one group and that is all that gets edited.
So select relevant geometry as you go, right click and select Make Group or Make Component. Double click a group/component to open it for editing.

Thank you! Just what I needed.