Adjusting the center of two joined objects for perfect mirroring

I am still working on the Grant Abbitt well tutorial, but I don’t know how do I re-center the center of the roof tiles and the beams of the well after joining them. How do I do that?

That picture shows my progress so far.

and in response…use Shift + S and select Cursor to World Origin that will get you a better Mirror option and you can then add an empty and use the empty as the mirror object …