1. Select a whole building first - all of it's levels, stairs, walls, roof, etc. Hold down the shift key when selecting additional things.
2. When you have the whole thing, hit ctrl-c to copy it to the clipboard.
3.Then click the 'save clip' button to save it with whatever name you want.
4. Then when you want to load it, click 'load clip' and move the cursor someplace and hit ctrl-v to paste it.
5. Paste it at the highest level that the building is in. So if the roof is at level 13, then paste it in there. It takes a little practice - don't expect to get it right the first or second time. And if you screw up, hit ctrl-z to undo.