How can I get rid of a yellowjacket nest under a flat roof without using poison
You might consider building a "bee vacuum" from a box with some screened holes and a shopvac, like the one in this video by Matthias Wandel: Wasp Sucker.  This would enable you to catch most or all of the adult wasps (you might need to do this a few times over a week or so, I'm not sure about the yellowjacket lifecycle) and then dispose of them through drowning in soapy water, controlled posioning (since your concern with using posion seems to mostly be about residue on-site), freezing in a block of ice, or whatever method suits you.