 Hmm, I'm curious. Usually, people consider a through-the wall installation to be more unsightly than through the roof. Can you share some pictures of the outside (or drawings, if the building's not there yet) so we get a better idea of what you're dealing with? You don't need to have a "chimney box" visible from the outside, it would just be a pipe sticking up through the roof with flashing around it. I assume this is a rubber roof? One thought that comes to mind if you want to avoid having the chimney at the edge of the roof would be to jog the pipe at 00 degrees the other way, toward the interior of the house, then up from there. That way the roof penetration will be less visible from the ground. 