I've been working with the attached model for a while and went to render it - to my dismay I received the "Object may not be a valid 2-manifold and may need repair!" .  I was able to narrow it down to the upper and lower window ledge brick applications on the side wall (see in-code comments) as the culprits.  This surprised me as these bricks appear to clearly intersect the STL wall core that I'm unioning them with.  Further, I do the same thing on the back wall without a problem. Does the group have any thoughts as to what I might be doing wrong that would cause this problem and how I could fix it? Thanks Bill Lugg