First:  when asking for help diagnosing a problem, please try to reduce it to a minimal case and ensure that all of the pieces are in place for the person helping you to test your program.  I had to fix several paths in the program to match what you supplied in the .ZIP file. Yes, you can use an imported STL for any purpose, if it's a valid STL. Diagnosis:  put a # in front of the "color("green")", F5, and then View/View All.  Note that your wall is about 40 units tall, and your wedge is at about 3000-3500 units up from the origin. Your problem is that your wall is inside a scale([HOScale,HOScale,HOScale]), but your wedge is not. Putting the wedge inside a scale(), or moving the scale() outside the difference(), will fix your problem. Note that when all three axes are scaled the same you can just use a single value, scale(HOScale). Also:  you have a section at the corner that extends further -Y than the rest of the wall and the wedge isn't big enough to take it out.