Just my 2 cents (Euro). a number of years ago I asked the same queston qnd Henry gave me the answer. The same as put here. I had made some 'velux' windows as 3D symbols in all the standard sizes they produce. Now I just choose whichever size I need, explode it so it is no longer a symbol, put it vertical to the ground plane (important otherwise your window will not be in the same plane as the roof), select it and make object. The object disappears and reappears in the objects palette. Then insert it as a 3D opening attatched to the roof assuming you used the roof tool to make the roof. Adjust the height, plus or minus to jet it flush.