Nice workaround okh
I checked it wasn't a regression introduced in a recent version, but it's not the case. When the bottom of an opening is lower than the highest point of a trapezoidal wall, Sweet Home 3D always computed "holes" in walls this way. I didn't think it could be important to take care of such cases, except if you draw two walls close to each other as you did.
It will be fixed in the next version, except in case the bottom of the opening starts to intersect the wall top line (like the 3rd and 5th vents in the image below).
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Emmanuel Puybaret, Sweet Home 3D creator
