The CopyAsNewFurniture plugin, like a few other ones, can't work with Java Web Start for security reasons. But for you, this plugin shouldn't be necessary from version 4 and its Beta version if you select the walls used to create a roof part before choosing 3D view > Export to OBJ format. In that case, Sweet Home 3D will prompt you if you want to export only the items selected in the plan, or all home items with a ground item at elevation 0, and you have to choose the Export selection option. This won't export the block you spoke about.
Don't worry for the double post and don't forget to sleep!
I'm looking forward to seeing your pictures.
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Emmanuel Puybaret, Sweet Home 3D creator
