No need to worry, it does work from SH3D. If you apply an invisible texture (a totally transparent .png) to your [font=courier new]ground_1[/font] material, the end shape (ground) will vanish. This, btw, is a nice general trick to make elements of some objects vanish.

ok

(You could also add transparency in the .mtl file, but then it is probably better to save the roof as an .obj only exporting selection).