Hello,
I'm using sh3d 5.4 on linux mint. The attached files show a house with a roof which was created by piece by piece with obj files I made separately and imported as furniture.
The images show artefacts in the 3D view which appear at the plane of the top or bottom of the respective pieces. Most of them don't appear permanently but only at specific angles in the 3D view. The artefacts show up in the rendered image even if the normal 3D view is clean (see attachment).
I observed these artefacts also in earlier versions ov sh3d.
If desired I can also provide the original file.
Regards
In Sweet Home 3D, the two sides of face may be visible or not.
In your case, I guess you imported roof parts from walls you exported to OBJ format, but you probably didn't select the walls to export and/or didn't choose the Export selection option when prompted.
You may fix this problem without exporting again theses walls : double-click on a wrong roof part to display its modification dialog box, click on the Modify button beside Materials option, then select the ground material in the list and chooose the Invisible option.