I created a model in Sketch-Up (Sketch-Up detects it as a solid, a closed hull, where all faces can be seen from outside).
Then i exported it as Collada .dae.
But Blender splits 5 faces off and converts them into separate objects, each one out of two faces, one face lies flat on the other one.
This is the model in Blender. Main part moved to set the separated objects :
Why does this happen?