Slicer is adding additional floor bed

Question

I'm trying to print an architectural model of a house with 2 floors and a room on the roof. It was designed in SketchUp by a friend. I removed inner walls and floors to print it upside down to get just the outer look of that building. All is good in SketchUp and in Meshlab (converted .skp to .3ds) and even after exporting it to .stl  format. But when I slice it and select preview, it is adding a floor on its own. Please take a look at the attached screenshots to get a better idea of what I am referring to. Any idea what's causing this and how it can be solved?

(Using Ultimaker Cura 4.1.0 for slicing at 0.2 mm layer height)

Trish · Answer

SketchUp has a problem with generating the normals of an STL in the right way. As you see in the slicer, the model claims, that the "tops" are overhangs, which means that the whole model is "inside out" with the normals.
To fix that, you'd need to open the file in a program that can fix that by recalculating the normals. I suggest blender:

Open Blender
Delete the cube by clicking on it and pressing the delete button
Import the STL via Drag & Drop or the menu
Change to edit mode in the top left corner
A to select everything
Ctrl + N to recalculate normals
N then under Mesh Display > Normals hook Faces

Now, your model should look like a hedgehog, each face having a blue line poking out.
It also should show if there are any faces remaining, which can be selected and deleted manually by clicking it, so that only it is highlighted, and then pressing delete

Slicer is adding additional floor bed

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