Fixing Non-manifold edges for 3D printing This is my first model for 3D printing, and I'm trying to fix non manifold edges. I included a screenshot, there are 6 non manifold edges. Things I already tried:
- recalculate normals
- remove doubles

To me it looks like there are no double edges left, so I'm clueless to what causes this problem. :) Who can help me?

The way I created this part is deleting two faces in the wall, extruding the floor face and merging the vertices to make it 1 piece again.
Non-manifold geometry is geometry where an edge is shared by more than 2 faces: 



This is what you should be looking for. It's easy to find using Select -> Select All by Trait -> Non Manifold 



Once you find it you can fix it by deleting some unnecessary faces, or separating the edges. You could use Edge split for that: 



Note that it splits  them all so you need to move one and then remerge the two you like connected(you can use Merge by Distance for that)