Noob problem - dark lines around ceiling in interior rendering.

[handz]

Hello guys,

I am an absolute noob so do not look at the modeling etc here (yep, the lamp is partially in the wall haha) It is just a quick visualization of a reconstruction of my apartment I made to see how it will look.  But then I started to play with it a little more, to finally learn some new skills about interior renders - but some things I am absolutely unable to find a solution for.  There are black lines around the whole ceilings - like if the ceiling does not connect to walls, but it dies. Also above windows, there a bit too dark areas - how to solve this?  Thank you for any help.

[TomG]

Do walls and ceiling have thickness, and are not just planes?

[maru]

Either there is a gap there, or the walls have no thickness, or:
- There is displacement applied to the walls and / or ceiling, so it looks different in the viewport and in the render
- There is some mesh problem here, like the whole room object is imported from CAD and then scaled 1000 times