Yes, it definitely has something to do with the angled ceilings. Deactiving the ceilings fixes it - thanks for that fast advice!

But is there another way to fix it while leaving the ceilings active?

Hi,

while building my house random polygons show up in the 3d view. This occurs especially often when i create triangular walls.

It is quite annoying, because these wrong polygons block the view to the room. Is there a way to fix this?

Thanks!