Try just grouping and raising the bottom face of walls a little. Hide the edges of the face.
Just yesterday I found myself having to provide an extra backdrop (simple face) behind some objects, because in LO Hybrid rendering was producing some trashy artifacts on curved surfaces. Nothing's perfect.
I would like to add to this here--I am seeing this now in 2015, so beware: In Vector rendering coarse short lines can appear--it seems mostly on smaller rounded surfaces. In the case above I guessed that something was "bleeding through" from further back in the model, but now I am not sure there fixes for this. This appears in Vector and Hybrid rendering. Incidently, I have mostly seen it in curved surfaces in my door and window components.
Another Vector rendering error is some hidden lines appearing. This could occur in previous versions of LO as well.
Screen Shot 2015-01-11 at 9.10.29 AM.png
