New to developing, have a question.

[dewness]

Hello everyone, my name is dewness. Recently I've gotten into developing maps for Amnesia; it's been going well so far, but I just want to know one thing: when I place floors using the primitives tool, it flickers when I move and looks glitchy. This doesn't happen with the ceiling, walls or other objects. This is probably a question that's been asked over and over but I couldn't find anything on it. Any help will be a appreciated.