I’m working on a new lot with a number of levels this week. Sometimes in build and buy modes there are these diagonal pieces that become visible. The lot has to be rotated at a certain angle to see them and they don’t appear to affect how the lot works, but they are really ugly.
In this screenshot, we see two pieces at the floors of two basement levels. These are phantom ceiling pieces that, as far as I can tell from a few searches around the interwebs, are a byproduct of the autoroof tool. Luckily they are easy to remove. Just use the sledgehammer tool to delete them. The tricky part of removing them is getting the sledgehammer in just the right place; sometimes you’ll only see a very small sliver of the edge, but if you’re careful, you can remove them.