Why does building make my light map wonky?

Hi there,

I’m new to UE4 and just following a SAMS book at the moment to pick of the basics. I’m on the lighting hour so early days, anyway, consider the 2 screenshots,

Lighting has not been built at this point

Lighting has been built at this point with “production” quality selected

Now I’m obviously doing something wrong, or my understanding is totally off. But considering this scene only consists of 2 cube primatives with default materials, and a single point light. Shouldn’t the build production light map look more like the first preview? I have all 3 actors set to static.

This one is after deleting the light source, dragging new one back in, and then dropping the cube down slightly so it’s protruding through the floor cube. The light map looked fine until I built it, then it went like this.

Okay got it, and would have found the answer sooner if I had ignored it and just read on in the book.

The problem is the light map res for the floor mesh was set too low.