Increase the lightmap rez of those surfaces.
In your world settings/lightmass settings, you can also lower the static lighting level scale to ,4 or ,2 instead of 1.
At the same place you can untick ‘‘compress lightmaps’’.
Raise the resolution to at least 2048 for a big surface like that.
Ideally make sure you only have the top part of the floor on your lightmap. The floor plane should use all the 0-1 space of your lightmap, or as much as possible. If your floor is a rectangle with thickness, just detach the top polygon in your 3d model software and assign it a proper lightmap.