Lighting issues

I don’t have much experience. I’m designing a level right now for a sort of sports/gladiator game.
The level is the “stadium” where the game will be held.

What I did to create the floor is there are multiple cubes that make up the floor. Planning to make each cube an interchangeable piece that can have different obstacles, etc in them.
To make the roof I just duplicated the bottom half of the stadium, rotated 180 degrees.

Since doing this I am now getting these light grid lines projecting onto the floor. The cubes are butting up to each other, light should not be getting through.

What am I missing?