Ok, you need static lights then 
Things that will cause unwanted shadows with moveable lights
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Inaccurately aligned meshes
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Bad normals on the meshes
To rebuild with static lights, you have to keep raising the lightmap res until the jaggedness goes away. Be aware, the trade off here is memory. Yes it runs better than moveable lights, but oops, you use a lot more memory.
Also, it’s better to use the whole UV space.
You will still probably get lines. You might be able to remove by tweaking the lightmass settings.
In both cases ( moveable and static ), the only sure fire way to get a smooth floor, is for it to be one mesh.