Moved to a new newly built barn and stall floors need some fixing up. Very uneven. Our old place had clay leveled/compacted under stall mats. I was thinking of using stone dust compacted. Any moisture that gets through mat joints will go through dust
My barn has cement floors. I bought and put in 3/4 rubber mats. My shelter (adjacent to barn) has an uneven floor (at the moment), but only one horse uses it as a stall--it's 16' x 19'--and it's flat enough in the middle.
I say even out the floor. I'm also a gardener. Just takes some muscle and elbow grease. =D