The National Oak Flooring Manufacturers Association (NOFMA) recommends that concrete slabs be sound and flat. To prepare the slab, grind off any high spots, fill low spots, and remove grease, oil, stain, and dust.
Concrete Slab Moisture Management
Concrete slabs must be tested for excessive moisture before installation. Testing options, which should be conducted in several spots on a slab, are the rubber mat method and the polyethylene film method.