I read that you said 1/4” over Berber carpeting was sufficient for a home gym. Would 1/4” be good over concrete, or should it be thicker due to the hard under-layer?

You can use any thickness of our Regrind Rolls over concrete. The hardness that you will feel is due to the rubber flooring itself and not the subfloor. Our 1/4" and 8mm thicknesses are typically used in home gyms. Our 3/8" thickness is popular with commercial gyms, and our 1/2" thickness is popular for anyone doing Olympic lifts.