I had a 5" floor installed in my garage. Also, it is fiberglass-reinforced concrete, not just plain concrete. It has so much fiberglass in it, that before they finished it off, it looked like a giant fiberglass mat. They guaranteed it not to crack, and said that it is much stronger than regular concrete. The price wasn't very much more, so you may want to consider that if you haven't already. Mine has gone through 4 PA winters and summers now, and there have been 9 cars, parts, and some engines sitting on it all that time, and there's not a sign of a crack anyplace.