If you think about it ,your shed floor will likely have to hold more than a ton of weight. And what about all that stuff? Do you want to load it in just to have to take it back out someday to make repairs to rot or damage. Answer’s no of course! And as the floor goes, often the rest of the shed soon follows. So your buying a shed, get the right floor the first time around. A little thought in advance will not only save time and trouble but also considerable expense later to repair a failing floor. Rule of thumb here is to always choose that the floor of your shed is constructed with pressure treated lumber when ever possible. Also be sure the joist will carry the intended load to be placed in the shed later. If necessary choose an option which offers additional joist’s,  12″ on center joists or those of greater dimension if necessary.