It is determined by where you are in the nation, the type of soil you are working with, and the estimated loading of the patio. In general, rebar is not required in the patio slab itself, but there may be some bending moments in the footers that would put them under stress... Therefore, a few inches of rebar in the footer hole can help prevent any potential problems down the road. The overall size of your patio will also play a role in determining whether or not you need rebar. Larger patios usually have stronger flooring materials that can support their own weight, so they often do not require any reinforcement. Smaller patios often use concrete for their floors which is much less strong than wood and should have a reinforcing bar placed every 16 inches or so to keep it from breaking under its own weight. Finally, if you live in an area that experiences earthquakes, then you should probably include some kind of seismic reinforcement in your patio. These bars are placed horizontally across the floor of the patio and connect to the foundation at each end. They work by taking the pressure off of the house's foundation instead of being crushed like a soda can under heavy furniture. Rebar is only necessary for the strongest patios out there who want to be sure nothing will break their floor in the future. For most patios, a couple of feet of vertical fencing will do the job nicely!