If you don't want to spend the money, a bottom is not necessary, but it does help with protection and keeping sand clean. If you are building one like the pallet sandbox above that is not too tall, then you can skip it. You can use proper lining instead. If you, however, have a project that is more complex/taller a wooden floor will add more support. Pressure-treated wood will keep your sandbox from warping or rotting over time.