Consider Skids

• When building a shed on uneven, shifting or soft ground, the more involved forms of foundations like concrete piers and slabs are required. However, a small and simple shed built on level, solid ground will be fine with a simple skid base over a layer of gravel, says Just Sheds. Digging a small depression to hold the gravel helps with drainage and prevents it from sinking into the ground. Skid bases allow the shed to be moved as you please. Pressure-treated lumber must be used for any part of the skid that comes in contact with the ground or gravel to prevent rot.
  
  

Concrete Piers

• Large sheds that are over 200 square feet require a more permanent base than gravel and skids due to the increased weight load, according to Popular Mechanics. You could sink posts into the ground to create supports for the joists, but concrete piers are a better choice. Pressure-treated lumber won't rot, but it could shift from frost heaving or other changes in the ground around it. Concrete piers are heavy enough that they will keep the shed upright and supported despite ground changes. Preformed piers are available, or you can dig and fill your own.
  
  

Dig Deep

• Sheds are often set on a base of material that rests on the top of leveled ground, which is called an on-grade foundation. While on-grade foundations are easier to build, they may not be appropriate in areas with very cold winter temperatures. The water in the soil below your shed can freeze and expand, causing the ground to break or shift in a process known as frost heaving. If this is a concern, a concrete slab or piers should be installed below the level of the ground that freezes each winter to prevent damage.
  
  

Add Space

• One of the biggest threats to a recently built outdoor shed is rot from excess moisture. The base of a shed is the most susceptible to this problem because of the water rising out of the ground. To prevent rot from ruining the floor of your new building, Popular Mechanics recommends leaving at least 6 inches between the ground and the lowest piece of lumber on the shed. Alternatively, you could use only rot-resistant pressure-treated lumber for the joists and bottom sills of the shed.