A gravel pad is the preferred method but placing a shed on concrete blocks is adequate in most cases. Having a properly prepared gravel pad can help:
Prevent water damage by allowing the rain water to drain into the gravel pad
Structural integrity by having fully supported runners
Gravel pads will help settling that can cause doors and windows to not function properly
Guidelines for Gravel Pad Preparation
A LEVEL SITE: Preparing a level site is critical. The pad must be perfectly level, square, and without voids or rises. If the pad is not level, we would add blocking to level the building. On gravel or stone pads, the blocks could sink faster than the rest of the building and become uneven over time. A level site is even more critical the larger the building.
MATERIALS: For the best results, use a 4x4, 4x6 or 6x6 treated wood perimeter. Fill the area with 3/4" crushed stone. Do not use smooth rounded stone or river bed stone. A concrete pad can also be used. The concrete pad should be as close to the ground level as possible. The concrete should be level. Any voids or rises may result in water pooling that could ultimately result in rot and decay.
SIZE: The stone pad should be at least 2 feet longer and 2 feet wider than your shed. This allows for proper drainage and will help keep vegetation, and the resulting moisture, away from your shed.