Any building construction project starts at the foundation. Getting the foundation level is critical to building a level and square shed. Most small sheds are built on a number of treated wood or concrete blocks. Levelling these supports requires the use of a carpenter's level and a straight board and can be accomplished by most do-it-yourself enthusiasts.
Stake out the location of the shed. Drive stakes in the ground at corner points. Take diagonal measurements between opposite corners. If these measurements are identical the location is square.
Mark the locations of the blocks on the two long sides of the shed. Plan the placement of the blocks so they are evenly spaced with gaps of about 4 feet between each block. Dig out the sod at the location of each block. Fill the excavation to ground level with gravel.
Determine the planned floor level of the shed. The top of the foundation will be 4 inches below this if 2- by 4-inch floor joists are used.
Place blocks at one of the corners to this height. Stack the solid concrete blocks, available in 2- and 4-inch thicknesses at the corner with the thicker blocks at the bottom.
Place a straight board, usually a 2- by 4-inch board, from the location of the first corner blocks to the next planned foundation. Place a 4-foot carpenter's level on the board. Stack the concrete blocks at the new location until the board between the two block locations is level. Remove or add gravel to the excavation to make smaller adjustments to the height of the foundation blocks.
Continue to level from foundation block to foundation block throughout the entire project. Use a longer straight board to confirm the level of multiple foundation blocks.
Check the level of the blocks themselves. Rest the level on the top of the concrete block foundation pier and check the level. Adjust the gravel base if the pile of blocks is not level. This insures the floor joists will rest on the entire block foundation, not just a high point of the block.