Smooth, Firm Site

• Choose a site for the shed which is fairly smooth, has solid (not sandy) soil and has good drainage, ideally a slope of at least 1/8-inch per foot away from the shed foundation for at least 10 feet on all sides. Shed foundations don't have to be as solid as those for houses, but they must be level and with soil firm enough that foundation elements won't sink into it. Once you've picked the site, level it; scrape off any obvious high spots and fill any low spots with the same kind of dirt as the rest of the area. If moisture is a problem, dig out the area and fill it with compacted gravel.
  
  

Square the Site

• A key step in building a shed foundation is squaring it. Do this by outlining the area with stakes and builder's twine, slightly larger than the area of the shed itself. Use a corner measuring system to make sure the outline is square. Measure diagonally from one corner to the opposite corner, then repeat with the other two corners. Those measurements must be the same. Adjust the outline as needed until it is square.
  
  

Choose Foundation Material

• Pick a type of foundation. You can pour a solid concrete slab, set concrete blocks or precast concrete piers, anchor wood posts in the ground, lay railroad ties or similar wood treated to resist rot and insects or just install a thick base of gravel. Masonry foundations are least subject to decay and movement; poured slabs are best for sheds which will be subject to a lot of dirt or moisture from tools and materials stored in them. Pre-cast piers come with built-in connections for floor joists. Most important is making sure corners are square and the foundation is level.
  
  

Support All Sides

• You need foundation support on all walls and in the center of the shed. Place concrete blocks, piers or wood posts at each corner of the shed, in the center of each wall and in the center of the shed area. You can set blocks in a solid line around and across the shed area if you wish. You can build right on a concrete slab. With piers and posts, add joists between the anchors to support a wood floor, usually plywood. Some shed builders set wood pallets on the anchors or a compacted gravel base to hold a floor; this is a cheap and easy alternative if the shed will not get heavy equipment or use.