An important part of framing a shed is deciding how to anchor it. Should you anchor it deep in the ground or set it on top? Unfortunately, the answer isn’t always straightforward and will depend on the shed’s size and intended use, the soil characteristics, the slope of the site, and what the building inspector will accept.
In-ground foundations are common for large sheds where the building department wants footings placed below the frost line (36 to 42 in. deep in northern states, and 24 to 30 in. in the mid-Atlantic, for example). Done right they will support a lot of weight, but you of course have to dig holes and mix concrete to lay the shed anchors.
The ultimate choice is a concrete slab with stemwall – the foundation used in most garages. While uncommon for sheds, it’s worth considering for a large one that will be heated, especially if the owners may want to convert it to a studio at some point.
Far more sheds are supported by posts placed every 6 to 8 feet, as well as at each corner. Some builders use concrete sonotubes, some pour a concrete pad at the bottom of each hole to support a pressure-treated post, while still others simply compact the base of the hole and drop the post into it. For large sheds on weak soil, the inspector may require a bell footing to spread the load and anchor the shed.