How to Anchor a Shed Based on Your Foundation

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.

While posts are great for a large shed, an on-grade or “floating” foundation should be more than sufficient for a small one on a flat site. And you don’t have to dig holes or mix concrete. Varieties include stacked blocks, wooden skids, and site-built timber frames. Joe Truini, a carpenter and author of to book Building a Shed, has used all of them and offers some tips.

Solid block. Solid, 2 in. or 4 in. thick concrete blocks can be stacked two or three high without mortar. The floor frame is then laid on top of them and adjusted with shims. “It’s hard to make a mistake with this method,” says Truini. For each block location you need only remove the sod and a bit of dirt to adjust the height, then lay the blocks on the ground or on compacted gravel. Spacing is the same as when for posts. Don’t use this method for large sheds, or where the ground is out of level by more than the thickness of two or three blocks.

Skids. Two or more pressure treated 4×6, 6×6, or 8×8 timbers are laid on the ground in parallel, then the deck frame placed on top of them. A couple of inches of gravel can be spread to aid drainage. Skid as shed anchors work best on sites that are nearly level, says Truini, but can also be used on a gently sloped site by supporting one end of the skid with solid concrete block.

Timber frame. Same idea as the skid, but it consists of a rectangular frame, made from 6x or 8x stock. Sides can be joined at the corners with half laps. Just stand the shed walls on top of if and screw them in place. The timber frame is best for small sheds with non-wood decking: Truini used if for a 4 ft. x 6 ft. shed and, rather than a wooden floor, used brick. If you do use wood, it’s not a bad idea to put gravel in the space first.