A Firm Foundation for a Backyard Shed

The success or failure of any outbuilding relies heavily on its foundation. No structure -- regardless of how well it’s designed or built -- will survive for very long on a weak or poorly made base. Therefore, choosing and building a proper foundation is the single most important construction step in the entire project.

On-grade vs. frost-proof foundations

Shed foundations fall into two basic categories: on-grade and frost-proof. On-grade foundations (sometimes called “floating foundations”) sit right on the ground and are sufficient for all but the very largest outbuildings. They’re also the quickest and simplest to build because they don’t require you to dig deep holes or pour concrete footings or piers. On-grade foundations are usually made of pressure-treated lumber or solid-concrete blocks.

Permanent, frost-proof foundations are more difficult to build, but they’re by far the strongest and longest lasting. These types of foundations are designed for cold-weather regions where ground movement caused by freeze/thaw cycles can affect a building.

The best foundation to build for your shed will largely depend on what the building inspector recommends, but keep in mind that it’s often based on three key factors: the shed’s size, the region of the country in which you live, and the type of shed floor you desire.

On-Grade Foundations

It’s no surprise that most sheds are designed to be built with an on-grade foundation. This base is quick and easy to build, relatively inexpensive, and adaptable enough to accommodate all but the most severely sloping sites. In addition, the components are small and light enough to easily set into place and shift around, making it very easy to get everything square and level. Although it’s not technically a “permanent” foundation, an on-grade foundation, when properly built, will probably outlast the shed it supports.

Solid-concrete blocks —

In this type of foundation, the shed is supported by a series of solid-concrete blocks, which are laid out in straight, evenly spaced rows. The number of blocks needed and the spacing between them is determined by the size of the shed and the lumber used for the floor joists.

It’s important to note that you must use only solid-concrete blocks for this type of foundation. Standard wall block or any other hollow block will eventually crack and crumble under the weight of the shed. If you have trouble finding solid blocks at a home center or lumberyard, visit a masonry supplier.

The blocks measure 8 in. wide by 16 in. long and come in 4-in.- and 2-in.-thick units. The thicker blocks are placed first, with the thinner “patio” blocks laid on top when you need to raise one block even with the others. In some cases, you may need to stack two or three 4-in. blocks on top of each other to raise the lowest corner of the foundation so it is even with the highest corner.

If the building site is high and dry, you can set the blocks directly on the ground. However, if there’s any chance that rain runoff will occasionally drain under the shed, you’ll need to use a shovel to remove a patch of grass under each block, compact the soil with a hand tamper, then cover the exposed dirt with 2 in. or 3 in. of gravel before setting the blocks. The gravel bed will ensure that the soil beneath the blocks won’t wash away or become soggy.

Precast pier blocks —

This building method is similar to the solid-concrete block foundation discussed above. However, instead of using flat blocks, a series of precast concrete pier blocks are used to support the shed’s floor frame. The pyramid-shaped blocks are designed for building decks, but they work great for sheds, too -- provided you choose the right type.

There are a few styles of pier blocks available, including one that has a square hole molded into the top through which a vertical 4x4 post can be inserted. Another type has a flat wood block set into the top so you can toenail a joist in place. For building shed foundations, I prefer to use Dek-Block piers. Each block measures 8 in. high by 11 in. sq. and weighs about 45 lbs. Molded into the top surface are a 3 1/2-in.-sq. recessed socket and a pair of 1 1/2-in.-wide slots. The socket accepts a 4x4 post; the slots are used to support a 2x floor joist. Because Dek-Block piers can accept either a joist or a post, they can be used on very uneven sites and badly sloping terrain.

Skid foundations —

When it comes to time-tested building methods, it’s hard to beat a skid foundation. Builders have been using this type of on-grade foundation to support outbuildings for more than three centuries. The technique is surprisingly simple in both concept and application: Two or more long, straight timbers (skids) are laid on the ground in parallel, evenly spaced positions. The building’s floor frame is then built on the skids, which are sometimes called runners or deadmen.

Skid foundations are still popular today, and it’s easy to see why: They’re very fast and easy to build; and they distribute the building’s weight evenly over a broad surface. Unfortunately, because the timbers are long and straight, this type of foundation is suitable only for sites that are relatively flat.

Originally, skids were nothing more than logs placed on the ground. Today, they’re usually made of pressure-treated 4x6s, 6x6s, or 8x8s. You can also make skids by gang-nailing together three or four 2x6s or 2x8s and setting them on edge.

Although skids are often set directly on the ground, I prefer to lay them on a bed of gravel. The stone creates a very stable base that’s not likely to settle or wash away. Begin by laying the skids in position on the ground, then mark around each one using spray paint or flour sprinkled from a can. Move the skids out of the way, then use a flat shovel to remove the sod and about 2 in. of soil from the marked areas. Check the excavated areas to make sure they’re close to being level. If they’re not, remove a little more soil from the high spots. Next, add 3 in. to 4 in. of gravel. Compact the gravel with a hand tamper or gas-powered plate compactor, then replace the skids.

Timber-frame foundations —

This foundation consists of little more than a rectangular wooden frame sitting on a gravel bed. The shed walls are built on the frame, and the entire weight of the building is transferred directly to the ground. However, the real advantage with this type of foundation is that you get to choose from a variety of flooring options. The floor area within the timber frame could be filled with gravel, concrete, crushed granite, marble chips, or slabs of bluestone or slate. Another flooring option is to nail pressure-treated 2x6s over the frame in a manner similar to that of a deck. Just keep in mind that the 2x6s must be installed before you erect the shed walls.

A foundation frame is typically made from pressure-treated 4x4s, 4x6s, or 6x6s. The timbers are joined with half-lap corner joints or stacked two or three high and fastened together with long landscaping spikes or screws.