A backyard shed is a useful storage space, but it becomes much less useful if it's falling over due to a failed foundation. Several foundation-building methods will keep a shed standing tall, but depending on where you live and the kind of shed you're putting up, your foundation options could be limited.
Although there are many different options for shed foundations, your choices will probably be limited by your local building codes. A building inspector can tell you how to meet code requirements for your foundation. The required foundation types vary from region to region. In a particular location, the types of foundation you'll be allowed to build will be determined by the size of your shed. The inspector can tell you what you're allowed to do and may also give you suggestions for the best type of foundation for your shed.
In many cases, a small shed can sit on a simple foundation resting on the surface of the ground. One option for an on-grade foundation is to rest the shed on concrete blocks that are themselves resting on 2- or 3-inch beds of gravel. If you build your foundation from concrete blocks, use solid blocks rather than hollow wall blocks; hollow blocks are not strong enough to support a structure and are likely to crumble under the weight of the shed.
Another option for a basic on-grade foundation is to position two parallel lengths of landscape timber, called skids, near the edges of the shed site and then rest the shed's floor frame on the skids. You can use four-by-sixes, six-by-sixes or eight-by-eights for the skids, but always use pressure-treated landscape lumber to prevent rot and insect damage to the foundation. Skid foundations are not as easy to level as block foundations, so they work best on flat sites.
A wood foundations consist of a rectangular timber frame, made from pressure-treated four-by-fours, six-by-sixes or eight-by-eights, on which the shed's walls sit. The foundation sits directly on grade, and the ground is the shed's floor. The advantage of this type of foundation is that it doesn't require a separate floor frame on top of the foundation.
Concrete Slab Foundations
A level concrete slab provides both a stable foundation and a durable floor for your shed. The stability and longevity of the slab depends on proper preparation of the base it sits on; the slab should be poured over a base of compacted gravel that rests on thoroughly compacted soil. Local building codes will specify the required thickness of the base, and they'll specify the thickness of the slab. Usually the slab should be 3 or 4 inches thick, and it should extend 2 to 4 inches above grade.
In colder climates, building codes may require that sheds sit on frost-proof foundations. A frost-proof foundation sits on concrete footers that extend below grade to the frost line. The footers may be made from poured concrete or, in some cases, pressure-treated lumber. If you're building a concrete slab foundation, the footers may be integrated with the slab in a technique called a monolithic pour.