The success or failure of your shed relies heavily on its foundation. Even the best-designed, strongest-built sheds won’t last very long if set upon a weak or poorly constructed base. Therefore, building a strong, durable foundation is arguably the single most important step in the entire project. Now let’s take a look at the best foundation options for your shed.
On-Grade vs. Frost-Proof
There are several different types of shed foundations, but they can be divided into just two categories: on-grade and frost-proof. On-grade foundations, which are sometimes called, floating foundations, sit right on the ground. They’re the quickest and easiest to build because you don’t have to dig holes or pour concrete. On-grade foundations are sufficient for most small- to medium-size sheds.
Frost-proof foundations are more difficult to build than on-grade types, but they’re much stronger and longer-lasting. For that reason, frost-proof foundations are often called permanent foundations. This type of foundation can be built anywhere but is especially well suited for cold-weather areas that experience frost heave. Frost-proof foundations are usually made by pouring concrete footings, piers, or a monolithic slab. But you can also create one with pole-barn construction, which uses a series of tall round poles or square posts set into deep holes. Frost-proof foundations are ideal—and often code required—for larger sheds.
Here are descriptions of three popular on-grade foundations:
Solid-concrete blocks—With 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 use solid concrete blocks, not hollow wall blocks, which would eventually crack and crumble under the weight of the shed.
The blocks most commonly used for on-grade foundations measure 4-inch thick x 8-inch wide x 16 inches long. There also 2-inch-thick blocks, called patio blocks, which can be set on top of the thicker blocks when it’s necessary to raise one block even with the others.
Tip: To prevent soil erosion, remove the grass under each block, cover the exposed dirt with 2 or 3 inches of gravel, compact the gravel with a hand tamper, then set the block in on top.
Skid foundations—This style of the on-grade foundation has been used to support outbuildings for more than three centuries. The technique is surprisingly simple: 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 top of the skids. Skid foundations are quick and easy to build, and they distribute the building’s weight evenly over a broad surface. However, because the timbers are long and straight, skid foundations are only suitable for sites that are relatively flat. Skids are usually made from pressure-treated 4x6s, 6x6s, or 8x8s, depending on the size of the shed.
Tip: If you can’t find long, straight timbers, make the skids by gang-nailing together three or four pressure-treated 2x6s or 2x8s, then set them on edge.
Timber-frame foundations—This easy-to-build foundation consists of little more than a rectangular wooden frame sitting on a gravel bed. The shed walls are then built on top of the frame. One reason timber-frame foundations are so popular is that it accepts a variety of flooring options. For example, the floor space within the frame can be filled with bricks, concrete, gravel, marble chips, or bluestone slabs. You can also simply screw down pressure-treated decking. The timber frame is usually made with 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 structural screws.
Tip: To ensure the timbers don’t rot prematurely, be certain to use pressure-treated lumber that’s rated for ground-contact use.
Here’s detailed information on three types of frost-proof foundations:
Concrete piers—A pier is simply a column of concrete poured into a hole in the ground. In cold-weather regions, the holes must extend below the frost line to prevent the adverse effects of frost heave. Two or more rows of piers support the shed’s floor frame. If you need to extend the height of the pier above the ground, line the holes with fiber-form tubes, which are commonly known by the tradename Sonotube, and then fill the tubes with concrete. The shed’s floor frame can be secured to the concrete piers with galvanized-metal hardware, such as post anchors or beam connectors. Just be sure to set the hardware into the piers before the concrete hardens.
Tip: Frost-proof foundations are typically required by code for sheds larger than 200 sq. ft. or taller than 12 ft. However, building codes differ from state to state, so be sure to check with the local building department for the exact requirements in your area.
Poured-concrete slab—A poured-concrete foundation is the best choice for larger outbuildings that will be used to store heavy equipment, such as woodworking machines, tractors, boats, motorcycles, snowmobiles, and cars. There are two basic ways to pour a concrete slab, but only one qualifies as a frost-proof foundation. It’s called a monolithic slab, and the shed floor and the perimeter foundation walls are all poured at the same time. The walls extend down to the frost line and are usually about 8 to 12 inches thick. The floor is only 4 to 6 inches thick, but it’s reinforced with wire mesh or metal reinforcing bars.
The second type of concrete floor is known as a floating slab or an on-grade slab. It’s simply a 4- to 6-inch thick layer of concrete that sits right on the ground. This type of shed floor should never be used when the building codes call for a frost-proof foundation.
Pole-barn foundations—All of the previously mentioned foundations are designed to support the shed’s floor frame. The floor frame, in turn, then supports the walls. A pole-barn foundation is completely different: it technically doesn’t even have a floor.
Pole-barn construction consists of several holes dug below the frost line around the perimeter of the foundation. Concrete footings are poured into each hole, and tall round poles or square timbers are set into the holes. Horizontal beams are fastened across the poles and then the walls are secured to the beams. The bare ground within the pole-barn foundation can be covered with processed stone, pea gravel, or wood chips. Or, you could pour a concrete floor.
Tip: Since the floor of a pole barn is nearly flush with the surrounding grade, it provides easy access for lawn tractors, boat trailers, farm machinery, and livestock.