Short answer: never set wooden framing straight on soil. For small or movable buildings, put ground-contact treated skids on a 4–6 in (10–15 cm) compacted gravel pad. For fixed buildings, use solid concrete blocks or piers on compacted gravel, or treated posts or piers below the frost line where frost heave is severe. Keep siding at least 6 in (15 cm) above finished grade, and slope the ground away from the building so water never pools under the floor.
Why buildings rot from the bottom
Decay fungi need wet wood. Forest Products Laboratory research puts the line at roughly 20% moisture content: wood kept below it doesn't rot, and wood held above about 30% does. A sill lying on damp soil wicks water all season and never dries. The same board lifted a few inches, with air under it and water draining away, can last decades. Almost every foundation choice below is a way to keep the lowest piece of wood dry.
Your options at a glance
| Foundation | Best for | Frost behavior | Effort |
|---|---|---|---|
| Treated skids on gravel | Coops, hog houses, feeders and sheds up to about 10 × 16 ft (3 × 5 m) that may move | Floats: rises and settles as one piece | Low |
| Concrete blocks or deck blocks on gravel | Small fixed sheds and coops on fairly level ground | Corners can heave unevenly on wet clay | Low |
| Gravel pad alone | The base under skids or blocks; a dry floor for open sheds | Drains water away, which reduces heave | Medium |
| Posts or piers below frost depth | Larger or taller buildings, sloped sites, heavy frost | Stays put | High |
Skids: the classic farm answer
Old extension plans put light buildings on skids for a reason. A pair of timbers spreads the load, keeps the floor frame off the dirt, and lets you drag the building to clean ground with a tractor. The Cattle Feeder stands on two treated 4×6 × 16 ft skids with upturned noses for towing. The Summer Range Poultry Shelter sits on 2×8 runners, and the Salt & Mineral Box on chamfered 2×6 runners.
- Use lumber tagged UC4A ground contact at minimum, and UC4B for skids that stay wet or would be hard to replace.
- Bevel or round the leading ends so they ride over the ground instead of digging in.
- Seal every saw cut with a cut-end preservative.
- Set the skids on gravel, not turf, wherever the building stays put for months.
How to build a gravel pad
- Mark the footprint and add about 12 in (30 cm) all round.
- Strip the sod and topsoil down to firm subsoil. Organic soil compresses and holds water.
- Fill with angular crushed stone (a mix with fines, such as ¾ in minus, compacts best), not rounded pea gravel, which rolls.
- Compact in 2–3 in (5–8 cm) layers with a plate compactor or hand tamper, to 4–6 in (10–15 cm) total.
- Level the top, or give it a slight fall to one side so water runs off.
- Edge it with treated timbers if you want it to keep its shape.
Blocks and piers
Solid concrete blocks are the quickest fixed foundation for a small shed or coop. Set each block on compacted gravel, never on topsoil, and level the building by adjusting the gravel rather than stacking loose shims. Where the floor frame touches concrete, the wood should be treated: concrete wicks moisture. Building codes require treated or naturally durable framing when it's within 8 in (20 cm) of exposed ground on a foundation.
Frost heave
Frost heave needs three things at once: freezing temperatures, frost-susceptible soil (silts and clays, not clean sand or gravel), and water feeding the freezing layer. Take away any one and heave mostly stops. That's why a well-drained gravel pad helps so much.
Two strategies work:
- Let it float. A building on skids or a continuous gravel bed rises and settles as one piece, and a light, rigid frame shrugs off a little movement. The International Residential Code exempts free-standing light-frame accessory buildings up to 600 sq ft (56 m²) with eaves no higher than 10 ft from frost-protected footings. Many farm buildings are exempt from permits entirely, but check with your local building department.
- Go below it. Posts or concrete piers set below your local frost depth don't move. Frost depth ranges from a few inches in the Deep South to 4 ft (1.2 m) or more in the northern states. Your building department publishes the local figure. The pole-framed Small Poultry House takes this route, with treated poles set in the ground.
The approach that fails is a rigid building on a mix of supports, some deep and some shallow. When the shallow ones heave, the frame racks, doors bind, and joints open.
Drainage finishes the job
- Build on the highest practical spot. The old extension sheets say an elevated, well-drained site.
- Grade the ground to fall away from the walls. Residential code asks for 6 in of fall in the first 10 ft (15 cm in 3 m), a good target for any building.
- Keep siding 6 in (15 cm) above the soil, and don't let mulch, manure or bedding pile against the walls.
- Add gutters, or a gravel drip strip under the eaves, so roof water doesn't splash back onto the sill.
Frequently asked
Can I put a shed directly on the ground?
Only on treated skids, and even then it lasts far longer on gravel. Untreated floor framing on soil can rot within a few seasons.
How deep should a gravel pad be?
4–6 in (10–15 cm) compacted is typical for small buildings. Go deeper on soft or wet clay.
Do I need footings below the frost line for a chicken coop?
Usually not for a small, light coop on skids or blocks. They make sense for larger pole-framed houses or where frost heave is severe. Local rules decide.
Code references are to the International Residential Code as commonly adopted; your local code and frost depth govern.