Pouring a Slab for a Shed, Carport or Barn? What Goes Under It Matters More Than the Concrete
Most cracked slabs we look at didn't fail because of bad concrete. They failed because of what's underneath it.
Concrete is strong when you push down on it and weak when the ground under it moves. Put a slab on soft dirt, a buried stump or a spot that stays wet, and the ground will shift and the concrete will crack along with it. So if you're planning a pad for a new shed, a carport, a workshop or a barn, most of the decisions that decide how it looks in ten years happen before the truck ever shows up.
Fall is a good time to do this, by the way. The ground is usually firmer than it is in spring, and you avoid pouring in the worst of the Georgia summer heat.
Step 1: Check whether you need a permit
This varies more than people expect. A lot of Georgia counties and cities let you put up a small storage shed without a building permit, often somewhere around 120 to 200 square feet, but the cutoff is different from place to place and some places want a permit for any size. Carports, garages and workshops with electrical work almost always need one.
Call the building department for the county or city where the property sits before you order anything. Ask about size limits, setbacks from your property lines, and whether they want a site plan. If you're on septic, ask whether environmental health needs to sign off on the location too. Some counties require it.
Step 2: Pick the spot like you'll live with it for 30 years
Because you will. A few things to walk through:
Stay off the septic system. Never pour over the tank or the drain field. You'll need access to the tank for pumping, and the drain field needs open soil to work. Find them on your county permit drawing before you stake anything out.
Avoid the low spot. If water collects there after a storm, it'll collect against your slab and under your shed door too.
Watch the trees. Roots can lift a slab, and big trees drop limbs on roofs. Give them room.
Think about the drive in. If you'll be backing a truck, a mower trailer or a tractor into it, make sure there's a way to get there that doesn't cross the septic field.
Mind the house. If the slab sits within 10 feet of your home, building code says hard surfaces there should slope at least 2 percent away from the foundation. That's roughly a quarter inch per foot.
Step 3: Get the dirt right
Here's the order we follow.
Strip it. Scrape off the grass, topsoil and roots. Topsoil is full of organic material that breaks down over time, and the slab settles as it does. Pull any stumps and big roots completely. A buried stump is a future sinkhole.
Level and fill. If the ground slopes, cut the high side and fill the low side. Fill goes in thin layers, roughly 6 inches at a time, compacted before the next layer goes down. Twelve inches of loose dirt packed only at the top will keep settling under your slab for years.
Add a gravel base. Four inches or so of compacted crushed stone gives the slab an even, firm bed and helps water drain away underneath. In Middle Georgia clay, which swells when it's wet and shrinks when it dries, that layer does a lot of quiet work.
Vapor barrier for enclosed buildings. If the slab is going inside a shed, shop or garage, a plastic sheet over the gravel keeps ground moisture from wicking up through the concrete. It's cheap insurance for tools and anything you store on the floor. For an open carport or a pole barn floor, it matters less.
Step 4: How thick, and what goes inside it
A common rule of thumb is about 4 inches for a shed, walkway or patio, and 5 to 6 inches where cars, trucks or equipment will park. If you're parking a loaded dump trailer or a tractor, go thicker still and talk it over with whoever's pouring.
The edges usually get thickened into a little footing that runs around the perimeter. That stiffens the slab and gives the walls something solid to anchor to. When a structure needs footings, Georgia's residential code sets them at least 12 inches below undisturbed ground. Frost is rarely the concern here. Soft or wet soil is.
Reinforcement, whether that's wire mesh or rebar, helps hold the slab together when it does crack. And it will crack a little. Every slab does. That's what control joints are for. Those are the straight grooves cut or tooled into the surface so the concrete cracks neatly along them instead of wandering across the middle. A good pour has them spaced close enough to do the job.
Step 5: Let it cure
Concrete keeps getting stronger for weeks after it's poured. In rough terms, you can walk on it in a day or two, drive a car on it after about a week, and it reaches its designed strength around 28 days. Keep it damp for the first several days if it's hot and windy, since concrete that dries too fast gets weak and crazes on the surface.
And please wait before you build on it. We know the shed kit is sitting in the garage.
Before any digging, call 811
Georgia 811 needs at least three business days' notice before mechanized digging, and it's free. Keep in mind they mark public utility lines only. If you've got a buried power line to an existing outbuilding, a well line or an irrigation system, you'll need to locate those yourself.
Want help with the ground work?
Area Solutions does the site work that makes a slab last, from clearing and grading to compacted fill, gravel base and the concrete itself. We work across Baldwin County and the surrounding area. Tell us what you're building and roughly where, and we'll come walk the spot with you.
Call (478) 251-5800.
Frequently asked questions
Can I pour concrete right on the dirt?
You can, and a lot of people do, but it's the most common reason small slabs crack and sink. At minimum, strip the topsoil, compact the ground and put down a gravel base.
Is a gravel pad good enough for a shed?
For a small, lightweight storage shed, a well-built gravel pad with a timber frame often works fine and costs less. Heavier buildings, anything with a vehicle or equipment inside, and workshops are better off on concrete.
When is the best time of year to pour concrete in Georgia?
Spring and fall are the easiest. Summer pours work too, but they need an early start and extra care to keep the concrete from drying too fast. Freezing nights in winter are the main thing to plan around.
Why did my slab crack?
The usual suspects are soft or poorly compacted fill underneath, organic material that rotted away, water collecting under the edge, or control joints spaced too far apart. Most of those trace back to the prep, not the pour.
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