How to Make a Concrete Slab for a Shed Foundation

You've got a new shed to build. Before you start framing walls, you need something solid underneath it. A concrete slab foundation is one of the most reliable ways to support a shed. And if you do it right, it'll last decades.

Below, we'll walk you through the full process, from planning to pour, so you can build a concrete shed foundation that holds up.

Why a Concrete Slab Is the Best Shed Foundation

A concrete slab gives your shed a flat, level, solid base that sits directly on the ground. It won't rot, it won't shift like pavers, and pests can't burrow through it. And if you're storing heavy equipment, a slab can handle the weight without flexing or sinking.

Compared to other shed foundation options, a concrete slab is hard to beat for a permanent structure. Here's why:

If you're building a permanent shed, especially one larger than 100 square feet or one that'll store anything with real weight, a concrete slab foundation is the right foundation for the job.

Before You Start: Planning Your Concrete Shed Foundation

A good pour starts long before you mix a single bag. Get the planning right, and everything else falls into place.

Check Local Building Codes

Most municipalities have building permits and codes that apply to sheds over a certain size. You may need a permit, and there are usually setback rules that dictate how close to your property line you can build.

One of the most important things to look up is your local frost line (the depth at which the ground freezes in winter). In areas with freezing temperatures, your foundation needs to extend below the frost line or sit on a properly prepared shed base to prevent heaving. Frost-proof foundations prevent the kind of cracking and shifting that ruins a slab over time.

Choose the Right Size

Your slab should match the footprint of your shed or extend a couple of inches beyond it on each side. Measure carefully. A 10×12 shed needs at least a 10×12 slab.

For a small shed used to store garden tools, a standard 4-inch-thick slab works fine. For larger sheds or shops where you'll park mowers, ATVs, or other heavy equipment, go thicker; 6 inches with heavier reinforcement.

Evaluate Your Soil Type and Drainage

Soil conditions make or break a slab. Clay-heavy soil holds water and expands, while sandy soil drains well but may not compact as firmly. It's important to know your soil type before you dig.

Proper drainage is just as important. If water pools under or around your slab, you're looking at erosion, settling, and potential water damage to anything you store inside the shed. Grade the surrounding ground so water flows away from the slab. A 1–2% slope away from the foundation is a solid target.

Tools and Materials You'll Need

Gather everything before you start. Nothing kills momentum like a trip to the hardware store mid-pour.

• Pressure-treated lumber (2×4s or 2×6s for forms)
• Wooden stakes and screws
• Crushed stone or gravel (for the base layer)
• Rebar or welded wire mesh
• Concrete mix
• Tamper or plate compactor
• Screed board
• Bull float or hand float
• Level (4-foot minimum)
• Edging tool

A note on mixing: If you're working with bagged concrete, a MudMixer can save you serious time and labor. It handles over 45+ bags (80 lb each) per hour with one person. No batch mixing, no overloaded wheelbarrows. You roll it right up to your forms and pour directly. For a shed slab, that kind of speed turns a full-day pour into a few hours of work.

Step-by-Step Instructions: Pouring a Concrete Slab for a Shed

Here's the process from dirt to done.

Step 1: Mark and Excavate the Area

Drive stakes at each corner of your planned slab and run a string line between them. Use the 3-4-5 triangle method to check that your corners are square: measure 3 feet along one string, 4 feet along the adjacent string, and the diagonal between those two points should be exactly 5 feet. If it's not, adjust until it is.

Excavate the area to a depth that accounts for your gravel foundation (typically 4 inches) plus your slab thickness (4–6 inches). For most shed slabs, you're digging about 8–10 inches below ground level.

Step 2: Build the Forms

Cut your pressure-treated lumber to length and set it around the perimeter of your excavation. The top edge of the forms should sit at the finished height of your slab.

Stake the outside of the forms every 2–3 feet and screw the lumber to the stakes. Check for level in all directions. If your slab isn't level at the form stage, it won't be level when the concrete sets.

Step 3: Add the Gravel Base

Spread 4 inches of crushed stone or compactable gravel inside the forms. This layer serves two purposes: it provides a solid base under the slab and it promotes drainage so water doesn't collect beneath the concrete.

Compact the gravel with a tamper or plate compactor until it's firm and level. Don't skip this. A poorly compacted base leads to settling and cracking down the road.

Step 4: Install Reinforcement

Lay rebar in a grid pattern (typically 12–16 inches on center) or use welded wire mesh across the entire area inside the forms. Set the reinforcement on rebar chairs so it sits in the lower-middle third of the slab, not resting on the gravel.

Reinforcement ties the slab together and controls cracking. Without it, temperature changes and ground movement will split the concrete over time.

Step 5: Mix and Pour the Concrete

This is where the job picks up speed, depending on your setup.

For a standard 10×12 shed slab at 4 inches thick, you're looking at roughly 1.5 cubic yards of concrete. That's around 60 bags of 80 lb mix. Mixing that by hand or in a barrel mixer is brutal. A MudMixer cuts that workload down dramatically. One person can mix a full yard per hour using its continuous mixing system, and the adjustable water dial keeps your mix from going too soupy or too dry.

In the Mix: Watch the MudMixer Pour a Concrete Slab Foundation:

Watch as a DIYer uses the MudMixer for the first time to pour a complete shed slab. From dropping the first bag to filling the entire form in just 15–20 minutes, he covers the full build from excavation to finish, and breaks down exactly what it cost him.

"While it's faster than mixing concrete with a shovel, it's much slower than using a MudMixer." (when comparing his old barrel mixer to the MudMixer)

Step 6: Level and Finish the Slab

Once the concrete is in the forms, grab your screed board and drag it across the top of the forms in a back-and-forth sawing motion. This levels the surface and pushes excess material ahead.

After screeding, use a bull float to smooth the surface and push aggregate below the top. Once the bleed water evaporates and the surface starts to firm up, run an edging tool along the inside of the forms to create a rounded edge. This prevents chipping.

Step 7: Cure the Concrete

Don't rush this part. Concrete reaches most of its strength in the first 7 days, but full cure takes about 28 days.

Keep the slab moist for the first 5–7 days. Spray it lightly with water once or twice a day, or cover it with plastic sheeting to retain moisture. Avoid walking on it for at least 24–48 hours, and don't place your shed on the slab for a minimum of 7 days.

Temperature matters here. If you're pouring in hot weather, the slab dries too fast and can crack. In freezing temperatures, the curing process slows dramatically. Aim for moderate conditions (50°F to 80°F is the sweet spot).

How Thick Should a Concrete Slab Be for a Shed?

This comes down to what you're putting inside:

If there's any chance you'll upgrade to heavier storage later, pour it at 6 inches now. It's a lot cheaper than tearing out a cracked slab and starting over.

A Solid Foundation Starts With the Right Pour

If you're tackling this project yourself, the biggest time sink is the mixing and pouring. That's where the right equipment makes a big difference. A MudMixer lets one person handle what normally takes a crew, with consistent mix quality from the first bag to the last.

You can purchase or rent one at over 2,000+ dealers across the country or order directly online at mudmixer.com.

Build it right. Your shed (and your back) will thank you.