Pouring a concrete slab for a shed is a great option if you want a permanent base for your shed and will provide a lifetime of service.
While there are many types of foundations possible for a shed, the concrete slab is generally recognized as the most stable type. When properly prepared, a slab is resistant to frost and even localized ground movement. It provides a good, level base for your floor joists, or you can use the slab itself as the floor.
This means that you won’t need a ramp for wheeled equipment to get in and out. You also won’t have to worry about your garden machinery being completely dry before putting it away, as wet tires won’t cause a problem on concrete. A raised floor is another choice, using treated lumber as joists. It allows for airflow between the slab and floor. This is good for moist climates, as it allows moisture to dissipate more quickly.
The drawbacks include the difficulty of moving the shed should it become necessary. This is why the location choice for your shed is doubly important if using a slab. Be sure of any restrictions in your locality for this, such as distance from the property line or other buildings. If you plan to use your shed as a workshop, you might consider a raised wooden floor.
If you spend any time in your shed during cold weather, the cold concrete will eventually make your feet cold. There are many other options for a shed foundation and you can read more about them in my article. What’s The Best Shed Foundation Option For You?
How Do You Prepare the Ground for a Concrete Slab?
Assessing the site you plan for your shed is your first consideration. Look at the lay of the land and see if water will flow toward your shed during heavy rains. Does this site sit in water during rainy weather? How do you plan to deal with the rainwater falling from the shed roof?
The soil type is also important to think about. Do you have a good depth of topsoil? Is the soil loose and sandy or does it contain a lot of clay that tends to become waterlogged? The soil type affects how deep you will need to dig for your foundation.
You can use the heel of your boot to test your soil. If the boot leaves a shallow imprint in the dirt, the ground will provide a fairly good base for your shed.
The frost line is another vital consideration. Frost causes moisture in the ground to freeze and thaw, expanding and contracting the soil. This movement will actually lift and lower your shed. Many town building departments require that all foundations extend below the frost line for this reason.
When you apply for your building permit, it would be a convenient time to ask about the regulations in your locality. You can read my article Do You Require a Building Permit For a Shed? to find out.
When preparing the ground, dig out the ground for the base to a depth of around four inches. You will probably need to make the excavation three or four inches larger than the actual slab will be.
Make sure the base is smooth and level. If your ground level is not even, this is where you make the base level. A two-inch layer of crushed stone goes next. This gravel will extend beyond your slab all the way around to help roof runoff to drain away from your shed.
If your soil is soft, you may have to dig down deeper to get below the soft layer. The additional depth should then be made up with additional gravel. The gravel should be tamped down to make the base more solid and level it. There are tools made to tamp down the gravel by hand, or you can rent a powered compactor.
If you needed to dig a deeper depth, you should tamp down the extra gravel in layers, measuring the level as you go. This makes the gravel layer more stable and helps you keep your gravel level. It’s much easier to level two 2-inch layers of gravel than one 4-inch layer.
How Do You Build Forms for a Concrete Slab?
The form is a wooden framework that will hold the concrete in place until it dries. You’ll want to use boards the width of the planned depth of your slab, as the top of the boards will be the level of your concrete.
Put each board in position, making sure the tops are all level with each other. You can use either 2×4 pegs of wood or steel pins to hold the planks in place. Since you’re driving these into the gravel and then the ground, the steel pins will probably be easier to work with.
How Thick Does a Concrete Slab for a Shed Need to Be?
The usual thickness for a shed slab is three to four inches. The smoother you can get the gravel base underneath, the thinner your slab can be, but it’s best not to try for thinness, especially with a larger size shed.
Do You Need Rebar for a Shed Concrete Slab?
Opinions vary on this. Some feel that using rebar actually encourages cracking. Concrete used by homeowners is often relatively porous, allowing moisture that will corrode the reinforcement. If you have a large slab to pour and want reinforcement, investigate various kinds of metal mesh. Stainless steel, for instance, resists rust.
You can also have synthetic fibers added to the concrete mix in order to strengthen it. Your supplier can answer questions about this.
Do You Have to Install a Vapor Barrier Under the Concrete?
It’s not absolutely necessary to install a vapor barrier, but it keeps moisture away from the slab. Concrete is porous, and ground moisture will eventually seep through the slab, possibly affecting any object that is in constant contact with it. This is why any joists or supports that are placed on the slab should be made of treated lumber.
If you are using the slab as your actual floor, the moisture will eventually affect items setting on it, such as wood, cardboard boxes or paint cans. You can eventually have mildew problems in the shed also. Not only can it ruin some items on the floor but can creep up your studs or wall materials. For more information read my article Do I Need a Vapor Barrier Under My Shed?
Another consideration is your flooring. If you plan to install a vinyl floor covering, for instance, even in dry climates the occasional moisture can seep up through the concrete and affect the floor covering.
It also prevents gases in the soil from seeping up through the slab into your shed. For instance, if your property was once used as farmland, there may be remnants of fertilizer or pesticides in the soil that produce gases, so this is a real concern.
Installing a vapor barrier really isn’t a huge job, and it will keep you from having concerns when you have an especially rainy spring. While a 6-mil thick polyethylene plastic sheeting has been used for years, this thickness has been found to be susceptible to tearing when the concrete is being poured. This can be overcome by using a thicker plastic 10 to 20 mils thick. The thicker plastic will also keep the concrete from curing too quickly.
What Is the Approximate Cost Per Square Foot of a Slab?
This will vary according to market pricing in various areas of the country. Some sources put the approximate cost at around $5.00-6.00 per square foot including labor for a 4-inch thick slab. Others put the cost at $7.50-$9.00 per square foot. If you’re hiring professionals to pour the slab, labor costs vary even more than materials costs depending on your location. If cost is a big issue check out my article What Are the Strongest Shed Floor Materials to Use? for other strong options.
Should I Hire Someone to Pour and Finish the Concrete Slab?
If your shed is going to be quite a large size or you have no experience with concrete, you may want to hire a professional to pour and finish the slab. Another solution that will save money is to have pre-mixed concrete delivered, then have it directly poured into your form.
If it’s not feasible to get the truck to the slab site, you can use wheelbarrows to deliver it from the truck and pour it. You’ll want to have help and a few wheelbarrows available. Ask your supplier what type of concrete you’ll need, as there are different types for different uses
You can use a simple formula to find out how much concrete you’ll need. Multiply the length by the width of your proposed slab in feet, using decimals, to figure the square footage. Convert the thickness you want to feet the same way. Multiply the thickness by the square footage to determine the cubic feet.
Multiply this number by 0.037 to convert this figure to cubic yards. This is the figure you’ll need to order ready-mixed concrete. Always plan on ordering a bit more than needed. This is no place to skimp. If it turns out you didn’t order enough you’ll have big problems.
If mixing your own you can figure out how many bags of concrete you’ll need. A 40-pound bag yields 0.011 cubic yards, a 60-pound bag holds 0.017 cubic yards, and an 80-pound bag holds 0.022 cubic yards. Of course, it’s better to end up with an extra bag than not enough, so be sure to get an extra bag. The various types should provide you with mixing instructions.
The easiest and least time- and effort-consuming way to mix the concrete yourself is to rent a mixer and use wheelbarrows to transport and pour it. You can mix it directly in the wheelbarrows, but this takes up more time, as you don’t want to mix very large batches due to the weight.
It also wears you and your helpers out more quickly. Be careful not to mix large batches in the wheelbarrow as one filled with concrete can weigh up to 400 pounds, so take that into consideration when mixing and loading.
Keep in mind that concrete starts to set as soon as it is mixed, so if you’ll need quite a few batches it may be better to buy the concrete ready-mixed and delivered. If mixing yourself, whether in a mixer or in the wheelbarrows, likely you’ll need a few friends with wheelbarrows to keep the concrete flowing to the slab site.
If you are adding reinforcing fibers to the concrete, you may need to rent a paddle cement mixer to ensure that the fibers are spread throughout the concrete evenly. Mixing by hand is not recommended, but do ask your supplier about the various brands, when to add them to the concrete mix and how to mix them.
How to Pour and Finish the Slab
Wet the gravel and board first. Pour the slab evenly from one side to the other. Fill your form to the tops of your frame boards, using rakes, shovels or hoes to move the concrete into the corners and throughout the frame. Tap the sides of the form from the outside to encourage the removal of air.
Then it’s time to screed or smooth the concrete. Use a 2×4 or 2×6, using the short side to smooth the concrete. It’s best to use a plank that’s a little longer than the width of your slab so the plank can rest on the tops of the form boards. You may need two people for this. Wear boots and go ahead and step into the concrete. Draw the screed board along with a sawing, scraping motion.
This will both smooth the surface and push the concrete into any gaps. When done, you may notice a sheen of water over the surface of the concrete. Wait until this bleed water disappears before the next step. The concrete will start to firm up during this, but that’s to be expected.
The next step is to use a float, which is a tool that draws up the cement and fine aggregate to the surface of the slab. There are various kinds, including hand floats for smaller jobs, but for larger ones, a bull float may be needed.
These are usually around eight inches wide by three to ten feet long and have a long handle. You may need to use some pressure if your concrete is starting to harden. If you’re satisfied with the look of the slab, you can stop here. If not, you can do further smoothing with a steel trowel. For a non-slip surface, you can finish up with a broom.
However you decide to accomplish pouring your slab, it’s a job you can be justly proud of when it’s all done. Plan for you and your helpers to sit down with a cold drink to admire the work and yourselves.