What’s the Best Concrete Mix for a Shed Base?


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So, you’ve moved into your dream home, or at least a home you plan to stay in for some time. Perhaps it’s your retirement home. Now that you’ve found out what taking care of your yard or land needs, you find that you have acquired more and more tools.

You’ve also found that you need a good, safe, dry place to store them instead of cramming more things into your garage or carport. You want a shed.

With a little research, you’ve found that whether you are building the shed or buying one pre-built and delivered, you need a good, solid base for it. You’ve decided on a concrete base.

Whether building or buying your shed, you need to get that base put in first, but you don’t have much, if any, experience with concrete. If you’re not hiring a professional to put in your concrete, you have a few questions. One of them is probably what is the best concrete mix for a shed base?

Your 3 options are pre-mixed concrete in bags or making the concrete mixture yourself from Portland cement, sand, the aggregate and water, or ordering pre-mixed concrete.

What is the Best Concrete Mix for a Shed Base?

You may want to mix your own concrete, as different uses require different mixtures and ratios of materials. You’ll need to mix sand and coarse aggregate, which is a form of gravel into your cement.

A Do-It-Yourself concrete base for a shed is usually mixed by volume at the rate of one part Portland cement, two parts sand, and three parts aggregate. If you are using ballast, this is usually the sand and aggregate already mixed.

In this case, you’ll use one part cement to five parts of ballast. There are also some reinforcing fibers you can add to the mix in case you think you’ll need some extra strength. However, there are concrete mixes on the market that come already mixed in the correct ratio, saving time and effort. Some even include reinforcing fibers mixed in.

You’ll need to wear some protective gear when mixing your concrete. Long sleeves and pants, along with gloves, boots, and safety goggles are recommended, as concrete is caustic. It can injure you if it comes into contact with your skin or eyes.

Cement can contain tiny particles which can fly up into your face when you’re pouring into your mixer, and the whole business can splash droplets onto you while you’re mixing.

How Thick Does a Concrete Slab Need to Be for a Shed?

Concrete bases for sheds are usually four inches, the same thickness as is used for sidewalks and concrete patios. If you plan to store really heavy things in your shed, you can go up to six to eight inches in thickness, but this is rarely necessary. If you plan a really large shed or plan to use the shed as a workshop with vibrating tools in use, you may want a thicker slab.

Weather can also play a part if your area is subject to heavy snow loads. The ground type is another factor. If your soil is prone to sinking in spots, you may want a thicker slab to prevent cracking from soil movement.

Some shed bases have spaces excavated more deeply than most so that the concrete is level with the surrounding ground, eliminating the need for any kind of ramp for your wheeled lawn tools such as mowers and tillers.

You do need to make sure that no wood of any kind in your framework or any siding will touch the ground when doing things this way, however. If you have a lot of water flow expected on your shed site, you may not want to construct your base in this way.

One tip is to make a thickened edge on your slab. This involves leaving less gravel fill around the edges of your site, sloping it downward so that the concrete here is deeper. This provides more support for the walls that will rest on the edges. It will also provide more depth in case you use tie-down bolts or other connectors that need to be drilled into the concrete.

Recommendations for the thickness of the edges range from six to twelve inches. Your concrete supplier can probably recommend an edge depth according to the size of your shed. The more unstable your soil, the deeper you’ll probably want your edges to be.

How Do You Mix a Concrete Floor for a Shed?

You can buy a pre-mixed concrete mix for your shed base in bags. Some products even already have reinforcing fibers mixed into the concrete. All you do is add water and mix. This saves a lot of work and also guesswork.

You’ll probably want to rent a concrete mixer. They come in different capacities, but you don’t need to mix all the concrete for the slab at one time. Making smaller batches will enable you to mix the concrete properly and thoroughly in a quicker time and eliminate the possibility of air pockets or some pockets of dry mix in the concrete. The mixer rotates to both mix the concrete and keeps it from drying prematurely.

You first measure out the amount of water you’ll need for your batch. Pour half of this water into the mixer first. Then add your concrete mix for that batch into the mixer. Allow it to mix for about a minute, then gradually add the rest of the water to make a consistency like thick oatmeal.

You may not need all the water you measured out or you may need a little more. Just be sure to add it gradually, as you don’t want to mixture to become too soupy or the concrete too weak. Mix it for about three to five minutes more before pouring the slab.

If you’re mixing the ingredients from scratch, you’ll need to make your own mix of the dry ingredients first, remembering that water is the first ingredient in the actual making of the concrete. You may want to use the mixer to form your dry mixture, then put it in another container such as a wheelbarrow so you can put the beginning water amount into the mixer itself.

How Many Bags of Concrete Do I Need for a Shed Base?

Many pre-mixed concrete brands feature charts printed on their bags to help you. There are also many calculators online that will help you here. Basically, you need to multiply the length by the width of your intended slab size and multiply the result by the thickness you plan.

This will give you the cubic footage of finished concrete that you need. For instance, a 10 x 12 slab that will be four inches thick will need 40 cubic feet of concrete, or 1.48 cubic yards. You’ll need 67 –  80-lb. bags of pre-mixed concrete for this.

If you’re mixing your own concrete dry mix from scratch, you’ll need to figure this number according to the ingredient ratio, one part Portland cement, two parts sand, and three parts aggregate. The 40 cubic footage you came up with before in the example is wet volume.

You’ll have to convert this to dry volume by multiplying by 1.54 to get 61.6. This is because wet sand particles move away from each other, causing what’s known as sand bulking. You’ll then multiply 1/6 times 61.6 for your cement volume, 1/3 x 61.6 for your sand volume, and 1/2 x 61.6 for your aggregate volume.

If you’re using ballast, or sand and aggregate already pre-mixed, you’ll multiply the cement the same way, and the sand and aggregate mixture as 5/6 x 61.6. The volume listed on the bags you plan to buy will help you figure out how many bags of each substance you’ll need.

Can You Pour Concrete Directly on Dirt?

You can pour your concrete directly on the soil once it’s excavated to be level and tamped down. However, this is not recommended. You’ll need a gravel base underneath for added strength and stability and to provide drainage. Dampness in the soil can interfere with the concrete drying and curing properly and evenly and may well lengthen the drying time.

In addition, the concrete won’t dry evenly, and this can lead to cracks, either while curing or later on down the road, as the uneven drying leaves weakened spots. Remember that concrete takes a long time to fully cure after it has dried.

You don’t want any weak spots which may come back to bite you later. You’re really taking a gamble if you do this and may have to start all over and put in the gravel anyway if your concrete cracks.

Is it OK to Pour Concrete Over Grass?

You really don’t want to pour concrete over grass. This would be even worse than pouring it directly over soil without a gravel base. Besides, it would also mean that you aren’t excavating a site for your shed, and that’s just not going to give you the results or stability you need for your shed.

If you’re not up to the excavation work, you probably won’t be up to the laying and tamping the gravel nor mixing and pouring the concrete and finishing it properly, either. In this case, it would be best if you hire professionals.

How Do You Prepare Ground for Concrete?

Drive in stakes at the four corners of the proposed slab location. Tie strings between the stakes all the way around and make sure they are level. You’ll then be able to see if your ground has a slight slope that wasn’t noticeable before or it is uneven and slightly bumpy.

Then remove enough soil from the site so that you can put in a six to eight-inch layer of gravel underneath the concrete. Remember to allow enough depth so that you can put in your deeper edges and still have a couple of inches of gravel underneath the edges. You may want to rent a compactor for this step to really do it right. It can compact the soil evenly and will be necessary anyway to get the gravel packed down firmly later. Check out my article How To Prepare The Ground For a Storage Shed for a detailed look at preparing your ground.

Do I Need Rebar for Shed Base?

Generally, if you’re pouring a concrete base that will be over five inches thick overall, you probably will want to add rebar for reinforcement. If you plan to use thicker edges, you can add a row or two of rebar around the perimeter to add support.

There are other types of reinforcement you can use. One is a welded wire mesh. It’s commonly used on concrete slabs. As mentioned before, you can mix reinforcing fibers into your concrete to add stability.

How to Form a Shed Base for Concrete

I’m assuming you have already located where you want your shed and have laid out the set backs from property lines and are ready to install the concrete forms.

Step 1] Once you have your soil excavated and level, and the gravel is installed and tamped you’ll need to make a concrete form by nailing  2 x 4s together to make the form. You need to cut some stakes to hold the form in place. You can use 1 x 4 or 2 x 4 for your stakes, make them around 16 inches long [depending on the type of soil you have you may need to use a crow bar to make the starting for the stakes].

Step 2] Now its time to position the form work and install the stakes, you’ll want to have stakes about 6 inches from the corners and then about 3 feet apart. once you have positioned the form and installed the stakes it’s time to level the form. Using a level find the highest point and nail or screw the stake in the form.

Next using the level go to the others corners and level from the corner that is set in place and adjust the height to level and fasten the stakes to the form. [do the outside corner stakes first and once there all set in place attach the other stakes to the form].

Step 3] Using a hand saw cut the stakes that are above the form work flush with the top of the form. Using some spare gravel fill in any spaces that are under the forms to prevent the concrete form pouring out. Using a string line or a straight 2×4 check the form for any bows and put a brace piece from the outside to straighten it.

Step 4] Install the vapor [if your using one] directly onto the ground and hold in place with some small rocks. If your not sure if you need to install a vapor barrier under your shed read my article Do I Need a Vapor Barrier Under My Shed? for more information.

Step 5] Install the re-bar or wire mesh, this is optional but worth the extra cost.

I have added a couple of videos for you:

Installing Wire Mesh

Installing Re-Bar

Pouring Your Concrete Slab

If you are planning a large shed, you’ll probably want to order a concrete delivery via a ready-mix company. Figure your cubic feet of concrete needed, then call the company at least a day in advance. Explain to the dispatcher that you need a concrete mix for a shed base, and he can advise you on the best mix for the project and the local conditions.

For instance, if you have a lot of below-freezing temperatures in your area, he may recommend a mix containing a few percent air entrapment. The tiny air bubbles help the concrete better withstand freezing temperatures.

You need to plan the route the truck will travel to get to your slab. For best results, it should be able to back right up to the concrete form. You’ll also need to keep an eye on the weather forecasts.

In case of rain, you’ll need to reschedule. Likewise, a spate of hot, windy days will harden the concrete before you have a chance to finish it properly. In fact, for a large slab in hot weather, you may want to divide your slab space into two sections and pour them on different days.

Remove your divider when pouring the second half. Have at least two large contractor wheelbarrows on hand and be prepared to rent them if needed. You’ll also need three or four helpers.

How to Finish a Concrete Slab

Once the concrete is poured, you’ll need to start finishing it. You’ll need good boots, as you’ll have to walk in the wet concrete. Drag a 2×4, or screen board, across the concrete, using the tops of the form boards as a guide, to level the concrete. You may need more than one pass. It helps to have someone who can add or remove concrete in front of the board as you pull it.

Next, push a bull float across the surface three or four times, pulling up on the blade a bit as you reach the edges of the slab. It’s a long-handled tool with a flat blade three to ten feet long used to smooth the surface. It also slightly embeds the aggregate. Don’t be surprised if some water comes up to the surface. Wait for it to disappear and the concrete to harden a bit before the next step.

Next comes an edging tool, used to round off the edges of the slab. If the edger sinks in more than 1/8 inch, the slab should harden a bit more before continuing this step.

If you are planning to use anchor bolts for the bottom wall plates, now is when you embed them, leaving around 2-1/2 inches of them sticking up. Bolt insertion should start at a foot from the slab corner, with subsequent bolts being placed every six feet. They should be at least six inches from the sides of your planned door opening.

If you have a large slab, you’ll need to cut control joints into the slab every ten feet. Wait until the slab can support your weight for this step. You can use squares of thick foam insulation to use as kneeling pads while you do this.

You’ll use a tool called a groover that will cut slots into the concrete to minimize cracking at the proper depth. Use a 2 x 4 as a guide, working in the tool as deep as it will go.

Use a magnesium float, which is a hand tool, next to further smooth out any humps, dips or marks left by edging. This brings a little cement slurry to the surface to allow trowel smoothing. Use a steel trowel to smooth the concrete. If you plan the slab to be your floor as well, you may want to skip the smoothing to eliminate slipping when walking on it. You can even use a push broom to roughen the surface further.

Keep the concrete moist as it dries so it dries slowly and evenly. You can also buy a curing compound to spray over it. Let the slab rest a day or two before removing the form boards, then another couple of days at least before you try building on the slab.

Here’s a Video of a Slab Being Done

Conclusion

If you’ve never worked with concrete before, this may all seem like a lot of work. It is, but if you’re handy with tools, it’s really not that big a deal. Doing the slab work yourself will save you money and you’ll end up with something to really be proud of.

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