How to Make a Concrete Slab Base for Your Shed

The base of your shed or garden building is one of the most important parts of the building. Not only does it give your garden shed a secure and safe place to stand, but it also protects it from flooding, pests, and other damaging elements.

If you want your shed, summer house, or log cabin for a long time, then you want the best base for it. And if you want the best base, then you want a concrete shed base.

Despite the simplicity of their design, concrete bases outshine the competition in many ways. In this guide, we’ll be exploring everything there is to know about concrete bases: advantages, disadvantages, how it compares – and even how to build one yourself! This is your one-stop-shop for concrete bases.

So, whether you’re just interested or ready to pour some concrete, scroll down to find everything you need!

Concrete Shed base in garden
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The Facts About Concrete Bases

Concrete slabs are one of the safest and secure garden building bases you can make or buy. Concrete slabs can be built to precise specifications and provide an extremely-stable, weather-proof base for any timber, plastic, or metal structure.

What’s even better is that it doesn’t matter what was there before – a concrete base can go practically anywhere. 

Of course, nothing is perfect, and a concrete base has a few flaws of its own. Instead of flying through them, and past all of their great advantages, let’s break down the pros and cons of concrete slab bases.

Concrete slab for shed base
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Concrete bases are very strong

We all know how strong and useful concrete is – we see it everywhere we go. A concrete foundation is just as strong and will provide any shed with a base that is basically indestructible (unless you’re trying to break it!). They’re more reliable and sturdy than anything else you can choose.

This can be especially useful if you have a garden building that is large or heavy. If you’re worried that it might break or squash whatever it sits on, then choose a sturdy and robust concrete shed base.

Concrete is water and weather-resistant

Come rain or shine, wet autumn or sunny summer, you’ll never have an issue with water or weather with a concrete base. Even the freezing cold of winter will do nothing to them!

A concrete surface is impermeable, meaning it will never fill with water or become soaked. Water that lands on it will bounce away, and water that tries to seep in from the bottom will be repelled, protecting the building on top regardless of the material. It also won’t suffer any water damage itself – although it might get dirty!

Rain on concrete slab
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Concrete bases last longer than any other base

The resistance and strength of concrete mean that it is able to withstand the years better than any other material. While erosion may slightly wear the slab over many, many years, it will never lose its strength or stability (provided it is laid correctly). In fact, concrete can survive for up to 100 years!

They can withstand immense weights

While this is useful for providing a base for a garden building of immense size as we’ve already mentioned, it is also useful for supporting whatever you put inside.

If you have a large shed that you want to use as a motorbike store or a full-blown garage, then concrete is the only way to go. A slab base will be able to support vehicles and bikes easily, making them the best base for those bigger buildings.

Concrete bases can be made to any size specification

Since concrete has to be laid, you can make a base to fit any size or shape you might need. You won’t have to worry about the base not fitting your shed, or being too big, as you will always get the right size for your garden building. Just make sure your measurements are correct first!

Measuring the ground using a red spirit level
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Concrete is the most expensive base material

Unfortunately, there’s no way around this one – whichever way you look at it, a concrete foundation is the most expensive kind. Not only is the material expensive, but hiring someone to lay the base will also be costly to do.

Even if you decide to do it yourself, like we’ll be showing you later, then you’re substituting the money you’d be putting in for time and hard work. For some of us, we relish the challenge. But if you’re not up for laying a whole slab of concrete yourself (and there’s no shame if you don’t), then just know that it will be costly.

They are hard to change or remove

 If you stop using your shed or garden building and want to remove the base, you might find that it is tricky to get rid of. Because concrete is so strong and sturdy, you’ll have a tough go of removing it! Your best bet will be to get a professional in, but unfortunately this won’t be cheap either.

Concrete with a crack running through it
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Concrete is bad for the environment

If you’re looking to be environmentally friendly in day-to-day life, or just in the building of this shed, then we recommend that you don’t make a concrete surface.

Concrete is made using cement, and the cement industry is one of the main producers of carbon dioxide – a direct contributor to global warming. The use of concrete as a hard ground surface can also contribute to a number of direct environmental effects. You can find a full breakdown by following this link.

Overall, concrete is a strong and reliable material that has some small but important drawbacks – none of which stop it from being the best base material out there!

Let’s Compare

You’ve seen us proclaim that concrete is the best, but you might be wondering why. Well, to clear up any doubts we’re going to quickly compare a concrete slab base to its main competitors: a timber frame base, and a paved base.

The table below tells you everything you need to know. Boxes with bold text mean they’re the best.



Concrete base


Timber frame base


Paved base




Materials and installation likely to be expensive


Cheap materials, cost-free installation if you DIY. Cheapest option.


Cost of slabs depends on the type you buy, relatively expensive.




Strong and durable, able to withstand any weight.


May break or splinter under heavy weight loads. May need strengthening.


Strong and can handle most weights – in extreme cases paving might break.


Water resistance


Impermeable and totally resistant to water. No chance of water ingressing.


Even with treatments, timber is not completely safe from water. It may also seep in from gaps.


Paving is impermeable, but water may find a way in through gaps between paving slabs.


DurabilityWill stand for many years without requiring maintenance.Needs yearly, if not seasonal, check ups and treatments. Shorter life span.


Good durability. Mortar between paving stones will eventually shrink and need replacing.


There you have it – with a score of three out of four, concrete is the clear winner! If you want a bigger breakdown or more information about another type of base, take a look at our general base guide here.

How to Build A Concrete Slab Base

BillyOh Portland Apex Metal Shed on Concrete Base
BillyOh Portland Apex Metal Shed on Concrete Base

Okay, now to the main event – building a concrete shed base.

Just a word of warning before we begin: building a concrete shed base is hard work. Even though the process is simple, it requires lots of hours of work and quite a bit of heavy lifting. Always be realistic about whether or not you can undertake hard manual labour. There’s no shame in looking after yourself and getting someone else to do it!

When working with concrete, always make sure to follow safety information. You can find concrete safety information here.

The tools you’ll need

Before we even start building, we need to make sure we’ve got everything we need to build the slab base! Here’s a list of what you’ll need:

  • Pegs and string for marking out the boundary
  • Building sand
  • Standard cement
  • Timber to make the box frame
  • Steel finishing trowel
  • Spirit level
  • Tape measure
  • Spade
  • Sweeping brush

Okay, now let’s move on to the step-by-step guide.

Step 1: Preparing the area

Yellow measuring tape
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Start by preparing the area you’ll be creating the base on. Make sure you’ve got enough space to pour the base, and that it’s not going to be intruding or breaking anything!

Measure out the area first. Mark out the area of your shed or garden building, then add 100mm (10cm) on all sides to create an edge. Add an extra 75mm (7.5cm) – or the width of your timber – to allow space for the timber frame you’ll be using to hold the concrete. 

Use pegs and string to mark out the area, then measure all the dimensions to make sure they’re equal. Start using the spirit level now to make sure you’ll end up with a level base.

Next, dig out the area. You’ll need to dig out an area with a depth of 200mm. 100mm of this will be the concrete floor, with another 100 below it for the sub-base. 

Step 2: Framing and the sub-base

Now it’s time to build your frame. Using timber cut to size, make a box or rectangle frame that fits perfectly to the dimensions you’ve measured. If you do this correctly, you should be able to fit it inside the area you’ve dug.

Once the frame is in place, measure it with a spirit level. The timbers need to be flat, or your base won’t be! Measure the timber again to make sure the dimensions are correct, including the corners – they should all be right angles.

Now comes the sub-base. Fill the first 100mm of your area with the material for your sub-base. This material should allow good drainage while providing a good foundation. We recommend using MOT type 1. Once you have laid and compacted this base, top it with a layer of builder’s sand.

The timber frame and sub-base should create a neat tray and level surface you can pour the concrete into.

Step 3: Mixing the concrete

Orange cement mixer in a garden
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Next, you have to mix the concrete. Unless you have extensive concrete knowledge, it’s a good idea to stick to the basic concrete mix. The basic mix is 1 part cement to 5 parts ballast. Ballast is usually aggregate and sand. This should produce concrete that is strong enough to support your shed. If you want to speed up the process, hire a cement mixer.

Take care during this process – but once it’s mixed, you’re ready to pour!

Step 4: Pour the concrete slab

Time for the important part! Start pouring your mixed concrete into the further corner of your frame. As you pour, spread it evenly using the shovel. Try to pour it 10mm higher than the wooden frame so that it compacts to the right height. Pour throughout the tray you’ve made until it is full.

If you have excess concrete at this stage, it’s best not to use it – just don’t let it set anywhere you don’t want it!

Once poured, compact the slab using a long piece of timber (longer than the framework) – make sure to cover the entire base. Compacting is better and easier with two people. Ensure there are no air pockets and that everything is compact before smoothing out all sides and edges using a finishing trowel. 

Step 5: Let it set

This is the easiest step – but often the most stressful! With the slab all compacted and finished, it now needs to rest and set. Warm weather and temperature does play a part in drying and setting the concrete, and temperatures of over 20° can make it dry too quickly. If it is going to be any hotter than this, cover it with plastic sheeting. Remember, don’t touch the wet concrete while it’s drying!

The concrete will take 48 hours of drying before it can be walked on – but even then it will not be completely set. To make sure your slab is wholly set, leave it to dry for at least five days. After that, your concrete base will be good to go!

Builders smoothing out concrete surface
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You’re All Set (Literally!)

Concrete bases are the best in class, and we’ll bet you feel the same way after reading this guide!

It outperforms all the competition, providing a strong, weather-proof base that lasts as long as you need it to. And, with a bit of hard work and the steps above, you’ll have your own finished base in no time!

Now you have a concrete base. You know what to do next – build a shed!

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