Pouring a concrete base sounds like the kind of job only a seasoned DIY warrior would take on, but it can be a very fulfilling DIY project once you understand the steps. Most people choose concrete because they want a shed base they never have to think about again. Once it’s down, it’s rock solid, weatherproof and strong enough to hold anything from a tiny tool shed to a full-blown garden workshop.

The trick to getting this right is simply knowing what you’re doing before you start mixing. A properly laid concrete slab will keep your shed level, dry and steady for years, while a rushed job can crack and sink, and turn into a massive DIY regret if you dive in too quickly without knowing what is required. The good news is that as long as you plan it properly, gather the right materials and take it step by step, pouring a concrete shed base is absolutely manageable.

So let’s walk through exactly how to make a concrete base for your shed, what you’ll need and how to avoid the mistakes that trip most people up.

A quick note on terminology (because this bit gets confusing)

Before we go any further, let’s clear up something that often causes a bit of confusion. When people talk about a “concrete garden shed base,” they’re usually referring to the foundation which is the solid slab that sits directly on the ground.

But, even with concrete, you will still need a support layer (also sometimes called a shed base) that fits between the shed floor and the slab itself. That might be:

• a timber frame
• plastic foundation grids
• adjustable risers
• timber skids
• concrete blocks

… or anything that lifts the shed slightly off the slab and allows air to move underneath.

This extra layer should never be skipped because even though concrete is solid, it still holds moisture. If your shed sits directly on it with no airflow, the underside will  be exposed to dampness and eventually you’ll be dealing with mould, swelling wood and rot.

So just to keep things crystal clear:

Soil → Concrete slab base (foundation) → Shed base → Shed floor

This guide explains how to build the concrete slab itself (the foundation layer) so you have a strong, long-lasting surface to put your shed and shed base on.

Why a concrete slab base is a great choice for your shed

Concrete is the go-to choice for people who want something strong, reliable and as low-maintenance as possible. It’s not the cheapest option and it’s definitely the most “serious” of all the shed bases in terms of skill needed and consequences when you get it wrong, but when you do get it right, you’ll end up with a foundation that doesn’t move or need attention and can handle almost anything you build on top of it.

Here are the main pros and cons:

The pros of a concrete slab base:

• Structural integrity: A concrete slab is the most solid and stable foundation you can give a shed. Once it finishes curing, it won’t shift easily and it keeps the whole building supported and wobble-free.
• Low maintenance: Unlike timber, gravel or other bases, concrete doesn’t rot, warp, settle or need topping up. Once it’s in, it’s in and it’ll stay that way for decades with barely any work from you. (Ironically, this is also the biggest con as if you don’t get it level moving it is a huge job).
• Suitable for all needs: A concrete slab can bear a heavy load, so it’s ideal for workshops, gyms, storage sheds and anything that will hold weight or machinery.
• Weather resistant: Concrete handles the wet, windy and cold UK weather far better than most other foundations. It doesn’t mind being rained on and it won’t sink just because the ground gets rather soggy in winter.

The cons of concrete bases for sheds

• Can’t be moved easily: Once you pour a concrete slab, that’s it. You’re committing to that spot. If you ever change your mind, moving it is… well, basically impossible without machinery, a skip and a full weekend of questioning your decisions. This is also why it is so important to know how to do this correctly before you begin as if you make a mistake the cost and hassle of starting over is going to be cumbersome.
• Requires more DIY know-how to install correctly: Concrete is absolutely DIY-friendly if you know what you’re doing, but it’s definitely not something to “wing” on the fly. If the slab isn’t level, thick enough, or compacted properly underneath, you can end up with cracks, dips and a potentially uneven surface that will impact the garden shed sitting on top of it. And unlike timber, you can’t just undo a mistake and try again. Removing a badly poured slab is expensive, time-consuming and makes most people forever swear off DIY. So if you’re new to this level of project, it’s a smart move to either work with someone experienced or take your time and make sure you follow the steps carefully.
• Cost: It’s one of the more expensive options when you factor in materials, tools and possibly hiring help. The upfront cost is noticeably higher than timber or plastic bases.

What is the cost of concrete bases for sheds

The cost of a concrete slab base is usually the thing that makes people hesitate, but having a ballpark figure upfront makes everything much easier to plan. For most UK gardens, a concrete base will sit somewhere in the £150–£1,200 range for DIY or £400–£2,500 if you bring in professionals.

The lower end of the scale covers small sheds (6×4 or 8×6). Once you move up to bigger buildings, the price climbs because you need more concrete, a thicker depth and sometimes reinforcement mesh to keep it stable.

One thing many people don’t realise until they’re knee-deep in unexpected quotes is just how many moving parts go into the true cost of building a garden shed. The shed itself is only one line on your budget. The foundation, delivery, ground prep, any drainage work and electrics all add up and that’s usually where projects derail. A little bit of planning goes a long way. When you map out the whole build from start to finish you can avoid expensive surprises and the entire project runs much more smoothly.

What tools and materials do you need to make a concrete base for a shed?

Here are the typical tools and materials you will need to make a concrete base for your garden shed:

Materials:

• Ready-mix concrete or cement, sand and aggregate
• Hardcore or MOT Type 1 sub-base
• Timber formwork boards
• Wooden stakes
• Damp proof membrane (DPM)
• Reinforcement mesh or rebar (optional but common)

Tools:

• Shovel
• Rake
• Wheelbarrow
• Spirit level
• Tape measure
• Hammer or mallet
• Circular saw (for formwork)
• Concrete mixer or mixing tray
• Tamping tool or straightedge (for levelling)
• Float or trowel
• Utility knife
• Safety gloves, goggles and a dust mask

How to calculate the volume of concrete you will need

Thankfully, this is one of the easiest parts of the whole project and you won’t need to relive any high school geometry trauma to get this right because you don’t need anything fancier than the basic measurements of your base.

To work out the volume of concrete you need, just use this simple formula:

Length × Width × Depth = Volume (in cubic metres)

That’s it.

A note on depth: this is usually around 75 to 150 mm, depending on your shed size. Convert that depth into metres, for example, 100 mm becomes 0.1 m and then multiply the three numbers together.

Whatever number you get is how many cubic metres of concrete to order or mix. It’s always smart to round up slightly to allow for small dips in the ground or any spills while pouring.

Safety precautions when laying a concrete shed base

Concrete work isn’t dangerous per se to work with if you take the right precautions, but it can be messy, heavy and irritating to skin. A few simple safety habits will save you a lot of discomfort later.

• Wear sturdy gloves and safety glasses whenever you’re mixing or pouring concrete because wet cement can irritate skin and the splashes sting like mad if they hit your eyes.
• Keep wet cement off your skin as much as possible. If it gets on you, wash it off straight away. It’s not like mud and can cause burns if left sitting.

• Use a dust mask when handling dry concrete mix because the fine powder puffs up everywhere and is not nice to breathe in.
• Set your cement mixer on flat, stable ground and keep kids and pets well away so nobody gets curious fingers or paws near the turning drum.

Steps for laying a concrete shed base

Right, let’s dive in to exactly what you need to do to get your concrete slab poured, level and ready for your garden shed:

1. Mark & clear the area

Start by marking out the exact size of your slab using stakes and string. Add at least 5–10 cm around the edges so the slab is slightly bigger than your shed.

Then clear everything inside that outline. Remove all grass, weeds, roots, stones and old soil. You want bare, solid ground so the concrete has something stable to sit on.

2. Excavate to the correct depth

Dig down roughly 150–200 mm (depending on the slab thickness). Aim for the top of your finished slab to sit slightly above the surrounding ground so water runs away from your shed instead of pooling around the edges.

You’re creating space for:

• 75–100 mm of compacted sub-base
• 75–100 mm of concrete

Try to keep the depth consistent and the surface roughly level as you dig. Uneven ground now will mean an uneven slab later.

3. Lay and compact the sub base

Tip in your hardcore or MOT Type 1 and spread it in a layer around 75–100 mm deep.

Then compact it until it feels completely solid underfoot.

You can use:

• A hired vibrating plate (quickest)
• A hand tamper (fine for smaller slabs)

This layer stops the concrete from sinking into soft soil over time, so don’t rush it.

4. Build the formwork

Your formwork is just a wooden frame that helps to shape the wet concrete.

• Use straight timber boards (usually 100 mm deep)
• Screw them together securely at the corners
• Check they’re level all the way around and across the diagonals
• Place the boards around the edges of the excavated area so they sit at the level where you want the finished slab surface to be
• Hammer stakes into the ground on the outside of the boards so they don’t bow out when you pour the concrete in

Good formwork will give you straight, neat slab edges

5. Add damp proof membrane

Roll out a sheet of DPM inside the formwork so it covers the entire area and comes a little way up the sides of the boards.

This does two things:

1. Stops moisture rising up into your shed
2. Helps the concrete cure evenly because the ground won’t suck water out of it

Tape any overlaps so the sheet doesn’t shift when you pour.

6. Mix the concrete

You can mix concrete:

• In a mixer
• In a wheelbarrow
• Or by ordering ready-mix (usually easiest for large slabs)

If you are mixing it yourself, use the standard UK mix for shed bases:

1 : 2 : 3 = cement : sand : aggregate

OR, if using ballast:

1 part cement to 5 parts ballast

This gives you a strong, workable mix suitable for shed slabs.

When mixed, the concrete should be workable but not sloppy. It should be wet enough to settle into corners but stiff enough that it holds its shape when you heap some on a shovel. If it’s running like soup, it’s too wet. If it crumbles or looks powdery, add a little more water, but always add water slowly because too much water will weaken the concrete and it won’t cure properly.

7. Pour the concrete into your formwork

Start at one end and work across. As you pour, use a rake or shovel to move the concrete into the corners and edges. Try not to trap air pockets when you pour and every so often make small taps on the formwork to help settle the mix. Aim to slightly overfill the formwork because you will level it down in the next step.

8. Tamp and level the concrete

Lay a long, straight piece of timber across the top edges of your formwork and pull it back and forth in a gentle sawing motion.

This does two important things:

1. Levels the surface
2. Brings excess water and fine cement to the top, which helps compact the concrete and brings fine cement paste to the top, making it easier to smooth

If any dips appear, add a little more concrete and tamp again. For a neater finish, use a float or trowel to smooth the surface after tamping.

9. Cover

Once you’re happy with the slab, cover it with plastic sheeting. This stops the wind and sun drying out the top too quickly, which prevents cracking.

Leave it alone for 24–48 hours so it can cure.

Concrete only reaches full strength at around 28 days, so be patient if you’re placing a heavy shed or workshop on top.

Common mistakes to avoid with concrete bases for sheds

Even when people follow the main steps correctly, there are a few easy-to-miss details that can cause headaches later. Here are the most common slip-ups and how to steer clear of them:

• Skipping compaction of the sub-base: If the hardcore isn’t properly compacted, the concrete will settle unevenly over time. You’ll end up with dips in the slab and a shed that slowly goes off-level. A few minutes with a tamper or vibrating plate saves you a lot of grief.
• Pouring on uneven ground: Concrete follows whatever shape you give it, so if the ground underneath the formwork isn’t level, the slab won’t be either. That leads to warped floors, sticking doors and constant adjustments later. It’s worth taking the extra 10 minutes to get everything even before you pour.
• Forgetting expansion joints for larger slabs: If your slab is big enough (usually anything over 3–4 metres in any direction), you need expansion cuts or joints. Concrete expands and contracts with temperature changes and without joints it will crack in random places.
• Pouring concrete in very cold weather: DIY concrete slabs are not a winter sport. Concrete doesn’t cure properly when it’s freezing. If the temperature drops too low, the water inside the mix can freeze and this will cause the slab to weaken or crumble. If you absolutely must pour in winter, you’ll need frost blankets and extra curing time but it’s really just much easier to just wait for a mild day.

Last thoughts

When it’s poured well, a concrete slab shed base gives you a foundation that stays level, drains properly and supports your shed for decades without needing any maintenance.

It takes a little planning, a bit of muscle and some patience while it cures, but the payoff is huge in a shed that sits exactly as it should, feels solid underfoot and doesn’t surprise you with tilting floors or damp creeping in from below.

If you’re going to invest time and money into a garden building, giving it a proper concrete base is one of the smartest decisions you can make.

Frequently asked questions

1. How much concrete do I need for a shed base?

You can work this out using a simple formula:

Length × Width × Depth = Volume in cubic metres (m³).

Just make sure the depth is converted into metres before calculating. For example, 100 mm becomes 0.1 m. Once you’ve got the volume, add a little extra (around 5–10 percent) to cover small dips or spillages.

2. How much concrete do I need for a 10m x 8m shed base?

If you’re pouring a 100 mm deep slab, calculate:

10 × 8 × 0.1 = 8 m³ of concrete.

Order around 8.5 m³ to allow for spillage, levelling and slight ground inconsistencies.

3. How deep should the concrete be for a shed base? 

For most standard garden sheds, 75–100 mm of concrete is enough.

If you’re building anything heavier, go for 100–150 mm. A thicker slab gives you better strength and avoids long-term cracking or sinking.

4. How do I mix concrete for a shed base?

You’ve got two options:

1. Mixing from raw materials: Use the classic 1 : 2 : 3 ratio (cement : sand : aggregate).
2. Using ballast: Go with 1 part cement to 5 parts ballast.

Add water slowly until the mix holds its shape. You want it workable but not runny.

If you’re ordering premix, ask for C25 grade concrete.

5. How long does it take for a concrete shed base to dry? 

Concrete is usually dry to touch after 24–48 hours, but that’s nowhere near full strength. It takes around 28 days to fully cure, so if you’re planning to put a heavy shed or workshop on it, give it the time it needs. It’s worth the wait.

6. How much is a concrete base for a shed?

A concrete base will sit somewhere in the £150–£1,200 range for DIY or £400–£2,500 if you bring in professionals.

7. How much concrete do I need for an 8m x 6m shed base?

If you’re pouring a 100 mm deep slab, calculate:

6 × 8 × 0.1 = 4.8 m³ of concrete.

Order around 5.3 m³ to allow for spillage, levelling and slight ground inconsistencies.

8. What is the best concrete mix for a shed base

For ready-mix orders, ask for C25 grade concrete, which is the strongest and most cost-effective option for garden sheds.