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How to Build a Concrete Shed Base
Published: 10/05/2021
As the name of this post suggests, we're devoting this page to showing you how to build a concrete shed base. Specifically, we're talking about using concrete mix for your shed base. Why? Because on the question of what material is best to put under your shed, we recommend concrete (ready-mix or made from scratch) on level ground. As well as tips on what you need for your concrete base, you'll find guides on using paving slabs for a shed base (and more) in the general information elsewhere on this site.
Concrete shed bases in detail
Here, we show you how to choose the right location for your shed and help you figure out concrete base depth (or thickness). We even provide tips on levelling, calculating the concrete mix, how much sand and cement you need, measuring dimensions, working out costs, pouring concrete to create the shed base, building the shed frame, finishing, edging and more. Note #1: If you have no choice but to place your building on a slope and you want to know how to build a shed base on uneven ground, concrete foundation blocks are a better option than using mixed or pre-mixed concrete. But we’ve written this article to help you if you intend to lay your concrete base on level ground. Note #2: The tips in this post work equally well if you want to know how to lay a concrete base for a summer house or log cabin.
Building a concrete base: benefits and drawbacks
For many experts, concrete is the most desirable shed base material for the following reasons:
Advantages
- Stronger and more durable
- Easy to maintain
- Lasts longer
- Provides ample support for any size garden building
- Suitable for storing wet items (such as garden machinery)
- Best surface for a metal or wooden garage base – especially if you keep a car in it
Due to the advantages listed above, a concrete shed base is far by the most solid and most reliable foundation you can use. However, concrete shed bases also come with some disadvantages:
Disadvantages
- Most expensive base material
- Difficult to remove (if you want to remodel later)
- Not kind to the environment [1]
- Cold to the touch (not good for bare feet)
- Take a long time to lay (at least 24-36 hours)
- Requires a lot of heavy lifting
If you’re still sure you want know how to build a concrete shed base after weighing up the pros and cons, keep reading…
What do I need for a concrete shed base?
Here's a list of everything you need to lay a concrete base:
- Ballast, cement and water (or ready-mix concrete)
- Compacted hardcore or brick rubble
- Earth rammer
- Cement/concrete mixer or cement mixing tray
- Tape measure
- Spirit level or set square
- Shovel
- Rake
- Tamping board
- Edging trowel
- Wheelbarrow and/or cement mixer
- Plastic sheet and wooden supports
- Breeze blocks or bricks
- Saw
- Pegs and string
Now you have a list of items to buy or find, let’s get started. If you already know how much concrete you need and how much it’ll cost (or you already have it), skip to step 2. And if you’ve already worked out the right location, go to step 3.
Step 1: Calculate the amounts and costs
It stands to reason that the larger your garden building, the larger your base – and therefore the more concrete you’ll need and the more it’ll cost. We’re going to help you figure all that out in this section…
Should a shed base be bigger than the shed?
Yes. We recommend that you lay a concrete base that’s approximately 30-40cm (roughly 12-16 inches) larger than the dimensions of your shed. So if you’re building a shed that’s 8x6 feet (96x72 inches), you need a base with dimensions of roughly 9x7 feet (108x84 inches).
Concrete mix for shed base
To work out how much concrete you need for your base, you need to work out the dimensions of the shed.
Concrete shed base thickness
In terms of how thick (or deep) your concrete shed base needs to be, we’d recommend it's a least 75mm deep – that’s enough for most garden sheds. With this in mind, you can work out how much concrete you need for your base using the following equation: Depth x width x length = amount of concrete for your base In the UK, we tend to measure concrete in cubic metres (m3). So for a base that is 9x7 feet, it’s easier to convert this into metres – this is 2.74 x 2.13m. For a base that is 75mm deep, we convert this to metres (0.075m) and follow this equation: 0.075 (depth) x 2.74 (width) x 2.13 (length) = 0.44m3 (rounded up to two decimal places) To ensure you have enough concrete to make your shed base, we recommend rounding up to the nearest first decimal place (in this case, it would be 0.5m3) and adding an extra 0.1m3 – which makes 0.6m3. Why? Because it’s always better to have too much concrete for your base than not enough!
How much sand and cement do I need for my shed base?
If you plan on using ready-mix concrete for your shed base, you don’t need to worry about how much sand and cement you need. But if you’re going to make the concrete from a dry sand and cement mix for your shed base (i.e. from scratch), you must factor in the amount of cement, aggregate and sand you require (this is what concrete is made from) [2]. To calculate the concrete mix for your shed base, use a ratio of one part cement to five parts ballast (a blend of sand and aggregate). You can buy 40kg bags of ballast at DIY stores and building merchants.
Work out the cost of the concrete
Yet much like sheds and any other commodity, the price of concrete is subject to change – perhaps due to availability, inflation or a wide range of other possible factors. For the most reliable information on the costs involved, take a look at the concrete shed base cost calculator on the Checkatrade website [3].
Step 2: Find the right location for your shed
To help work out the location of your shed, think about what you’ll be using it for. Then try and imagine the position in your mind’s eye to ensure you have clear access to and from the shed.
Things to be sure of before you lay concrete
It’s unlikely you’ll need to seek permission to build it – but to make sure, visit our page on planning permission for sheds. As we say earlier, a concrete base is impossible to move once you lay it (you’ll need to remove it and start again). As a result, it’s vital to work out the right location for your shed beforehand. To choose the right location, it’s important to do the following: Find an area that’s as level as possible: The screw holes in the panels are likely not to line up if the area isn’t level. Choosing a relatively flat location also helps you avoid extra work that comes with laying a concrete base on a slope (or otherwise uneven ground). Avoid swampy areas: If you build your shed on swampy (or particularly wet) areas, excessive moisture may damage the shed, the tools and other equipment inside. Look out for trees and foliage: Trees and foliage can block light into the shed – potentially making it dark and dingy inside. In the autumn, trees also shed their leaves – leaving moisture to build up on top of (and inside) the shed. By avoiding trees and foliage, being in your shed should be more pleasant and practical. To avoid rot and potential damage to the equipment inside the shed, also keep an eye out for young trees that are likely to grow and overshadow the shed in future. And this brings us neatly on tour next tip… Dodge shady spots: Whether you want to spend time in your shed, or you’d just like to see what you’re doing while inside, it’s important that you have plenty of natural coming into your shed (unless of course it has electricity and a supply of light). However, you should also be wary of too much direct sunlight – this can cause the shed and any sensitive equipment inside to overheat. Ensure there are big enough gaps between the shed and fencing and walls: Leave a gap of at least one metre (approx. 3 feet, 3 inches). This makes the shed easier to build, allows clear access and helps ensure it’s easier to maintain.
Step 3: Remove vegetation
Remove any weeds and grass from the area where you’re going to lay your base. This will help prevent air pockets from appearing in the concrete; it will also stop moisture from building up around the shed, which in turn could help prolong its lifespan.
Step 4: Mark out the dimensions
Once you’ve decided where to position your base, use your tape measure, pegs and string to mark out the dimensions of your base. As we state earlier in this post, we recommend making the base 30-40mm larger than the shed on each side. As we say in this article, to ensure the area is square and not sloped, measure the lengths of the area at opposite ends. If the base is square and level, these lengths will be equal.
Step 5: Excavate and level off the area
To create a level base, excavate the earth in the marked area to a depth of six inches (150mm). Lay approximately 75mm (three inches) of compacted hardcore/brick rubble to act as the foundation – leaving space for 75mm of concrete on top. If the area requires further levelling, add some compacted sand. Use your earth rammer to compact the stone/sand to help firm up the base, before removing the pegs.
Why can't you lay the concrete shed base on soil?
Because it's never a good idea to build a concrete base or any kind of soft ground, which means it needs some kind of sub-base.
Step 6: Build a shed frame
A shed frame (we describe the frame as timber rails elsewhere on the Tiger site) acts as a way to contain the concrete while it sets, while also maintaining the shape of the base. To build this frame, make sure the inner dimensions of the frame match those of the base. Make the frame 100mm (approx. 4 inches) thick and use your spirit level or set square to make sure it’s level all the way around. By doing this, you’ll ensure that when the rails are full to the brim, and the concrete will also be level all the way around.
Step 7: Lay the concrete
To learn how to get the right consistency, go back to the “concrete mix for shed base” section of this article. To lay the concrete, pour a layer of concrete into the shed frame – spread it evenly and be sure that it goes into the corners and edges. We recommend applying the concrete one layer at a time until it overflows the frame slightly. While pouring the concrete, use the tamping board to compact the wet concrete. Taking care to remove excess concrete, go backwards and forwards over the whole surface with the tamping board until it’s flush with the top of the frame (use a spirit level to help achieve this).
Step 8: Round off the edges
Once you’re happy with the finish you’ve achieved, take your edging trowel along the frame to round off any concrete and prevent potential chipping.
Step 9: Cover the concrete shed base
To protect the concrete while it dries (and to prevent cracking), add wooden supports and cover the concrete with a plastic sheet, before keeping it in place with breeze blocks or bricks.
How long does a concrete shed base take to dry?
Leave the concrete base to set for at least 48 hours before constructing your shed. When it sets completely, you can then build the shed on top of it. If you’ve built the base in warm weather (or it becomes warm during the drying process), spray the base with water and/or cover it with a damp cloth to prevent cracks appearing. Now you’ve made your concrete shed base, you’re ready to build your shed.
How do you secure a shed to a concrete base?
For the most part, it isn't necessary to secure your shed (or 'anchor' it) to the concrete base - largely due to a lack of extremely strong winds in the UK (of the kind you get in the US, for example). But if you must anchor your shed for whatever reason, oneprojectcloser.com is a good source of information [4]. For more help and advice, go to the help section of the Tiger Sheds website.
References
[1] https://www.theguardian.com/cities/2019/feb/25/concrete-the-most-destructive-material-on-earth#:~:text=Concrete%20causes%20up%20to%208,our%20relationship%20to%20the%20planet. [2] https://wedc-knowledge.lboro.ac.uk/resources/e/mn/056-How-to-make-concrete.pdf [3] https://www.checkatrade.com/blog/cost-guides/concrete-pouring-cost/ [4] https://www.oneprojectcloser.com/anchor-shed-concrete/
