How To Build A Shed Base: Timber, Concrete, Patio & Plastic

Constructing a shed in your garden can’t be completed without a suitable and sturdy base. Sheds need to be built on a flat, level surface in order for them to be secure and to ensure the overall stability and longevity of the structure. A good shed base will also protect against moisture or rising dampness, prevent shifting or settling and prevent weeds or other plants from growing through the floor.

Within this post, I’m going to talk you through three of the most common long-lasting shed bases and also discuss and few budget options. I will compare each shed base type so that you can make an informed decision as to which is the best for you, your shed and your garden.

Whether you are building your own shed from scratch or you’re building a pre-fabricated flat pack shed you’re going to need a suitable base. Here’s how to accomplish this.

Most common shed bases

Here are the three most common shed base types. Each of these shed bases provides a strong, permanent platform for your shed.

Some of you may also like to know:

How do you build a simple base for a shed?

Here are three of the most basic types which are also the cheapest way to build a shed base.

Let’s take a look at each of these shed bases in more detail.

Timber frame shed base

How to build a timber frame shed base

A wooden frame or timber frame shed base is ideal for those who are working on uneven ground or would like something less permanent. It’s also a slightly cheaper and easier base to construct, especially if you don’t want to buy or rent a cement mixer or other heavy machinery. Let’s take a quick look through the process to build one of these bases.

Step 1. Clear the ground and mark out your area

The first thing to do is select the perfect spot for your shed. Ideally, you’ll want to leave some space around all sides of the shed for painting and maintenance access. I tend to leave about 50cm. You can go closer to a wall or neighbouring fence but make sure to allow for the roof/eaves overhang.

Clear the area of debris and any plants that could potentially grow into your shed. Measure your shed and mark out the area on the ground with some string and four pegs. You can also use spray paint or pieces of timber to mark out your area.

Step 2. Fit the support posts – Foundations

The next task is to decide how many support posts are required and start digging the holes. The holes act as the support foundations for your shed. Your timber frame will attach to the support posts and should be constructed using a good solid timber.

I’d recommend using 100x47mm timber for the frame and 100mm timber posts for the support. You should install one support post at least every 1.2m. This gap should be decreased if you are planning to have a lot of weight in your shed.

Starting with the four corner posts, dig a 200x200mm wide hole in the ground. This should be about 500mm deep. Fill the first 200mm with rubble. Something like 20mm Limestone chippings is ideal. Compact this down with one of the support posts.

Fill half of the hole with clean water, then place the 100mm post in the hole. Now pour Postcrete cement into the hole up to the water line. Use a spirit level to ensure the post is upright before the cement sets. Postcrete sets quickly, in just a minute or two.

When the Postcrete is set (10-15 minutes) you can start fitting the frame. The support posts can be cut to the correct height after fitting the outer frame.

3. Secure the outer frame

Line up the outer frame and secure each piece of timber with some strong screws and fixings. I would recommend using 80mm x 5mm carbon steel wood screws. Ensure you use a spirit level before securing each piece. Use 3 or 4 screws per fixing point. It can be a good idea to pre-drill holes to prevent the wood from splitting.

4. Add the joists

You can now add the joists to the frame. I tend to use the same method used to support the frame to the posts, but some might opt for using joist hangers. Joist hangers add extra strength for those that require support for heavy loads.

No part of the timber frame should be touching the floor.

5. Add joist bridges

Finally, you should add some joist bridges to secure the joists and strengthen the whole structure. These can be set at 1m intervals and screwed into place using the same method above.

6. Add weed protection

It’s important to protect the underside of your shed from growing weeds and plants. Add a layer of weed protective material to stop anything from growing under your shed.

7. Add OSB, plywood or simply build your shed

The final stage of this timber framed shed base is to make it fit for building your shed on to. You can build your shed directly on top of this frame like in the above picture. Simply lay your bearers on top of this frame and build your shed.

If the shed base has been built to the exact size of the shed footprint then you can add a layer of OSB or plywood for some extra support. You will just need to ensure the boards are covered and aren’t exposed to the elements.

This completes the timber frame shed base. Now let’s take a look at the step-by-step build for a concrete shed base.

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Concrete shed base

What are the pros and cons of a concrete shed base?

Pros of a concrete shed base:

1. Durability: A concrete shed base is strong and durable, and will not rot, warp or be affected by moisture.
2. Low maintenance: Once a concrete shed base has been constructed, it requires little maintenance and will last for many years.
3. Suitable for heavy loads: A concrete shed base is strong enough to support heavy equipment or machinery, making it a good choice for those who need to store heavy items in their shed.
4. Pest-resistant: Concrete is not attractive to pests such as termites, making it a good choice for areas where termites are a problem.

Cons of a concrete shed base:

1. Cost: A concrete shed base is more expensive to construct than a timber-framed shed base, which can make it a more expensive option for those on a budget.
2. Time-consuming to construct: Constructing a concrete shed base can be time-consuming, as it requires preparation, mixing and setting time.
3. Difficult to modify: Once a concrete shed base has been constructed, it is difficult to modify or change.
4. Heavy: A concrete shed base is a heavy, permanent foundation for your shed. It will be near impossible to move or remove without being broken up. This can be a time-consuming task and could require heavy machinery to help.

How to build a concrete shed base

1. Clear the area and level the ground

For the concrete slab to be constructed you will need a clear, level area. If you have a garden located on a slope or uneven ground take a look at my post about ‘how to build a shed base on uneven ground.’ This will help you prepare the area for your concrete shed base.

2. Mark out the area

Use some sticks and string to mark out the area you wish to build your shed base. Alternatively, you can use some timber or spray paint. Ensure the area you mark out has perfect right-angle corners. To do this you can measure diagonally from corner to corner and make sure the length are the same.

This video explains how to make a perfect rectangle.

3. Build a supporting frame

Building a level supporting frame will be essential to keep the cement within the designated area and form a solid shape. This can be made from 150x22mm timber and 4 small stakes. The frame isn’t a permanent fixture but should be strong enough to hold the cement in place whilst pouring and smoothing over the top.

For larger bases use more small stakes to support the weight of the concrete. One every 600mm should be sufficient.

4. Excavate the ground

Once you have the area marked out you’ll need to excavate to about 150mm. Some of you may decide to build your shed base above ground level but you’ll need to ensure the hardcore sub-base is still partially below ground level. In this case, you should only excavate down 75mm.

5. Fill and compact 75mm of hardcore

The sub-base consists of hardcore and a topping of building sand or ballast. You will need roughly a 75mm depth of compacted Type 1 MOT (hardcore). Here is a great calculator for working out how much Type 1 MOT you’ll need:

The Type 1 MOT and sand will need to be sufficiently compacted to ensure a strong base. To do this you should use a compactor for this task. These are also known as wacker plates. You can hire or buy them for a very reasonable price.

You could also manually compact the sub-base using a hand held rammer but this can be a very time-consuming process, depending on the size of your shed base.

6. Pour 75mm of cement

Mixing your cement should be carried out in a cement mixer. These can be purchased or rented from a local hire company. Alternatively, you can mix the cement on a cement mixing tray. This can however be a very strenuous and time-consuming task.

You will need 1 part cement to 8 parts ballast for the perfect shed base. (1:8)

Here is a great calculator for working out how much cement and ballast you will require.

Pour in your concrete mix into the timber frame and spread it over the sub-base. Moving the mix from the mixer to the base can be done using a wheelbarrow.

7. Level the cement

Using a long flat piece of timber you should smooth over the top of the cement mixture. This process can be easier with two people but if you are on your own you should wear wellies whilst standing in the cement. Tap and draw the timber across the surface to smooth. If you’re struggling with working with concrete up to this point, you may want to try some concrete classes with Rock Up Group. 

Further smoothing can take place with a straight trowel. This can be useful to prevent the build-up of water in grooves on top of the concrete slab.

8. Leave to set for 24 hours

Your concrete slab will take several hours to go off (set) but it’s advised to leave it for up to 20 hours before building your shed. You can also remove the timber frame and tidy the edges.

This completes the concrete shed base. Now let’s take a look at the step-by-step build for a patio slab shed base.

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Patio slab shed base

What are the pros and cons of a patio slab shed base?

Pros of a patio slab shed base:

1. Durability: A patio slab shed base is strong and durable, and will not rot, warp, or be affected by moisture.
2. Low maintenance: Once a patio slab shed base has been constructed, it requires little maintenance and will last for many years.
3. Suitable for heavy loads: A patio slab shed base is strong enough to support heavy equipment or machinery, making it a good choice for those who need to store heavy items in their shed.
4. Pest-resistant: Patio slabs are not attractive to pests such as termites, making them a good choice for areas where termites are a problem.

Cons of a patio slab shed base:

1. Cost: A patio slab shed base is more expensive to construct than a timber-framed shed base, which can make it a more expensive option for those on a budget.
2. Time-consuming to construct: Constructing a patio slab shed base can be time-consuming, as it requires preparation, mixing and setting time. You’ll also need specialist equipment to complete the job. A cement mixer and a compactor will be required.
3. Limited design options: Patio slabs come in a limited number of sizes and shapes, which can limit design options.

How to build a patio slab shed base

A patio shed base will start off in a very similar way to a concrete slab. You’ll need a nice level surface, and a sub-base finished with some patio-style slabs.

1. Clear the area and level the ground

For the patio-style base to be constructed you will need to clear and level the area. If your garden is located on a slope or uneven ground take a look at my post about ‘how to build a shed base on uneven ground.’ This will help you prepare the area for your concrete shed base. Use a large spirit level to ensure the surface is level or close to level. You’ll get away with a little bit of rise or fall. Up to 3cm per meter is acceptable.

2. Mark out the area

Use some sticks and string to mark out the area you wish to build your shed base. Alternatively, you can use some timber or spray paint. Ensure the area you mark out has perfect right-angle corners. To do this you can measure diagonally from corner to corner and make sure the length are the same.

3. Excavate the ground

You’ll need to excavate the ground for the sub-base. The sub-base will consist of hardcore rubble, known as type 1 MOT. It’s essentially broken-up rocks measuring between 40mm and smaller. When compacted it helps provide a very strong surface that can support weight evenly.

Dig down about 50mm ensuring the ground is relatively level throughout. This excavation will provide the footing for the shed base and hold all the hardcore in place.

Excavation can be done by hand with a spade and shovel. This process can also be carried out with heavy machinery but in most cases, this is not required. It largely depends on the size of your shed.

4. Fill and compact the sub-base

To ensure minimal or no movement throughout the base it’s important to compact the type 1 MOT. Fill your excavated area with 50mm of hardcore and compact it using a wacker plate or manually with a handheld rammer. For a small job, I made a makeshift wacker out of an old thick patio slab and a sledge hammer. It work very well for this smaller patio area:

I also used a heavy roller to help compact the area and smooth the surface ready for the slab installation.

Use a large spirit level to check you have a nice even surface. You can also use a long, flat piece of timber with a smaller spirit level.

You can manually move hardcore around to find the perfect level. Ballast can also be added in places to fill in any smaller gaps.

5. Mixing cement and laying slabs

You have a huge choice of slabs for this project but as most of them will be covered by the shed it might be sensible going for the cheapest option. You can pick up 400x400mm slabs for just £4.20 from your local B&Q.

Mixing your cement for laying patio slabs

You’ll benefit hugely from having a cement mixer for this job. You can mix the cement manually in a wheelbarrow or on a mixing tray but it’s a massive time saver with a cement mixer. These can be purchased or hired for the day.

You’ll need to mix 4 parts sand with 1 part cement.

Add a little water until you have a reasonably firm consistency. The mix should sit comfortably on a towel without sliding off at more than 20 degrees.

Laying slabs

With the mortar mixed, use a trowel to place 4 blobs of mortar in each corner of each slab, followed by one in the centre. (Blobs should be placed on the floor, not the slab). With the blobs of mortar laid, place a slab on top and level it with a spirit level. You’ll start from one corner and work your way out.

Use tiles spacers to line up each slab. I went for a 4mm tile spacer.

Keep on placing slabs one by one checking each is level as you move. Use a rubber mallet to tap each slab to the correct level as you go. You will want to aim for a slight slope on your patio to allow for water runoff. Ideally, this will be about 2cm for every meter.

6. Pointing your patio slabs

You’ll need to fill all the gaps in your patio base to make it water tight. Here’s how to do it.

1. Clear the area: Remove any obstacles from the patio, and sweep it thoroughly with a broom to remove any dirt or debris.
2. Prepare the joint filler: Mix the joint filler according to the manufacturer’s instructions. This is normally the same as the mortar mix mentioned above but without the same amount of water. Just add a tiny bit of water.
3. Apply the joint filler: Use the pointing trowel to scoop up the joint filler and apply it to the gaps between the paving stones. Press the joint filler into the gaps using the trowel, making sure that it is pushed down to the bottom of the gap.
4. Fill all the gaps: Continue filling all the gaps between the paving stones until all the joints are filled with joint filler. Use the trowel to smooth the joint filler as you go, ensuring that it is level with the surface of the paving stones.
5. Brush off the excess: Use a masonry brush to brush off any excess joint filler that may have spilled onto the surface of the paving stones. This will help to prevent any staining or discoloration.
6. Allow the joint filler to dry: Let the joint filler dry for about 24 hours.

Here’s a great video of the process:

This should complete your patio-style shed base. It’s now ready to start building your shed.

This concludes the three main shed base types.

Now, let’s take a quick look at a few budget shed bases.

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A plastic grid shed base

What are the pros and cons of a plastic grid shed base?

Pros of a plastic grid shed base:

1. Lightweight and easy to install: Plastic grids are lightweight and easy to install, making them a good choice for those who want to set up a shed quickly.
2. Cost-effective: Plastic grids are relatively inexpensive, making them a cost-effective solution for those on a budget.
3. Drainage: Plastic grids have a permeable design, which allows water to drain away easily, reducing the risk of water damage to the shed.
4. Versatile: Plastic grids can be cut to size, making them suitable for a wide range of shed sizes and shapes.

Cons of a plastic grid shed base:

1. Not as durable as other materials: Plastic grids can become brittle and crack over time, which may cause problems with the shed’s stability.
2. Limited load-bearing capacity: Plastic grids have a limited load-bearing capacity, which may make them unsuitable for heavy equipment or machinery.
3. May not be suitable for all types of ground: Plastic grids may not be suitable for all types of ground, such as areas with heavy clay soil or uneven surfaces.
4. May require additional support: If the ground is uneven, plastic grids may need to be supported with additional materials, such as gravel or concrete, which can increase the cost of the project.

How to install a plastic grid shed base

A plastic grid shed base can be installed on most surfaces, including a solid surface like a patio area. When installed on a solid surface the plastic base helps raise the shed off the ground and prevents rising moisture from the ground.

If you are installing the plastic shed base on a turfed area then you should follow these basic instructions for the best results.

1. Preparing the ground

Ensure you are working on a level surface. Slight slopes will be ok but nothing more than 4 degrees should be built on without levelling the ground first.

Remove any turf from the desired area. Avoid digging any deeper than the height of the plastic base.

Then add 10mm sharp sand to level the area. Use a spirit level to find a perfectly flat surface. You won’t need any runoff with this type of base as water will drain through the base.

You should now rake over another 10mm of pea gravel on top of the sand and compress this with your feet or a heavy roller.

2. Add a weed proof barrier

Lay out a suitable weed-proof membrane and cut it to size.

3. Add the plastic base

The plastic base can now be installed. The plastic panels should easily clip together to form the desired size of your shed. Ensure the base is large enough to cover the total footprint of your shed. Some plastic grid shed bases can be cut to size but you can also keep an overlap.

4. Cover the plastic base with more pea gravel

This will add a porous layer for water to easily drain through the ground. It also hides the plastic base and makes for a much nice finish to the shed and surrounding area.

Here is a quick video that demonstrates this process.

This completes the build guide for this simple plastic shed base. Plastic shed bases are ideal for small to medium size sheds. You can build wooden and plastic shed on these bases but it’s not a good idea to build a metal shed on them because they don’t tend to come with bases. The best plastic sheds like Keter and lifetime are both suitable for this type of base. Just ensure the base is completely level before starting construction. Also, small wooden sheds are suitable for this type of base.

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Pre-made timber frame shed base

Pre-made timber frame bases are usually made from pressure-treated timber and come in a range of sizes to suit different shed sizes. They are designed to be easy to assemble and provide a level and stable base for your shed.

One advantage of using a pre-made timber frame shed base is that it can save time and effort compared to building a base from scratch. You don’t need to worry about cutting and assembling the frame and you can be sure that the base will be level and stable. The pre-made base also helps to prevent dampness from the ground, which can prolong the life of your shed.

To install a pre-made timber frame shed base, you will need to prepare the ground by removing any vegetation and making sure the area is level. A layer of weed protection is also a great idea to prevent weeds or other plants growing into your shed. Then, simply place the frame in the desired location and bash it into the ground using a sledgehammer. Just be careful not to damage the frame. tap each corner bit by bit. Make sure the frame is level in all directions and adjust as necessary.

When choosing a pre-made timber frame shed base, make sure to check the size and weight capacity to ensure it is suitable for your shed. You may also want to consider the type of timber used and any treatment to ensure it is durable and long-lasting.

Overall, pre-made timber frame shed bases can be a convenient and effective option for those looking to build a shed base quickly and easily. With a little preparation and assembly, you can have a sturdy and reliable base for your shed in no time.

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Plinth timber framed shed base

The final option for building a shed base is a timber-framed base on plinths. This type of base is often used on sloping or uneven ground and involves building a timber frame on top of concrete or brick plinths to level the base.

To build a timber-framed shed base on plinths, you will need to prepare the ground by removing any vegetation and making sure the area is level. You will then need to create a series of concrete or brick plinths at regular intervals to support the timber frame. The plinths should be set into the ground and level with each other to provide a stable and level base. The timber frame I built in this project was relatively level and I used patio slabs distributed around the base for a secure hold.

Once the plinths are in place, you can begin to assemble the timber frame. This usually involves constructing a series of joists and beams to support the weight of the shed. The joists should be spaced at regular intervals to provide adequate support and be levelled on top of the plinths. The beams should then be attached to the joists to complete the frame.

Once the frame is complete, you can add a layer of plywood or other suitable material to the top to create a flat and level surface for the shed. You may also want to add a layer of damp proofing material to help protect the base from moisture.

One advantage of a timber-framed shed base on plinths is that it can provide a stable and level base on sloping or uneven ground. It can also be a more cost-effective option than building a concrete base, as the plinths can be made from inexpensive materials such as bricks or concrete blocks.

When building a timber-framed shed base on plinths, it’s important to ensure that the plinths are level and secure, and that the frame is constructed correctly to provide adequate support for the shed. You should also consider the size and weight of your shed to ensure that the base can support its weight.

Overall, a timber-framed shed base on plinths can be a good option for those building a shed on sloping or uneven ground. With careful planning and construction, you can create a stable and level base for your shed that will last for years to come.

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Conclusion

So there we have 6 different ways to build a base for your shed. Each have their own pros and cons so it’s up to you which is best for your desired finish. If you are thinking of building a timber-framed shed then also think about how to weatherproof your shed and base. There are certain techniques that will dramatically increase the life of your shed.

Before you get started with your shed base. ensure you are building your shed in a place that doesn’t require planning permission. Most sheds won’t need planning permission but it’s important to check before you get started as having to take it all down will be a huge disappointment, waste of time and money. Check out my post about this here:

Finally, think about the use of your shed. If you plan on using it for hobbies all year round you might want to add insulation to your shed and shed base. It’s far easier to insulate a shed base during the build process, rather than trying to do it retrospectively. Have a quick look at my post all about insulating your shed here:

Thanks for reading. Hopefully, you have found all the answers you need to get started on your shed build. Feel free to drop a comment below and I will respond asap.