How To Build A Concrete Base For A Garden Shed?

If you are investing in a new shed then you must have a solid base for the new shed to be placed on. If you do not put your shed on a concrete base then your shed will not last the test of time.

Building A Concrete Shed Base - Example
Building A Concrete Shed Base – Example

Once it rains and the ground becomes wet the shed will absorb the rainwater and start to rot.

Making A Wooden Frame/Mould

Wooden Frame For A Concrete Base
Wooden Frame For A Concrete Base

Before starting your concrete base for the new shed you must measure the surface area. If it is a 3m x 4m shed then you need to make a wooden frame one inch over the required size so the shed fits correctly.

Remember you are filling the wooden frame so be sure to use inside measurements when cutting your wooden frame to size.

  • Measure the surface area of the shed
  • Build your frame using internal measurements
  • Allow an extra inch or 2cm extra on all sides for movement
  • Use 3″ by 2″ timbers for the frame
  • Use screws to hold the corners in place

TOP TIP: Oil the inside of your frame so it does not remove the corners or edges when your base is dry and you remove the frame!

Preparing The Ground For Your Base

When you have measured your frame and you are happy with the position of the shed, then it is time to start digging your area.

Use your wooden frame mentioned earlier and position it where the shed will be living. Place the frame on the ground and use a shovel to mark the area of soil being removed, making sure the corners are at 90°.

Hardcore Options For Garden Shed Base
Hardcore Options For Garden Shed Base

The depth will depend on the size of the shed, for an average shed you want to dig down 5″. 2″ for the hardcore and 3″ for the concrete.

You do not need to add any steel bars to hold the concrete together, but it will not hurt and will strengthen your overall concrete structure.

  1. Place the wooden frame in the desired shed position
  2. Mark it with a spray or go around with a shovel and cut out the area
  3. Dig down 5″ so you have 2 inch for hardcore and 3″ for concrete

Adding Your Hardcore

When your hole has been dug you need to cover the “soil” area with hardcore.

Hardcore can be broken bricks, stones or scalpings, do not put large rocks in, make sure your hardcore is 1/2″ or 1″ in size.

When you have covered the area using a thumper or large piece of timber to beat the hardcore into the ground. You do not need to go overboard with the thumping.

Thumping will prevent your shed base from sinking over time.

Compress Hardcore With A Thumper Or Earth Rammer
Compress Hardcore With A Thumper Or Earth Rammer

Once you have covered the area with hardcore you need to add your wooden frame over the hardcore.

Check if the frame is flush with the grass or other surface in the garden.

If your wooden frame is flush and sitting true “use a level” to check and then remove the wooden frame.

Use A Membrane To Avoid Rising Damp

Cut a sheet of plastic so your base does not absorb the moisture and rain from the soil underneath the hardcore.

Cover the whole hardcore with your membrane, leaving a few inches over, this will be cut back later on when the concrete is dry.

  1. Add your hardcore around 2″
  2. Beat your hardcore into the mud with a thumper or large piece of wood
  3. Check your wooden frame is flush and level
  4. Add a membrane to protect the concrete base from rising damp
  5. Add your wooden frame on top of the membrane
  6. Leave a few inches all the way around when cutting the membrane
  7. Once the concrete is set you can cut the membrane so you do not get patches of damp
Concreting Examples
Concreting Examples

Top Tip: Adding steel mesh is optional, you can add steel mesh at this point if you have a large shed. Steel mesh will greatly strengthen your concrete shed base but is optional for normal size sheds.

Filling Your Concrete Shed Base

Levelling Your Concrete Shed Base
Levelling Your Concrete Shed Base

Once you have levelled off your wooden frame and it’s flush with the surrounding grass or patio area then you should start mixing your concrete.

For this concrete base, we will be using the 1-2-3 method. These can be measured in bucket loads or shovel loads.

  1. One cement
  2. Two sand
  3. Three aggregate

You must keep this mixture the same every time, or you will notice different loads will set differently. The colour also changes so be sure to keep everything the same and be consistent.

The more water you add to the mix the weaker the concrete base will be, so be sure to remember how much water has been added to each load.

TOP TIP: Always dry mix your concrete and then add the water last this avoids pockets of sand and cement at the bottom of your mixer. Plus you will use less water making your shed base even stronger when set.

Before you start filling in your concrete shed base be sure to oil the inside of your wooden frame, this will make it much easier to remove. If you do not oil the wooden frame it might take the corners and edges with it when you remove the wooden frame because it has become set to the concrete.

Finishing Your Concrete Shed Base

Once you have filled your concrete base use a length of wood and continuously rub it across the wooden frame until all areas have been filled.

This will give you a level surface once the concrete base sets. Just before the concrete has set rub your trowel around the edges so it fills the wooden frame and you get a nice crisp edge when you remove the timber frame.

TOP TIP: Once your concrete shed base has set, grab some PVA sealer and seal your concrete. This will add protection against moisture from rain or garden run-off.

  • Use a straight edge to level your concrete
  • Your wooden frame will act as a level guide
  • Use your trowel to fill any dips
  • Before it sets use a trowel and smooth the edge
  • This will give the concrete better corners and crisper edges

Conclusion – Concrete Shed Base

Starting with a strong/solid and damp free concrete shed base will vastly improve the overlook and feel of your garden shed.

A solid will prevent your shed from becoming damp and rotten over time.

If you would like any more information when building a shed base then use the comment section below and we will advise ASAP.

This method can also be used for driveways, pathways and patios, just be sure to use a membrane or your concreting will become damp, green and full of algae.

Damp weakens the concrete and makes it crack and deteriorate within a few years!

Happy Concreting 😉

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