How to prepare the ground and build concrete foundations for your garden shed

A combination of brick and concrete foundations provides the best base for your garden shed. Concrete foundations ensure that your shed is stable, level, less prone to insect infestation and most important – dry.

Unfortunately, concrete foundations are also the most expensive option, they take much longer to build than a wooden base (concrete takes 7 days to cure) and they require some skill, especially with ground preparation and levelling.

Concrete foundations are ideal for any size and any type of shed, whether it is wooden, metal or plastic, providing the ground in your chosen spot is reasonably firm, drains at least moderatly well and is not prone to flooding.

You must also be reasonably sure of your chosen spot. A concrete slab, once build, cannot be moved to another location. You will have to use reasonable force to break it up and remove it or you will end up with an unsightly concrete patch on your lawn.

Preparing the ground and laying concrete foundations for your garden shed

I have found an excellent video guide prepared by Lafarge Cement. While Lafarge DIY cement is a very good brand of cement that you can certainly use on this project, but you can use any other brand you have.  Just prepare the cement mixture according to the quantities and mixing instructions on the packet and you are good to go.

Outline for base, brick wall enclosing concrete floor/base

If concrete is used as a base for the retaining wall, this foundation should be at least twice the width of your bricks (8″/20cm) and a minimum of 4″/10cm thick for a small shed and for larger buildings at least 10″/25cm wide and 6″/15cm thick. If you skimp and go much smaller than these dimensions you are wasting your efforts.

Concrete is also used for shed floor and needs to be at least 3″/8cm thick or it will not be strong enough. On spans over 6’/2m aim for at least 4″/10cm thick on firm ground and if the ground is soft or you intend to raise the floor level due to bad ground drainage, you need to use some aggregate.

Two rows of brick are  sufficient for smaller sheds, but for larger sheds or where the surrounding ground slopes, is very soft, or drains slowly after rain, three or more rows would be better.

Preparing the ground

• First check the ground for size and level.
• Dig out high spots, ignore hollows for now.
• Mark out the area for your shed using string and pegs. You will need to know the exact external measurement of your shed . Double check your measurements to make sure that the corresponding sides are even.
• From my own experience digging neatly with a normal digging spade to the depth of the spade should give a reasonable trench for the foundations. As the foundation needs to be at least a brick depth below the surface level measure from the lowest point.
• Although the bottom of the trench does not have to be perfectly level, the top of the concrete does. Use wooden pegs banged into the bottom of the trench as guidelines. Insert a peg in each corner and at least one in each side. Use a straight edge or a piece of string stretched across the tops of the pegs to check if they are level. Use a spirit level, don’t assume anything. If you don’t get that part 100% right, you will have problems with assembling your shed or later on .

Mixing and pouring the concrete

• If you are using pre-pack concrete, follow the instructions on the bag. If you have bought cement, sand and gravel separately, the mixing ratio is 1 – 2 – 4 (one part cement, two parts of sand to four parts of gravel).
• If you are mixing the above ingredients by hand, mix dry ingredients first then add water until the mix is of a consistancy you could almost pour from a bucket. If using a mixer put a bucket of water in first followed by the gravel.
• Once the concrete is mixed, fill your trench to the level of the pegs with the concrete.
• Smooth and leave to cure for at least five days.

Building the base brick wall

• YouNow reset your strings, they should be in line with the top outer edge of your proposed brick level.
• Mix your mortar follow the instructions on the bag if using premix or mix in a ratio 4 sand to 1 cement.
• The mortar should be moist enough to stick to the brick but not so wet that it runs off again.
• Now lay the first row of your bricks starting at the corners.
• The second and third row should be overlapping the row below by half a brick.
• You may need to cut some bricks in half to fill the gaps.
• Leave  the wall to dry for a few days.

Constructing and damp proofing the shed floor

• Next is the floor. Use small stones/blinding to level up, dig off high spots. Use a heavy duty polythene layer between the ground and the concrete to produce a much dryer floor. Mix and pour your concrete, tamp a level and leave to cure.

• If the weather is really hot, dry, wet, or cold (frost) covering the drying concrete with cloth sacks/sheets is a must, this also has the benefit of keeping off pets.
• Once the floor is dry, dry trim the polythene.

Your concrete foundations are now ready and you can start assembling your shed.