With normal soil conditions, 4" (100mm) concrete over 4" (100mm) clean well-compacted hard blinded with 2" (50mm) well-compacted sand or ash should be sufficient. The perimeter should be thickened to a minimum of 10" (250mm) by 10" (250mm), below ground level. To increase the strength of the base, steel mesh (A142 – 6mm) can be added, and glass-reinforced concrete may be used.
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Will I need Local Authority Approval for my concrete garage base?
If the building is over 30 square meters in an enclosed floor area, then you will need Building Regulation Approval, and you will need to have the base design inspected and approved by your local Building Control department. Your local agent has a table of models with the enclosed floor area already calculated and will be able to advise on the size of your chosen model.
In any event, the base design should be to suit local soil conditions, the position of the base, trees, etc.
There are other conditions that may apply, dependent on locality. For instance, you may require Building Regulation Approval if the proposed building is within 1 meter of a boundary. You should check with your Local Authority if in doubt.
Should my concrete garage base be level with the surrounding ground?
Ideally, no. It should stand out from the surrounding ground, to ensure that water does not drain from the ground onto the base, where it will attempt to get under the garage panels and through the sand and cement mortar fillet which is laid when new concrete garages are installed.