How thick really should my concrete garage foundation be? With typical soil conditions, 4" (100mm) concrete over 4" (100mm) clean up properly compacted hardcore blinded with two" (50mm) properly compacted sand or ash should be enough. The perimeter should be thickened into a minimum 10" (250mm) by ten" (250mm), below http://sectional-garage-leith74951.dailyhitblog.com/9597923/foundation-information-and-facts-for-new-concrete-garages