re: DIY Outdoor fireplace

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Here are some general guidelines: 4-inch thick slab: A 4-inch thick concrete slab can typically support about 2,000 pounds per square foot [1]. 6-inch thick slab: A 6-inch thick concrete slab can support up to 4,000 pounds per square foot

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I think you are going to be fine with a 4 inch pad. The plan is calling for 20 bags of concrete and some rebar, so it sounds like it has the pad in the plans.