Reply To: Weight on a Petworth Porch?

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Concrete has enormous strength in compression, but is relative weak in expansion. The rebar in the concrete helps absorb some of the expansion force from deflection of the slab, and taller structures in DC use post-tensioned steel cables through the slab to reduce net expansion forces. So keeping in mind that the mass of the porch is probably greater than the mass of the rain barrels, as long as the load of the porch is bearing directly on something (say, a well-mortared brick ledge) and not toward the middle of the slab you should be alright. (Disclosure: Lector emptor – reader beware)