Porch slabs against wood framing

Does anyone have a great detail for installing a concrete porch slab against an existing wood frame house? The top of the slab would have to be about 6″ below the existing interior finished floor, which means that the slab would be against the rim joist. Is there a flashing technique that might make this work? The rim joists in this house are already rotting because they are in direct contact with the ground, but my client insists on having a concrete slab porch.
Thank you.