The definition of “ground contact” as it applies to pressure-treated lumber seems to be vague. Does “ground” mean dirt or accumulated water? What if the treated lumber is in contact with concrete or a correctly installed paver base at ground level that is sloped for drainage, such as would be the case with stair stringers? Adding to the confusion, many lumber suppliers do a poor job of labeling or even stocking ground-contact lumber in sizes typically used for stair stringers. I often see prefabricated stair stringers with only 0.19 ACQ retention levels, and rarely see 2x12 stock with the 0.40 pcf recommended for ACQ lumber.
From online comments