It's not quite that simple. If it's floor joists in a crawl, the rule is clearance of less than 18" requires an approved protected lumber ** (see list below). If beams in a crawl, 12". If wood siding/sheathing, 6". Framing members that rest on concrete/masonry, 8". And sills/sleepers on concrete/masonry only need a moisture barrier. I don't believe there is a category that covers skirt boards on a pole barn, and that aspect of the barn is likely not covered by code anyhow (or it would be the inspector's call). Note in the types of lumber below that there are numerous categories that are acceptable to code that are not ground-contact rated (only C4 is ground contact rated). So anyhow, you don't necessarily need it to be ground contact rated, and the 8" number is specific to framing resting on concrete/masonry. ** Approved lumber is one that is pressure treated in accordance with AWPA C1, C2, C3, C4, C9, C15, C18, C22, C23, C24, C28, C31, C33, P1, P2 and P3, or decay-resistant heartwood of redwood, black locust, or cedars. The only thing I would add is that my barn passed code review and final inspection with UC3B skirt boards, which are not rated for ground contact, and the plans examiners raked me over the coals. They would have jumped all over that if it was a problem.
