A. Glenn Mathewson, a building inspector in Westminster, Colo., responds:Your memory serves you well, except that the required distance varies for different conditions. IRC Section R317.1 (the 2009 edition, which I administer) lists seven circumstances where protection from decay is required, and four of them could apply to your description of wood attached to concrete or masonry, depending on the details. In general, the code provisions deal not only with the wetting potential of the wood, but how easily it can dry. In some conditions, being "inside" actually creates more decay potential than being "outside," where wood might be better able to dry out. An interior condition where wood stays damp is a great recipe for decay.
The code also addresses the type of wood you can use. In most cases, either pressure-treated lumber or naturally durable lumber (defined by the IRC as the heartwood of redwood, cedar, black locust, and black walnut) is acceptable. In some cases, separating wood from concrete with a water-impervious membrane or vapor retarder is all that's...
to read the full article.