So many things the homeowner must consider when trying to protect wood from decay! According to Build Green: Wood Can Last for Centuries, water pooling on the hard surface of a porch, sidewalk, or patio as a result of draining from the roof presents another decay hazard.
Decay problems arise in the base of porch posts, entryway trim, and door jambs that rest directly on sidewalks, driveways, or porch and patio slabs. Wood resting in pooled water results in water wicking into the end grain of the wood through capillary action. Bases of wooden posts need protection, particularly in regions with high hazard for decay.
Preservative-treated wood is recommended in these applications. Otherwise, be certain that roof runoff is directed away from these structures and that posts, trim, and door jambs do not rest directly on the hard surface.