today’s wood columns
are made to vent top and bottom. The column sits on a plinth (flat pedestal) routed or molded at the bottom of each side to allow airflow into the column. Note that even traditional wood-column manufacturers supply an aluminum plinth, or a molded urethane base with plinth; once painted, it’s unnoticeable. The top may be vented through the face of the column (on a protected side facing the house), out through a hole in the top of the capital, or through a hollow entablature above the column. Screen vent holes on the inside.
Old columns sitting directly on masonry, and Doric columns with no base or plinth, should be set on a thick lead plate, which is chiseled to conform to the base and with vent gaps cut in. —Patricia Poore