The key to designing exterior columns is to proportionally relate the column to the building. Traditional designs call for a beam resting on top of the column and underneath the roof. Even if the beam isn't required for structural support, it's necessary to create an architecturally complete exterior. A roof sitting directly atop columns will appear to overwhelm the columns, creating a sense of flimsiness. An intervening beam, however, bridges the mass disparity between column and roof; by appearing to add mass to the roof support, the beam helps the whole assembly project sturdiness.
— Brent Hull is a millwork consultant and author of the book, Historic Millwork. Reach him at www.hullhistorical.com.