A: A header is doubled 2-by material above the door to distribute the weight of the structural framing above. Always make the header exactly as thick as the door frame—-for instance, two 2-by-4 boards sandwiching a plywood board cut to the same size. Plywood or drywall is usually used as a middle board to make the doubled header the same thickness as the door framing. The size of the header will be determined by the span of the door, and whether or not it is a load-bearing wall. Assuming you're using a species of wood at least as strong as No. 2 Douglas fir, a doubled 2-by-4 (4-by-4) is adequate for a load-bearing opening of 48 inches or less. The required header size increases as the span gets wider.