To calculate the span of a header, first determine where the header is to be used and make the appropriate calculations for the bearing load and header length. Floor beams, roof beams and roof-and-floor beams all have different specifications. Once the header is identified, refer to a chart to determine the beam size, such as the charts available on BetterHeader.com and SouthernPine.com.
Construction professionals typically choose double 2 x 8 headers above windows and door frames, as these are the standard for residential loads. However, for more complicated and larger structures, different beam spans can support stronger loads. Headers are designed to transfer the weight of larger loads from the top of the structure to the bottom through a series of supports.
Beams must be strong enough to not break under the load, which is the Fb value, and also rigid enough so that it does not transfer too much of the load, which is the E value. These two factors need balance to prevent the support system from failing. For an efficient distribution, the loads need to be set evenly along the span of the header or focused on specific load-bearing points along the header, which are themselves supported by other systems.