Though they have some local variations, building authorities throughout the U.S. follow the International Residential Code (IRC). The International Code Council, a non-profit founded in 1994, maintains the IRC and other construction regulations, including the International Building Code, the International Fire Code and the International Energy Conservation Code. By having one source for all building rules, structures throughout the United States meet the same standards for durability and safety, and all construction professionals work to the same specifications.
The allowable maximum opening spans for headers depend on their sizes, the snow load, the building width and what they’re supporting. For example, assume an exterior wall that supports a roof and ceiling, with a ground snow load of 30 pounds per square foot. Two 2-by-4 inch headers can span 3-1/2 feet if the structure is 20 feet wide and 34 inches if the structure is 36 feet wide. Increasing the size to three 2-by-8 inch headers increases the allowable span to 76 inches for a 20-foot width and 89 inches for a 36-foot width. This assumes the headers are made of Douglas fir, southern pine or spruce.
Increasing any of the factors, such as load or number of floors, typically reduces the allowable span. For example, increasing the snow load to 50 pounds per square foot reduces the allowable span for two 2-by-4 inch headers to 38 inches for a structure that is 20 feet wide or to 2-1/2 feet if the structure is 36 feet wide. Having the girder support an additional floor, plus the roof and ceiling, reduces the allowable span for a snow load of 30 pounds per square foot to 37 inches for a 20-foot width and 29 inches for 36 feet.
Spans for the same types of headers go down slightly if the opening is in an interior, non-load-bearing wall, such as in the doorway to a walk-in closet. According to the IRC, for one floor, two 2-by-40 inch headers can span 37 inches for 20-foot-wide structures and 29 inches for 36-foot-wide buildings. Increasing the header size to two 2-by-8 inch pieces increases the span to 69 inches for a 20-foot width and 53 inches for a 36-foot width.