A roof rafter span table reveals the distance of a roof that can be supported by rafters without installing another beam for more support. Roof rafter span tables are based on three variables: the size of the rafters, the distance between each rafter and the pressure of the roof's maximum designed snow load. One of these variables, usually snow load, is considered a set variable in order to display the table in two dimensions.
A roof rafter span table can be expressed in standard or metric measurements. In the United States, it is usually expressed in feet, inches and pounds per square foot due to the fact that lumber is measured using the Imperial system. In addition to the lumber size, the type of wood from which the rafters and beams are made determines their span capabilities.
Another element to consider in designing a roof is the amount of insulation to be used for the roof. If the building has no attic or the owner is considering turning the attic into a living area, the long width of a rafter is the maximum thickness the insulation can be. Since "2-by-6" rafters are really 5.5 inches wide on their longer edges, many builders in cold climates opt for 2-by-8 or 2-by-10 rafters, and a roof rafter span chart is used to determine the distances between the rafters based on the support each size provides. Thus, climate is a key determinant in roof design for two reasons: snow load and the need for insulation.