The online AWC Span Calculator performs calculations for ALL species and grades of commercially available softwood and hardwood lumber as found in the NDS Supplement. There are also several other online span tables available for various regional species and grades of lumber and loading conditions:
Rafter span tables are tabulated based on allowable deflection limits of either L/240 or L/180.
The L/240 limitation allows for attachment of a ceiling to the underside of the rafter by limiting possible cracking. Cathedral ceilings are an example of this type of application. The L/180 deflection limitation would apply to a rafter with no ceiling directly attached, so deflection is not a concern.
Roof live and snow loads range from 30 psf to 60 psf. Roof dead loads range from 10 to 20 psf.
Typically a user will know the span and desired spacing for a given application. The user can select a species, size, and grade for trial use. Using the Design Values for Joists and Rafters, the user determines the modulus of elasticity and bending design value of the species, grade and size of lumber. Bending design values are already adjusted for load duration and repetitive member factors in the Design Values Supplement. With this information, the user enters the span table with the given bending design value and determines the allowable span for an estimated size and spacing. If the allowable span is greater than that required for the application, the species, grade and size selected should then be verified for stiffness or deflection. At the bottom of each table required modulus of elasticity for specific joist spacing are tabulated. If the modulus of elasticity is less than the tabulated value then the user can either select a deeper member, decrease the joist spacing or select a high lumber grade, thus a higher modulus of elasticity. For either option, bending design strength has to be re-evaluated to ensure that strength limits are maintained.
Decking Span Tables
Redwood – Click “Deck Construction” to see their updated version
Western Red Cedar
National Lumber Grades Authority’s rule #126 under all species that have been graded according to NLGA (which includes Ponderosa Pine) for Exterior Patio Decking provides grade rules as follows:
“Exterior patio decking rough or unsurfaced – kiln dried, air dried, or unseasoned 5/4″ to 2″ thicknesses, 4″ and wider for flatwise load applications where spans are not to exceed 16″ on centre.”