In each of these cases, the stress rating applied to a given grade is based on the weakest species in the group. Since the Redwood Inspection Service and Southern Pine Inspection Bureau grade only redwood and southern pine respectively, their grade stamps do not have a species designation. Southern pine is actually lumber obtained from several pines found primarily south of the Mason-Dixon Line. In some areas, Virginia pine is used. The wood of these species is virtually identical in appearance and very similar in strength properties.
Some mills produce lumber from several different species.
It may not be economical for that mill to separate out each individual board by species. These mills can apply the species symbol for western woods, an all-encompassing term, to their lumber. The stress ratings are based on the weakest species in all the western woods, even though much of the lumber will be much stronger. RIS and SPIB publish rules for California redwood and southern pine respectively. WWPA provides coverage for the 13 western states while WCLB covers the Pacific Coast states.