Softwood Lumber Grades
Bruce E. Cutter
School of Forestry, Fisheries and Wildlife
Every year, a considerable volume of softwood lumber is used in residential, commercial and industrial construction in the United States. The lumber is used either as primary framing members, temporary scaffolding, or decorative finish. When purchasing lumber, the buyer must realize that certain grades of lumber are intended for specific uses.
For example, there are specific grades of lumber to be used as scaffolding. Scaffold planking has very specific requirements that are not met by most lumber. This publication will explain the basic divisions of softwood lumber and their intended uses.
To most people, softwood means wood from trees like pines, cedars, redwood, firs, hemlocks, etc. (evergreens). This is correct, but there is at least one instance when a hardwood (aspen) is placed in this broad category. In some circumstances, aspen is graded the same as softwood construction lumber, which means that it can be used in construction.
It is also important to remember that when people refer to a 2 x 4, the actual size of this piece of material is something less than 2 inches by 4 inches. These sizes are commonly referred to as nominal dimensions; this means in name only. The actual dimensions are usually going to be less. This applies to most softwood lumber. But there are always exceptions (such as scaffold plank).
Now let's look at some ways of classifying softwood lumber. One method is by its intended use; either in construction (houses, etc.) or remanufacture into other products (furniture, millwork, etc.). Construction lumber is graded based on the assumption that it is going to be used in the same size in which it was produced. If the lumber is going to undergo significant changes in its shape, size, appearance, etc., before it is sold as a product, it is called remanufacture lumber. Because many of these uses are very specific to a certain industry (stiles and rails for doors, crossarms for utility poles, etc.), they will not be discussed in this publication.
Lumber also can be classified by size. Softwood lumber usually is manufactured in lengths of 2-foot multiples (8,10, and 12 on up to 24 foot), although it can be made in 1-foot increments. It also is usually between 2 and 16 inches in width. Lumber less than 2 inches in nominal thickness is referred to as boards. When the nominal thickness is more than 2 inches and less than 5 inches, it is referred to as dimension lumber. Lumber 5 inches or more in nominal thickness is referred to as timbers. Softwood lumber also can be classified based on whether it is rough-sawed, planed, or patterned. Rough-sawed lumber has all the saw marks still visible and has not been surfaced to a final dimension. Most lumber that people buy for construction and framing is planed to its final dimensions (all the sides are smooth). This lumber is also referred to S4S, surfaced 4 sides. Patterned lumber has had something extra done to it. Perhaps a tongue-and-groove pattern has been machined into the sides.
Within this category, softwood lumber is classified in three ways: stress-graded, non-stress-graded, and appearance lumber. Almost all softwood dimension lumber found in a typical retail/wholesale lumberyard is stress-graded. This means that there should be a grade stamp on the piece and a set of allowable design stresses assigned to that grade of lumber. The American Softwood Lumber Standard, PS 20-70, provides a National Grading Rule for dimension lumber (from 2 up to but not including 5 inches in nominal thickness). Special products such as scaffold planks are excluded from these rules (they have their own rules).
The national grading rules establish lumber classifications and grade names for the visually stress-graded lumber as shown in Table 1.
National Grading Rule lumber classes
(2 to 4 inches thick, 4 inches wide)
|Structural light framing
(2 to 4 inches thick, 2 to 4 inches wide)
(2 to 4 inches thick, 2 to 4 inches wide)
|Structural joists and planks
(2 to 4 inches thick, 6 inches and wider)
(2 to 4 inches thick, 2 to 4 inches wide)
Sizes are nominal.
These grades are based on the assumption that the lumber is going to be used as is with no remanufacturing. In other words, the grade assigned to a 12-foot 2 x 4 is based on the location and size of defects over the entire length. grades themselves are based on location, size and placement of knots, slope of grain, manufacturing defects, wane, warp and other factors that must be considered. Within each grade, there is a ane of values that are allowed. Thus, not all stud grade studs are exactly the same in appearance, but they all have the same structural design capabilities.
Sample grade stamp.
The grade assigned to an individual piece of dimension lumber is stamped on each piece about 18 to 24 inches from the end. It is called a grade stamp, and for lumber graded under PS 20, it must contain certain information. A sample grade stamp is shown in Figure 1. This stamp contains the following information:
- The grading agency that wrote the rules and issued the grade stamp.
- The species of lumber.
- The grade itself.
- The mill identification (name or number).
- The moisture content of the wood at the mill when the stamp is applied.
In the U.S., there are six associations that publish grade rules: Redwood Inspection Service (R1S), Northeastern Lumber Manufacturers Association (NELMA), Northern Hardwood and Pine Manufacturers Association (NHPMA), Southern Pine Inspection Bureau (SPIB), West Coast Lumber Inspection Bureau (WCLB), and Western Wood Products Association (WWPA). Several other agencies are licensed to use these grade rules and apply stamps of their own (there is a similar set-up in Canada, and their rules are essentially the same as ours).
Of all the aspects of the grade stamp, this may be the most confusing to consumers. When a log is sawed into lumber, the wood contains very large amounts of water. Wood is much like a sponge in that regard. If we wring the sponge out, the sponge is relatively dry. You wring the water out of the wood by drying the wood. The weight of the water that you "wring out of the wood" divided by the oven-dry weight of the wood is the fractional moisture content. Multiply this by 100 and you have the moisture content expressed as a percent. The oven-dry condition is 0 percent moisture content. Typically, freshly cut logs have moisture contents of 80 to 150 percent, depending on the species of wood.
After the lumber is sawed into whatever sizes are desired, the wood is dried. Lumber is dried for several purposes. These include reducing the risk of insect or fungal damage, reducing shipping weight (and therefore shipping costs), controlling the amount of shrinkage that takes place, and making gluing and finishing more feasible. Notice that drying only controls shrinkage; it does not eliminate the fact that wood shrinks and swells in response to changes in the relative humidity of the surrounding air.
Under the National Grading Rule, there are three moisture content conditions: S-GRN, S-DRY, and MC 15. S-GRN (surfaced green) means that the moisture content is above 19 percent. Most lumber is probably dried to the S-DRY (surfaced dry) condition. This means that the moisture content is less than 19 percent. MC15 means that the moisture content is less than 15 percent. Remember, this is the moisture content at the time the grade stamp has been applied at the mill, not after the lumber has sat in a puddle of water for two months!
Species and species groups
Dimension lumber is the principal stress-graded lumber product available at most retail lumberyards. This lumber probably will be used where strength, stiffness and size uniformity are important. Floor joists, ceiling and roof rafters, and studs are common examples. Although there are many sizes and species of material produced, not every lumberyard stocks every item. A lumberyard usually will have only the most commonly requested items in stock in a few species groups. "Hem-fir," "S-P-F" and "DF-L" are typical species that you might find. Hem-fir stands for western hemlock and true firs. These woods are very similar in strength and appearance and it is not economical to separate them. S-P-F represents spruce-pine-fir, a common grouping for some of the eastern softwoods. DF-L refers to Douglas fir and western larch. 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. Sometimes referred to as "southern yellow pines," the principal species used are loblolly, shortleaf, longleaf and slash pine. In some areas, Virginia pine is used. The wood of these species is virtually identical in appearance and very similar in strength properties.
Under the WWPA rules, there is a species grouping referred to as "Western woods." 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. NELMA publishes rules for species found in the New England and Middle Atlantic States while NHPMA covers those species found in the lake states. WWPA provides coverage for the 13 western states while WCLB covers the Pacific Coast states. Obviously, there is some overlap between the various agencies.
Dense included in the grade. This has to do with the growth rate and the amount of latewood (also known as springwood). Remember, these are the grades permitted under the National Softwood Grading Rule.
There is a grade of lumber known as Economy that is not permitted under these rules. Lumber with this grade stamp on it usually is not permitted for use in load-bearing walls, roofs, etc., in areas where there are building codes. If you plan to use this material, check with your local building inspectors first.
Mills and companies pay for the right to place a given grading agency's grade stamp on their lumber. When the mill subscribes, they are assigned an identification number by that agency. No other mill or company may use that number without permission. Some companies stamp their name or trademark on the lumber as well. This is primarily done for advertising purposes.
When you go to your local retail lumberyard, remember that the grade was placed on the lumber at the mill — not at the store! The grade is placed on the lumber for the protection of both the consumer and the producer. If you have questions about which grade of lumber to use, ask the salesperson.