Chapter 4.  Lumber

Chapter 4.  Lumber

Nominal and Manufactured Lumber Sizes

        To avoid confusion in interpreting information in this chapter, terminology must be clarified regarding the dimensions (thickness, width, and length) of lumber. The term nominal is used for dimensions as customarily communicated in com­merce, such as referring to a piece of softwood lumber as a 2x4. The term manufactured refers to the dimensions in product specifications that correspond to nominal dimensions. Manufactured dimensions depend on the state of manufacture; a surfaced-dry softwood 2x4 is required by the American Softwood Lumber Standard (USDC 1970) to be a minimum of 1.5 inches by 3.5 inches. Actual dimensions are obtained by measuring individual pieces with calipers. Actual dimensions will differ slightly from the manufactured dimensions re­quired in specifications due to minor variations in drying shrinkage and milling accuracy. Information and examples in this chapter are based on nominal and manufactured dimensions. A mill could substitute its own actual dimensions for the manufactured dimensions required by product specifications.

        As discussed in Chapter 2, the manufactured sizes of North American lumber are generally different from the nominal sizes. Exceptions are special orders and some lumber exports, particu­larly those manufactured to metric specifications. The board foot measure of lumber volume is, how­ever, based on the nominal size. Consequently there can be a significant discrepancy between the cubic volume of lumber implied by board foot measure and the true cubic volume of wood present in the corresponding manufactured size. In many other parts of the world lumber is measured in metric units, with negligible difference between the nominal and manufactured size.

Softwood Lumber Sizes

        Softwood lumber is marketed by species, grade, and form of manufacture. For many products, the American Softwood Lumber Standard is a basic reference. Softwood lumber can be viewed as falling into two broad categories of use: con­struction lumber and lumber for remanufacture.

Construction Lumber.     Softwood construction lumber falls into three general categories: stress rated, nonstress rated, and appearance. The first two are used where structural integrity is the prime consideration. In the latter group, structural function, while important, is secondary to aesthetic considerations. Construction lumber is typically used in the form (sawn, planed, dried) and grade it receives at the sawmill.

        Table 4-1 shows nominal and minimum manu­factured thickness and width required of green and surfaced-dry softwood construction lumber. The nominal and manufactured lengths, in one-foot increments, are the same. Even, two-foot lengths are the most common. Also shown are metric equivalents and nomenclature.

Lumber for Remanufacture.     There are numerous groups (Factory or Shop, industrial clears, molding stock, etc.) of remanufacture lumber. To become integral parts of other products, these often undergo major transformation by secondary manufacturing after leaving the primary sawmill. Grading for remanufacture attempts to describe the yield obtainable from subsequent operations, and the lumber is offered in sizes needed to fit the partic­ular end product requirements.

        Table 4-2 shows thickness standards for one group of softwood remanufacture lumber. Note that the manufactured thickness required for a given nominal size is different and generally greater than for construction lumber (Table 4-1). Factory lumber is usually random width and random length. The notes at the end of Table 4-2 indicate some of the variations that occur in practice.

        The appropriate lumber grading agency (Appendix 5) should be contacted for the size requirements of other specialized lumber products.

Hardwood Lumber Sizes

        Most hardwood lumber is produced for remanu­facture into products such as furniture and cabinets or is made directly into flooring, paneling, millwork, and so forth. The three principal market categories are factory lumber, dimension parts, and finished market products. In addition, several hardwood species can be marketed under the procedures outlined in the American Softwood Lumber Standard; currently the volume produced is quite small.

        This section focuses on factory lumber, which dominates hardwood production. Details on sizes required for other hardwood products are contained in the grading rules obtainable from the National Hardwood Lumber Association (NHLA 1987).

Table 4-2.  Standard nominal and dressed sizes for Factory and Shop lumber under Western Wood Products Association rules.

 

	Nominal			Dry dressed			Metric nomenclature
	(in)	(quarters)		(in)	ALSb		(mm)d
	1	4/4a		3/4	S4Sc		19.05	19
	1-1/4	5/4		1-5/32	S2S		29.37	29
	1-1/2	6/4		1-13/32	S2S		35.72	36
	1-3/4	7/4		1-19/32	S2S		40.48	40
	2	8/4		1-13/16	S2S		46.02	46
	2-1/4	9/4		2-3/32	S2S		53.18	53
	2-1/2	10/4		2-3/8	S2S		60.32	60
	2-3/4	11/4		2-9/16	S2S		65.09	65
	3	12/4		2-3/4	S2S		69.87	70
	4	16/4		3-3/4	S2S		95.25	95

Source: WWPA (1981) and USDC (1970); metric values added by author.

^a4/4 is generally produced S4S in widths the same as listed for dimension in Table 4-1. All other thicknesses are generally produced S2S in random width; the average width is the nominal width plus 1/8 inch. For example, S2S 6-inch wide 8/4 actually ranges from 5.5 to 6.5 inches wide; the average width is 6.125 inches.

^bThe surfaced-dry thicknesses are based on the American Lumber Standard (ALS) thicknesses; in grading pieces under this rule, planer skip is not allowed. However, industry practice is to produce 5/4 and 6/4 stock as "heavy." Heavy means that these items are produced to be full-sawn (manufactured size = nominal size) when surfaced but grading this stock allows planer skip.

^cS4S  =  surfaced on all four sides.  S2S  =  surfaced on two sides (faces).

^dLength in meters begins at 1.3 m and increases in 0.3 m multiples.

Table 4-3. Nominal and manufactured thicknesses of hardwood lumber.

 

	Nominal rough-dry		Manufactured rough-green	Surfaced-dry (S2S)	Tolerance
	(in)	(quarters)	(in)	(in)	(in)
	3/8	4/4		3/16	1/8
	1/2	4/4		5/16	1/8
	5/8	4/4		7/16	3/16
	3/4	4/4		9/16	3/16
	1	4/4	1-3/16	13/16	1/4
	1- 1/4	5/4	1-7/16	1-1/16	1/4
	1- 1/2	6/4	1-11/16	1-5/16	1/4
	1- 3/4	7/4	1-15/16	1-1/2	1/4
	2	8/4	2-7/16	1-3/4	3/8
	2- 1/2	10/4	2-9/16	2-1/4	3/8
	3	12/4		2-3/4