Types of Lumber: Sizes, Grades, Species and Panels

Lumber is classified three ways, by tree type, by size and by grade, and most confusion about types of lumber comes from mixing them:

• By tree type - softwood from conifers such as pine, fir, spruce and cedar, and hardwood from broadleaf trees such as oak, maple, cherry and mahogany. Marling's lumber guide groups them this way; our hardwood vs softwood page explains why the names do not always match hardness.
• By size - the USDA Wood Handbook splits softwood lumber into boards (less than 2 in nominal thickness), dimension (2 in up to, but not including, 5 in) and timbers (5 in or more in the least dimension).
• By grade - a stamp that describes strength (for framing) or appearance (for boards and hardwoods).

A piece of lumber has all three at once: for example, a No. 2 Southern pine 2x6 is a softwood, a dimension piece and a structural grade.

Width and thickness are sold by nominal size, and the actual piece is smaller. The Handbook's Table 6-6 (from American Softwood Lumber Standard PS-20) gives minimum dressed dry sizes:

• 1 in boards: 3/4 in thick
• 2x4: 1-1/2 by 3-1/2 in
• 2x6: 1-1/2 by 5-1/2 in
• 2x8: 1-1/2 by 7-1/4 in
• 2x10: 1-1/2 by 9-1/4 in
• 2x12: 1-1/2 by 11-1/4 in
• 4x4: 3-1/2 by 3-1/2 in

Green lumber is surfaced slightly larger (a green 2x4 is 1-9/16 by 3-9/16 in) so that it shrinks to about the dry size at roughly 15% moisture. Length, by contrast, is the actual length: softwood comes in 1 ft multiples, usually even 2 ft steps, and retail dimension lumber runs 8 to 18 ft.

Hardwood lumber is sold differently: random widths, lengths from 4 to 16 ft, and thickness in rough sizes that surface down, for example 1 in rough to 13/16 in surfaced two sides (Table 6-4).

The Handbook divides softwood lumber by use into yard lumber, structural lumber, and factory and shop lumber.

• Dimension (framing) lumber is stress graded under the National Grading Rule, so grade names are the same nationwide even though allowable strengths vary by species. Retail yards stock 2x4 and wider in Select Structural, No. 1, No. 2 and No. 3; a 2x4 may also be sold as Construction, Standard, Utility or Stud grade. Grading can be visual or by machine.
• Boards are graded mainly for serviceability. Retail grades are usually No. 1, No. 2 and No. 3 Common. The higher grades go into siding, shelving and paneling; the lower grades are graded for usability, not appearance, and end up as sheathing, subfloor and concrete forms.
• Factory and shop lumber is graded by how much clear material can be cut out: Factory Select and Select Shop must yield 70% clear cuttings, No. 1 Shop 50%, No. 2 Shop 33-1/3%.

The stamp also names a species group. The AWC deck guide, for example, gives separate span tables for Southern pine; Douglas fir-larch, hem-fir and spruce-pine-fir (SPF); and redwood, western cedars, ponderosa pine and red pine.

Hardwood lumber is graded under National Hardwood Lumber Association rules by how much of each board can be cut into clear pieces (Handbook Table 6-1):

• FAS - boards at least 6 in wide, 8 to 16 ft long, yielding 83-1/3% or more clear cuttings.
• F1F - FAS on one face; also at least 6 in wide.
• Selects - at least 4 in wide, 6 to 16 ft long.
• No. 1 Common - at least 3 in wide, mostly 66-2/3% clear.
• No. 2 Common - at least 3 in wide, down to 50% clear.

The upper grades leave fewer defects to cut around, which matters for long furniture parts; small parts can come out of Common grades. For choosing a species, start with oak vs maple.

• Pressure-treated lumber is ordinary pine or fir with preservative forced in, sold by use category; see treated wood for decks.
• Naturally durable lumber such as cedar and redwood heartwood; see best wood to use outdoors.
• Plywood - veneers glued with the grain of adjacent layers at right angles, in an odd number of layers; see types of plywood.
• OSB, particleboard and MDF - panels made from strands, particles or fibers.
• Engineered lumber - LVL, glulam, I-joists and CLT, covered in wood building materials.

• Brazilian rosewood (Dalbergia nigra) - listed in CITES Appendix I, the most protected category, on 11 June 1992, so international trade is restricted (Wikipedia).
• American chestnut - chestnut blight, first found in 1904, killed an estimated three to four billion trees in the first half of the 20th century (Wikipedia). Chestnut sold today is mostly reclaimed.

If you find an old board you think is one of these, our types of wood guide and WoodLens can help you narrow it down before you buy or sell.

The types of wood for building are mostly softwoods: Wikipedia's lumber article says about 80% of lumber comes from softwood, chiefly pine, fir and spruce (sold together as spruce-pine-fir), cedar and hemlock, and that softwood dimensional lumber is typically used for construction while hardwood boards more often go into cabinets and furniture. Here is what goes where in a typical wood-framed house:

• Wall, floor and roof framing - softwood dimension lumber; Wikipedia's framing article names spruce, pine and fir as the preferred species for linear structural members. Studs are usually 1-1/2 by 3-1/2 in (a 2x4) or 1-1/2 by 5-1/2 in (a 2x6), commonly 16 in on centre, changed to 12 or 24 in depending on load. Which species group spans farthest is covered in the grades section above.
• Sheathing and subfloor - plywood or oriented strand board (OSB); see types of plywood.
• Long spans and headers - engineered lumber: laminated veneer lumber (LVL) for large openings, wooden I-joists for floors, glulam beams, and finger-jointed lumber, which Wikipedia says can be made up to 36 ft long. Our wood building materials page compares them.
• Outdoors and ground contact - preservative-treated lumber, or the heartwood of naturally durable species; see treated wood for decks and best wood for outdoor use.
• Interior trim, cabinets and floors - hardwoods, or MDF, which the USDA Wood Handbook says is widely used for mouldings, interior trim and furniture; see types of MDF.

Wood that has been in a house for decades may not be what is sold today. Wikipedia notes that chromated copper arsenate (CCA), once the most common wood preservative in North America, began to be phased out of most residential uses in 2004, so treated deck and fence lumber older than that may be CCA. As a precaution, wear gloves and a dust mask if you cut or sand it, and do not burn it.