Terminology

• Softwood / hardwood: conifer wood of tracheids / broadleaf wood with vessels (ring-porous or diffuse-porous)
• Heartwood / sapwood: inner decay-resistant wood / outer less resistant wood
• Nominal size: historic rough size; a 2x4 is at least 1-1/2 by 3-1/2 in. dressed
• Specific gravity (G12): density relative to water at 12 percent moisture; density = 1,120 × G12 kg/m³
• LVL: laminated veneer lumber, typically 1-3/4 in. thick in depths of 9-1/2 to 24 in.
• Glulam: glued laminated timber made from 2x4 or 2x6 stock
• Kiln-dried / air-dried: drying methods; seasoning defects cause splits, bowing, and honeycombing
• EN 338 classes: European strength classes (C16 to C30 softwood, D24 to D70 hardwood)

History and demand

First uses in construction

• Wood has been used for construction for about 10,000 years; Neolithic European longhouses were mainly wood
• 1593: Cornelis Corneliszoon of Uitgeest invented the first wind-powered sawmill, about thirty times faster than hand sawing
• 1924: American Lumber Standard set dimension, grade, and moisture specifications; 1961: standard fixed dressed sizes (1-inch board 3/4 in., 2-inch lumber 1-1/2 in.)

Sourcing

• 2023 industrial roundwood removals were about 1.92 billion m3 and sawnwood output 445 million m3 (FAO)
• Industrial roundwood by country, FAO-derived listings: United States 18 to 19 percent, China 9 to 12, Russia 9 to 10, Brazil 7, Canada 6 to 7

Demand trend

• Almost 4 billion m³ of wood harvested in 2023; forest growing stock about 630 billion m³ in 2025; about 80 percent of lumber is softwood
• Timber framing has grown in United States popularity since the 1970s

Composition and production

Raw materials

• Cell walls: cellulose 40 to 50 percent, hemicellulose 15 to 25 percent, lignin 15 to 30 percent; cellulose microfibrils carry tension, lignin and hemicellulose matrix resist compression

Manufacturing process

• Logs sawn to lumber, kiln or air dried, planed

Product forms

• Dimension lumber, timber, LVL, wood I-joists, finger-jointed lumber (up to 36 ft in 2x6), glulam, trusses, plywood, and cross-laminated timber

Grades and types

• Softwood: about 80 percent of lumber; species table and Janka hardness in the Softwood note
• Hardwood: vessel-bearing wood; European classes D24 to D70; species table and Janka hardness in the Hardwood note
• Machine-graded lumber: for trusses and I-beams
• Engineered wood: LVL, I-joists, glulam, finger-jointed, trusses

Properties

Physical

• Density at 12 percent moisture: about 310 kg/m³ (19 lb/ft³, northern white cedar) to 880 kg/m³ (55 lb/ft³, live white oak)

Mechanical

• Compressive strength (12 percent moisture): about 27 MPa (3.9 ksi, northern white cedar) to 70 MPa (10.2 ksi, black locust)
• Flexural strength: about 45 MPa (6.5 ksi) to 139 MPa (20.2 ksi, hickory)
• Moisture effect: drying increases strength most in crushing strength; a dry spruce block carries about four times the permanent load of a green one

Thermal and moisture

• Moisture content: air-dried 8 to 16 percent; wood shrinks and densifies as it dries

Durability

• Decay: needs moisture above 25 percent; 19 percent is the maximum safe level for untreated wood in service
• Heartwood: more decay resistant than sapwood

Standards

Material specifications

• American Lumber Standard: dimension, grade, and moisture specifications with inspection (American Lumber Standard Committee)
• DOC PS 20: lumber grading
• ANSI/APA A190.1, ASTM D3737: glulam
• ASTM D5456: structural composite lumber
• ASTM D5055: I-joists
• ANSI/APA PRG 320: cross-laminated timber
• EN 14081, EN 338: European grading and strength classes

Uses in buildings

Building elements

• Wood-Framed Walls, Wood-Framed Floors, Wood Trusses, Wood I-Joists, Shear Walls: light-frame lumber, joists, trusses, and structural panels
• Glulam Beams, LVL and PSL, Mass Timber Walls: engineered and mass timber members
• Flush Wood Doors, Stile and Rail Wood Doors, Overhead Sectional Doors: wood door leaves
• Wood and Clad Wood Windows: wood sash and frames
• Hardwood Flooring: solid and engineered flooring
• Wall Cladding, Exterior Decking, Interior Millwork: siding, decking, and trim

Pros and cons

Pros

• About one cubic meter of lumber sequesters roughly one tonne of CO2
• Heartwood resists decay
• Engineered products extend spans and sizes

Cons

• Shrinks, swells, and loses strength when wet
• Decay when moisture exceeds 25 percent
• Resin-saturated "fat lighter" is very flammable

Design considerations

Movement

• Wood shrinks and densifies on drying

Sustainability

Recycled content and end of life

• United States wood waste (2018, EPA): 67 percent landfilled, 16 percent incinerated with energy recovery, 17 percent recycled

Manufacturing and cost

Main manufacturers and suppliers

• Weyerhaeuser
• LP Building Solutions
• Boise Cascade
• Huber Engineered Woods

Trade associations: APA – The Engineered Wood Association; American Wood Council.

Sources

• Wikipedia: Wood, https://en.wikipedia.org/wiki/Wood
• Wikipedia: Lumber, https://en.wikipedia.org/wiki/Lumber
• UpCodes: 2024 IBC Chapter 23 wood, https://up.codes/viewer/department-of-defense/ibc-2024/chapter/23/wood
• Wikimedia Commons: Pine plank fence 2019 G1.jpg, photo by George Chernilevsky (Public domain), https://commons.wikimedia.org/wiki/File:Pine_plank_fence_2019_G1.jpg