Wood & Engineered Wood Products

Wood is the oldest structural material in continuous use and remains the dominant material for residential construction in North America. It is renewable, workable, lightweight for its strength, and aesthetically warm. But wood is also a natural material with inherent variability — its strength depends on species, grain orientation, moisture content, and defects. Understanding these variables is essential for using wood safely and effectively.

Wood is composed primarily of cellulose (provides tensile strength), lignin (provides compressive strength and rigidity), and hemicellulose (binds cellulose fibers together). These components are arranged in a cellular structure of long, parallel fibers aligned along the tree's growth direction — the grain.

Grain Direction Matters

Wood is an anisotropic material — its properties differ depending on the direction of loading relative to the grain:

• Parallel to grain: Strongest direction. Loads applied along the length of the fibers (axial compression in a column, bending in a joist) use wood's strength most efficiently.
• Perpendicular to grain: Much weaker. Loads applied across the fibers (bearing at a support, crushing at a connection) can cause crushing. Perpendicular-to-grain compressive strength is only about 25% of parallel-to-grain strength.
• Shear parallel to grain: Wood is weakest in horizontal shear (fibers sliding past each other). This governs beam design near supports.

Moisture Content

Moisture content (MC) is the weight of water in wood expressed as a percentage of the oven-dry weight:

MC = (wet weight - oven-dry weight) / oven-dry weight × 100%

Moisture profoundly affects wood:

• Green lumber: Freshly sawn, MC can be 30-200% (depending on species and whether it is heartwood or sapwood).
• Air-dried lumber: MC of 15-20%.
• Kiln-dried (KD) lumber: MC of 19% or less (stamped "KD" or "S-DRY").
• Kiln-dried to 15% (MC15): Used for applications requiring greater dimensional stability.

Key threshold — 19% MC:

• Above 19%: Wood is susceptible to decay, mold, and dimensional changes (shrinking as it dries).
• Below 19%: Generally stable and resistant to decay.
• Below 12%: Required for finish carpentry, cabinetry, and interior trim to prevent gaps and warping.

Shrinkage: Wood shrinks as it dries below the fiber saturation point (~28-30% MC). Shrinkage occurs primarily across the grain — tangentially (6-10%) more than radially (3-5%), and negligibly along the grain (<0.1%). This differential shrinkage is why wood warps, cups, and checks (develops cracks).

Lumber is graded to classify its strength, appearance, and suitability for different uses.

Visual Grading

A trained grader inspects each piece of lumber for defects that reduce strength:

• Knots: Where branches grew from the trunk. Knots reduce strength because they interrupt the grain direction. Larger knots and knots near edges reduce strength more.
• Slope of grain: If the grain angles relative to the edge of the board, the lumber is weaker.
• Checks and splits: Cracks caused by drying. Reduce shear strength.
• Wane: Missing wood or bark on the edge of a board. Reduces the effective cross-section.
• Warp: Deformation including bow (curvature along the length), crook (curvature along the edge), twist (rotational distortion), and cup (curvature across the width).

Common visual grades for framing lumber (2×4 through 4×4):

Machine Stress Rating (MSR)

Each piece of lumber is mechanically tested by passing it through a machine that measures stiffness (modulus of elasticity). Pieces are then assigned to stress grades based on their measured properties. MSR lumber is more precisely graded than visually graded lumber and is used where consistent, predictable strength is needed (trusses, engineered applications).

Designation example: 1650f-1.5E means the lumber has an allowable bending stress of 1,650 psi and a modulus of elasticity of 1,500,000 psi.

Different wood species have different strengths, densities, and properties. Common species groups used in U.S. construction:

Wood that will be in contact with the ground or exposed to continuous moisture must be pressure-treated with preservative chemicals to resist decay and insect attack.

Process: Lumber is placed in a sealed cylinder, air is evacuated (vacuum), and preservative solution is forced in under high pressure, penetrating deep into the wood cells.

Common preservatives:

• ACQ (Alkaline Copper Quaternary): The most common residential preservative since the phase-out of CCA. Contains copper (fungicide) and quaternary ammonium (insecticide).
• CA-B (Copper Azole Type B): Similar to ACQ. Contains copper and tebuconazole.
• CCA (Chromated Copper Arsenate): Historically the most common preservative. Still used for industrial/commercial applications but banned for residential use since 2004 due to arsenic content.
• Borate: Water-soluble preservative for interior use only (not suitable for ground contact or exterior exposure).

Retention levels: Expressed in pounds of preservative per cubic foot (pcf). Higher retention = more protection.

• Above ground (UC3A): 0.06 pcf (ACQ) — decks, railings.
• Ground contact (UC4A): 0.15 pcf (ACQ) — posts, ledger boards.
• Ground contact, heavy duty (UC4B): 0.31 pcf (ACQ) — critical structural members, permanent wood foundations.
• Fresh water immersion (UC5A): 0.40 pcf (ACQ) — docks, marina piles.

Important: ACQ and CA-B are highly corrosive to standard galvanized fasteners. Use hot-dip galvanized (HDG), stainless steel, or ACQ-rated connectors with treated wood.

Engineered wood products are manufactured from wood fibers, veneers, or strands bonded with adhesives. They overcome many limitations of solid lumber — they are more consistent, available in longer lengths, and can be made stronger and stiffer than equivalent solid lumber.

Plywood

Plywood is made from thin layers (plies or veneers) of wood glued together with adjacent layers' grain oriented at 90° to each other (cross-lamination). This makes plywood strong in both directions and resistant to splitting.

• Structural plywood: Used for roof sheathing, wall sheathing, and subflooring. Grades include CDX (C face, D back, exterior glue — the most common structural grade) and structural rated panels.
• Span rating: Stamped on panels (e.g., 32/16) — indicates the maximum rafter/truss spacing (32") and joist spacing (16") the panel can span.
• Thickness: Common structural thicknesses: 3/8", 15/32" (nominal 1/2"), 19/32" (nominal 5/8"), 23/32" (nominal 3/4").

Oriented Strand Board (OSB)

OSB is made from rectangular wood strands arranged in layers and bonded with waterproof resin under heat and pressure. Like plywood, alternating layers are oriented at 90° for cross-directional strength.

OSB is less expensive than plywood and has largely replaced it for wall sheathing, roof sheathing, and subflooring. Performance is comparable to plywood for most applications, though OSB is more susceptible to edge swelling when exposed to moisture.

Laminated Veneer Lumber (LVL)

LVL is made from thin veneers (typically 1/10" to 1/8" thick) layered with grain running parallel and bonded with adhesive. The result is a strong, stiff, dimensionally stable beam or header material.

• Common sizes: 1-3/4" wide (single ply) in depths from 5-1/2" to 24".
• Uses: Headers over windows and doors, beams, hip and valley rafters, scaffold planks.
• Advantage: Consistent strength with no knots, checks, or warp. Available in long lengths (up to 66 feet).

Glued Laminated Timber (Glulam)

Glulam is made from multiple layers (laminations) of dimensional lumber glued together with structural adhesive. Laminations are typically 1-3/8" or 1-1/2" thick.

• Sizes: Width matches standard lumber widths (3-1/8", 5-1/8", 6-3/4", etc.). Depths can be manufactured to virtually any size.
• Uses: Large beams, arches, columns, exposed timber structures. Common in churches, gymnasiums, and commercial timber-frame buildings.
• Advantage: Can span longer distances than solid lumber. Can be manufactured in curved shapes. Naturally beautiful for exposed applications.

Wood I-Joists (Engineered I-Joists)

Wood I-joists have a cross-section shaped like the letter "I" — two flanges (typically LVL or solid lumber) connected by a web (typically OSB or plywood).

• Depths: 9-1/2", 11-7/8", 14", 16" (and deeper for commercial applications).
• Uses: Floor joists and roof rafters. The dominant floor framing member in modern residential construction.
• Advantage: Lightweight, long spans (up to 30+ feet), consistent depth (no crown to set), pre-punched knockouts for wiring and plumbing.
• Limitation: The thin OSB web is vulnerable to fire — burns through faster than solid lumber. Must not be cut, notched, or drilled except at manufacturer-designated locations.

Cross-Laminated Timber (CLT)

CLT is a mass timber product made from layers of lumber boards stacked crosswise (each layer perpendicular to the adjacent ones) and bonded with structural adhesive.

• Panel sizes: Typically 3 to 10 layers, with finished thicknesses from 3-1/2" to 15" or more. Panels can be manufactured up to 60 feet long and 10 feet wide.
• Uses: Floors, walls, and roofs in multi-story buildings. CLT is enabling the construction of "mass timber" buildings — wood structures 8 to 20+ stories tall.
• Advantage: Prefabricated panels enable rapid erection. Lower carbon footprint than concrete/steel. Excellent fire performance (CLT chars predictably, maintaining structural integrity).