Comprehensive Notes on Wood as a Building Material

Wood as a Building Material

• Wood is considered the most beloved building material.

• It is the only renewable building material available for construction.

Tree Growth and Structure

• Trees: The source of all wood products.

• Tree Growth Components (from exterior to interior):

  • Dead bark: Outermost protective layer.

  • Live bark: Inner bark layer.

  • Cambium: A thin layer of living cells between the bark and sapwood, responsible for producing new wood (xylem) and bark (phloem).

  • Sapwood: The living outer region of a tree's wood, containing living cells and responsible for water and nutrient transport.

  • Heartwood: The older, inactive, and darker wood in the center of the tree, providing structural support but no longer transporting sap.

  • Growth rings (Annual rings): Visible concentric rings in a tree trunk, each representing one year of growth.

  • Pith: The small, soft core at the very center of the tree trunk.

• Growth Rings – Specific Components:

  • Springwood (earlywood): Formed during spring, characterized by larger cells and lower density, as the tree grows more rapidly.

  • Summerwood (latewood): Formed during summer, characterized by smaller, denser cells, as the tree's growth slows.

Softwoods and Hardwoods

• Classification:

  • Softwoods: Come from coniferous trees (cone-bearing, typically evergreen, e.g., pine, spruce, fir).

  • Hardwoods: Come from broad-leafed trees (typically deciduous, e.g., oak, maple, cherry).

• Common Applications:

  • Softwoods: Primarily used for framing, structural panels (e.g., Alpine fir, Black spruce, Douglas fir, Eastern white pine, Loblolly pine, Ponderosa pine, Sitka spruce).

  • Hardwoods: Primarily used for trim, paneling, cabinetwork, furniture (e.g., American beech, Birch, Black cherry, Black walnut, Elm, Hard maple, Red oak, White oak, Yellow poplar).

Lumber Production and Characteristics

Lumber Sawing Methods

• Plainsawn (or flatsawn):

  • Produces boards with a broad, irregular grain figure.

  • Cut tangent to the growth rings, resulting in wider face grain patterns.

• Quartersawn (or radial-sawn):

  • Produces a more tightly spaced, regular, and often striped grain structure.

  • Cut perpendicular to the growth rings, increasing stability and often preferred for appearance.

• Pros and Cons: (Implicitly, quartersawn offers more stability and better appearance but is more expensive; plainsawn is more economical but prone to more shrinkage/cupping).

Seasoning (Drying of Wood)

• Moisture Content (MC): The amount of water in wood, expressed as a percentage of the weight of the dry wood.

  • Formula: $\text{MC} = \left( \frac{\text{Weight of water}}{\text{Weight of dry wood}} \right) \times 100\%$

• Typical Moisture Content Values:

  • Growing wood: Varies from about $30\%$ to $200\%$ or more.

  • Equilibrium Moisture Content (EMC): The moisture content wood reaches after a period of time in a particular environment.

    • Exterior wood in North America: Averages around $12\%$ EMC.

    • Interior wood in North America: Averages around $8\%$ EMC.

  • Standard framing lumber: Typically $19\%$ MC at the mill.

  • For structural applications requiring better control of shrinkage: $15\%$ MC, often labeled "MC 15".

Moisture Shrinkage

• Wood shrinks and swells as its moisture content changes, particularly below the fiber saturation point (above which shrinkage does not occur).

• Shrinkage Directions: Significant shrinkage occurs tangentially (around the growth rings) and radially (across the growth rings), but very little longitudinally (along the grain).

  • Tangential shrinkage is typically about twice as much as radial shrinkage for the same change in moisture content.

  • The provided graph illustrates this: Tangential shrinkage is consistently higher than radial shrinkage across various moisture content percentages.

Lumber Defects

• Growth Characteristics (common):

  • Knots: Places where branches joined the tree trunk.

  • Knotholes: Holes left when loose knots fall out of the wood.

  • Decay: Deterioration caused by fungi.

  • Insect damage: Damage from boring insects.

• Manufacturing Characteristics (usually caused by non-uniform shrinkage or processing errors):

  • Splits and checks: Cracks in the wood, often due to shrinkage stresses.

  • Crooking: Warping along the length of the board, causing it to curve sideways.

  • Bowing: Warping along the length of the board, causing it to curve longitudinally (like a bow).

  • Twisting: Warping along the length, causing ends to rotate in opposite directions.

  • Cupping: Warping across the width of the board, causing it to curve into a C-shape.

  • Wane: An irregular rounding of edges or faces, caused by sawing too close to the log's perimeter.

Lumber Grading

• Purpose: To classify lumber based on its structural performance or appearance qualities.

• Types:

  • Structural grading: Based on strength and stiffness, often rated by species group or species combination, as different species share similar structural properties.

  • Appearance grading: Based on aesthetic qualities, such as freedom from knots or other blemishes.

Structural Properties of Lumber

• Wood possesses significant strength in both tension and compression, generally stronger along the grain than across it.

• Strength and stiffness values vary considerably among different wood species and grades of lumber.

Lumber Dimensions

• Nominal Dimensions: Standardized sizes in inches (e.g., $2 \times 4$) used for lumber designation.

• Actual Dimensions: The true, dressed size of kiln-dried lumber, which is smaller than the nominal dimension due to surfacing (planing) and drying.

  • Pieces less than $2$ inches nominal thickness ($38$ mm actual) are boards (e.g., a $1 \times 4$ is actually \frac{3}{4}\" \times 3\frac{1}{2}\").

  • Pieces $2$ to $4$ inches nominal thickness ($38$ to $89$ mm actual) are dimension lumber (e.g., a $2 \times 4$ is actually 1\frac{1}{2}\" \times 3\frac{1}{2}\"; a $2 \times 6$ is 1\frac{1}{2}\" \times 5\frac{1}{2}\"; a $2 \times 8$ is 1\frac{1}{2}\" \times 7\frac{1}{4}\").

  • Pieces nominally $5$ inches ($114$ mm actual) and more in thickness are timbers (e.g., a $4 \times 4$ is actually 3\frac{1}{2}\" \times 3\frac{1}{2}\"; a $4 \times 12$ is 3\frac{1}{2}\" \times 11\frac{1}{4}\").

• Board Foot: The unit of measurement for pricing lumber in the United States.

  • It is defined as a solid volume $12$ square inches in nominal cross-sectional area and $1$ foot long.

  • Calculation is based on nominal dimensions, not actual dimensions.

  • Example: An $8$-foot-long $2 \times 4$ has $\frac{2 \text{ in.} \times 4 \text{ in.} \times 8 \text{ ft}}{12} = 5.33 \text{ board feet}$.

Veneer

• Definition: Wood produced in very thin sheets, typically about $1/8$ inch ($3$ mm) or less in thickness.

• Rotary-cut (or rotary-sliced) veneer: Produced by spinning a log against a long knife edge, peeling off a continuous sheet. Commonly used for structural plywood panels.

• Hardwood veneers for fine architectural woodwork: Often produced by slicing, which can yield various aesthetic grain patterns.

Engineered Wood Products

These products are designed to maximize efficiency, strength, and utility of wood resources.

Glue-Laminated Wood (Glulam)

• Description: Beams made by joining many smaller strips of solid lumber together with glue.

• Advantages:

  • Available in larger sizes than solid wood, making longer spans possible.

  • Stronger and stiffer than equivalent solid wood members.

  • Can be manufactured in curved shapes.

  • Efficiently uses high-grade pieces in high-stress areas and lower-grade materials in less critical sections.

• Reasons to laminate: Size, specific shapes, and improved quality/performance.

Cross-Laminated Timber (CLT)

• Description: Structural panels formed by laminating solid lumber layers, with the orientation of members in each layer alternating ($90$ degrees) from those above and below.

• Applications: Used for walls, floors, and roofs.

• Performance: Structurally competitive with concrete structures, offering good strength and stiffness in two principal directions.

Structural Composite Lumber (SCL)

• General Description: Made from wood veneers or fiber strands and glue.

• Applications: Used as beams, headers (short beams over openings), studs, and non-structural rim boards.

• Types of SCL:

  • Laminated Strand Lumber (LSL): Made from shredded wood strips (about $12$ inches or $305$ mm long) pressed and glued. Similar process to PSL but with longer strands than OSL.

  • Oriented Strand Lumber (OSL): Similar to LSL but uses shorter wood strands. Potential for off-gassing of volatile organic compounds.

  • Parallel Strand Lumber (PSL): Produced by cutting logs into long, thin strips of veneer, drying them, treating with resin adhesive, aligning them parallel, and then microwave heating and pressing into solid billets.

    • Minimizes wood waste.

  • Laminated Veneer Lumber (LVL): Made from full-depth wood veneers (typically $1/10$ to $1/4$ inch thick) glued together with their grains running in parallel directions.

    • Superior strength and consistency compared to solid lumber.

Wood I-Joists

• Description: Engineered joists resembling an "I" cross-section.

• Components:

  • Top and bottom flanges: Made from solid lumber or composite material (often LVL).

  • Webs: Made from OSB (Oriented Strand Board) or plywood.

• Advantages: Can span further than solid lumber while using less raw material.

Wood-Plastic Composites (WPCs)

• Description: Mixtures of wood fibers and plastics (e.g., recycled polyethylene, polypropylene, or PVC).

• Characteristics:

  • May be prefinished.

  • Less stiff than solid lumber.

  • Greater thermal expansion and contraction compared to solid lumber.

• Applications: Often used for exterior decking, exterior railing systems, and interior/exterior finish trim.

Plastic Lumber

• Description: Composed of greater than $50\%$ plastic content.

• Applications: Both finish and structural applications.

• Advantages: Durable, maintenance-free, and decay-resistant.

• Disadvantages (similar to WPCs):

  • More flexible than solid wood, requiring support at more closely spaced intervals.

  • Expands and contracts more significantly with temperature changes, necessitating greater allowances for thermal movement during installation.

Wood Panel Products

• General Characteristics:

  • Standard panel dimensions are typically $4$ by $8$ feet ($1220$ by $2440$ mm).

  • Panels exhibit more nearly equal strength in their two principal directions compared to solid wood.

  • Shrinking, swelling, checking, and splitting are significantly reduced.

  • Make more efficient use of forest resources than solid wood products.

• Primary Uses: Sheathing on framed walls and roofs, and subflooring over floor framing.

Types of Structural Wood Panels

• Plywood:

  • Constructed from laminated softwood veneers.

  • Layers are typically arranged with alternating grain directions to improve stability and strength.

  • Structure: Face (outermost veneer), Crossbands (veneers with grain perpendicular to face), Core (central veneer), Back.

• Oriented Strand Board (OSB):

  • Made of long shreds (strands) of wood compressed and glued into three to five layers, similar to LSL strands.

  • Due to the length and controlled orientation of the strands, OSB is the strongest and stiffest type of non-veneered panel.

• Fiberboard:

  • Made from very fine-grained wood particles.

  • Generally limited to interior uses due to lower density and strength compared to other panels.

  • Manufacturing is more intensive than particleboard.

• Composite Panels: Feature face veneers bonded to a solid core.

• Particleboard: Made from glued wood particles, which are smaller than OSB strands but larger than fiberboard particles.

Specifying Structural Panels

• Span Rating: A numerical designation indicating suitable structural applications.

  • Example: A panel with a span rating of $32/16$ means it can be used as roof sheathing over rafters spaced $32$ inches ($813$ mm) apart or as subflooring over joists spaced $16$ inches ($406$ mm) apart.

• Bond Classifications (Exposure Durability): Describe the panel's ability to resist moisture exposure.

  • Exterior: For permanent exterior use, requiring full exterior glue.

  • Exposure 1: The most common classification.

    • Features exterior glue and medium quality veneers.

    • Suitable for interior use, but permits prolonged (temporary) exposure to moisture during construction.

  • Interior: For interior use only, with less durable glue.

• Plywood Veneer Grades: Describe the quality of the individual veneers used in plywood construction.

  • Grades range typically from A (highest) to D (lowest).

  • C-Grade Veneer: Allows tight knots up to $1\frac{1}{2}$ inch, knotholes up to $1$ inch (and some up to $1\frac{1}{2}$ inch total width), synthetic or wood repairs, discoloration, sanding defects (not impairing strength), and limited splits/stitching.

  • D-Grade Veneer: Allows larger knots and knotholes up to $2\frac{1}{2}$ inches width across grain (or $1/2$ inch larger within limits), limited splits, and stitching. Limited exclusively to Exposure 1 or Interior panels due to lower quality.