Wood as a Building Material

Wood as a Building Material

A

Sawn Lumber Characteristics

• Common Use: Sawn lumber is predominantly utilized for residential light construction.

• Lightness and Strength: Lumber is renowned for its relatively light weight compared to its strength.

  • Structural Composition: Analogous to a bundle of straws composed of tracheid cells fused by lignin (the glue).

• Production Location: Wood cells are generated in the roots, branches, and under the bark layer (cambium).

Forest Dynamics (1920 – 2011)

• Growth and Removals: Annual removals measured in billions of cubic feet.

  • Source: USDA Forest Service, 2013.

Sustainable Practices in Lumber Production

• Replanting Assurance: Ensured through certified lumber.

• Guidelines: Sustainable practices, like LEED (Leadership in Energy and Environmental Design) for homes, endorse lumber from certified or reclaimed forests.

• Certification: Forestry Stewardship Council maintains a chain of custody for certified lumber.

Tree Anatomy and Wood Growth

• Cambium Layer: Wood growth varies seasonally in the cambium layer.

  • Springwood (Earlywood): Fast growth, lower density, larger cells.

  • Summerwood (Latewood): Slow growth, higher density, smaller cells.

  • Heartwood vs. Sapwood:

    • Sapwood: Outer, lighter-colored wood that transports water and nutrients; contains living cells; less decay resistant.

    • Heartwood: Inner, darker-colored wood that is no longer metabolically active; provides structural support and decay resistance due to extractives.

B

Log Cutting Techniques

• Tree Rings Formation: Alternating springwood and summerwood creates tree rings.

• Ring Orientation: Orientation of these rings impacts wood behavior.

• Two classifications:

  • Plain-sawn (flat-grained):

    • Rings are roughly parallel to the wider face of the board.

    • Produces a distinctive cathedral grain pattern.

    • More prone to cupping and warping due to tangential shrinkage.

    • Most economical to produce, maximizing yield from a log.

  • Quarter-sawn (edge or vertical grained):

    • Rings are largely perpendicular ($45^\circ$ to $90^\circ$) to the wider face of the board.

    • Exhibits a straighter, more uniform grain pattern.

    • More dimensionally stable, resisting cupping and warping better.

    • More expensive to produce due to lower yield from a log.

C

Types of Wood: Softwoods vs. Hardwoods

• Type Classification:

  • Softwoods: Evergreens, Conifers, or Gymnosperms with exposed seeds. Typically characterized by needles and cones (e.g., pine, spruce, fir).

  • Hardwoods: Broadleaves, Deciduous, or Angiosperms with enclosed seeds. Typically characterized by broad leaves that commonly shed seasonally (e.g., oak, maple, cherry).

• Hardness Misconception: Hardwoods aren't necessarily harder than softwoods (example: Balsa is a hardwood). However, softwoods are extensively planted and more economical for construction due to faster growth and ease of processing.

• Lumber Sourcing: Framing lumber in the Northeast is primarily sourced from softwoods like spruce, pine, and fir (SPF).

D

Lumber Classification: Boards vs. Dimensional Lumber

• Classification and Grading:

  • Boards: Up to 1 inch thick for aesthetic purposes like window trim, flooring, siding. Graded based on size, knot type, wane, and blue stain presence.

  • Dimensional Lumber: 2 to 4 inches thick primarily for structural purposes like studs, joists, and rafters. Graded based on defects like splits, wane, shake, and knot holes.

  • Timbers: 5 inches or thicker in their smallest dimension, used for heavy construction.

E

Lumber Grading

• Grade Types: Divided into boards, dimension lumber, and timbers according to usage. Lower grades allow for more defects.

• Visual or Mechanical Grading: Done primarily visually for boards and dimension lumber for strength. Mechanical stress grading also used for structural lumber.

• Types of Knots (examples of defects impacting grade):

  • Tight Knots: Firmly embedded in the wood, often indicating a healthy branch during growth. Generally less detrimental to strength.

  • Loose Knots (or Encased Knots): Not firmly grown into the surrounding wood and may fall out, leaving a hole. Significant strength reducer.

  • Sound Knots: Free from decay and appear solid across their face.

  • Unsound Knots: Contain decay.

  • Spike Knots: Appear on the face of a board when a knot is cut lengthwise, diminishing structural integrity.

F

Size Definitions: Nominal vs. Actual

• Sawn Dimensions: Lumber is initially cut to nominal sizes and later dried and planed to actual (dressed) sizes. The drying and planing process reduces the initial size.

  • Example: A nominally sized $2 \times 4$ board typically measures $1.5 \text{ in} \times 3.5 \text{ in}$ after drying and planing.

  • Example: A nominally sized $2 \times 6$ board typically measures $1.5 \text{ in} \times 5.5 \text{ in}$.

  • Example: A nominally sized $1 \times 4$ board typically measures $0.75 \text{ in} \times 3.5 \text{ in}$.

G

Drying and Defects in Lumber

• Cup Defect: Rings while drying tend to “straighten”, leading to cupping.

• Cost Consideration: Vertical grain lumber, while more expensive, resists cupping better than flat grain lumber.

• Installation Dilemma: Deckboards can be installed either “smiling” (with the curvature facing up) or “frowning” (with the curvature facing down).

• Shrinkage Characteristics:

  • Wood is an anisotropic material; it shrinks and swells differently in different directions (tangential, radial, longitudinal).

  • Tangential shrinkage (around the growth rings) is typically the greatest (approx. $6-10 \%$).

  • Radial shrinkage (across the growth rings) is about half of tangential shrinkage (approx. $3-6 \%$).

  • Longitudinal shrinkage (along the grain) is negligible (approx. $0.1-0.2 \%$).

• Critical Moisture Contents:

  • Fiber Saturation Point (FSP): The moisture content at which free water has left the cell cavities, but the cell walls are still saturated with bound water (typically $25-30 \%$ MC). Shrinkage begins as wood dries below the FSP.

  • Equilibrium Moisture Content (EMC): The moisture content wood reaches when it's in equilibrium with the relative humidity and temperature of its surrounding environment.

H

Board Foot Calculations

• A board foot is a unit of measurement for the volume of lumber. It represents a piece of wood that is 1 inch thick, 12 inches wide, and 12 inches long.

• Formula: $\text{Board Feet} = \frac{\text{Nominal Thickness (in)} \times \text{Nominal Width (in)} \times \text{Length (ft)}}{12}$

• Example: A $2 \times 4$ nominal board that is 8 feet long contains: $\frac{2 \times 4 \times 8}{12} = \frac{64}{12} \approx 5.33 \text{ board feet}$.

I

Pressure Treated Lumber

• Purpose: Lumber is pressure-treated with chemical preservatives to enhance its resistance to rot, decay, and insect infestation, especially when exposed to moisture or ground contact.

• Health Concerns: Early pressure-treating chemicals contained Chromated Copper Arsenate (CCA). Due to arsenic content, CCA-treated wood was phased out for residential use in 2003 because of potential health risks from leaching into soil/water and contact.

  • Modern treatments typically use Alkaline Copper Quaternary (ACQ) or Copper Azole (CA), which are considered safer but still require careful handling (e.g., wearing gloves, avoiding inhalation of sawdust, proper disposal).

J

Plywood and OSB

• Plywood: An engineered wood product made from multiple thin layers (veneers) of wood glued together, with adjacent layers having their wood grain rotated up to $90 \text{ degrees}$ to each other. This cross-graining enhances strength and dimensional stability.

  • Veneer Grades: Graded with letters (A, B, C, D) based on quality, from best (A) to worst (D) for appearance on face and back veneers. For example, 'AC' plywood has an 'A' face and a 'C' back.

• Oriented Strand Board (OSB): An engineered wood product made from compressed layers of wood strands (flakes) arranged in specific orientations, bonded with adhesive resins.

• Span Ratings: Both plywood and OSB sheets carry a span rating (e.g., $32/16$ \text{ or }$48/24$ ), indicating the maximum recommended spacing in inches for roof rafters (first number) and floor joists (second number) when the panel is installed with its long dimension perpendicular to the supports.

K

Engineered Wood Products (EWPs)

• Definition: Products created by binding together wood strands, particles, fibers, or veneers with adhesives to form a composite material.

• Common Types: Glued Laminated Timber (Glulam), Laminated Veneer Lumber (LVL), Parallel Strand Lumber (PSL), I-Joists, Plywood, OSB.

• Advantages:

  • Improved Strength and Stability: Often stronger and more dimensionally stable than solid lumber, resisting warping, twisting, and shrinking.

  • Efficient Resource Use: Made from smaller, faster-growing trees and wood waste, maximizing forest yield.

  • Larger Sizes Available: Can be manufactured in custom or longer lengths and wider widths than are typically available from solid sawn lumber.

  • Consistency: More uniform in strength and quality, with fewer defects than solid wood.

• Disadvantages:

  • Cost: Can be more expensive than conventional lumber for some applications.

  • Moisture Susceptibility: Adhesives can be sensitive to moisture, potentially leading to delamination or reduced strength if exposed to severe or prolonged wet conditions.

  • Fire Performance: Some EWPs may behave differently in fires compared to solid wood, and charring rates can vary.

  • Aesthetic: May not have the natural aesthetic appeal of solid wood for visible applications.