Wood as a Building Material

Introduction to Wood

Wood is the first of four basic building materials discussed, along with steel, concrete, and masonry.
Wood is unique due to its quirks like warping, splitting, knotting, burning, and changes in shape/size over time.
Wood is susceptible to rot, insect attacks (termites), and fungus/mold.
No two pieces of wood are identical.
Wood has an orientation due to the vertical growth of trees, leading to different shrinkage rates in different dimensions.
It is the least dense structural building material.

Understanding Wood Structure

Wood can be visualized as a bundle of straws (vertical cells).
It's stronger vertically than in other dimensions due to the orientation of cells.
Freshly cut wood shows ends of vertical cells; fallen wood reveals the linear, stringy nature of timber.
Microscopic view reveals parallel nature of cells in wood like red oak and black walnut.

Living vs. Dead Cells in Wood

Living cells are found in the cambium and sapwood.
Bark is the outermost, dead layer.
Cambium is living, producing new bark and cells. Cutting around the cambium can kill a tree by preventing the transport of sugars and water.
Sapwood carries sugars and water up to branches/leaves.
Heartwood is dead but provides structural support; generally darker than sapwood. In rot-resistant woods (redwood, chestnut, cypress, cedar), the heartwood is rot-resistant.
Pith is the center of the tree.

Springwood vs. Summerwood

Trees alternate producing springwood (in spring) and summerwood (in summer), creating rings.
Springwood grows faster but is weaker; summerwood grows slower but is stronger.
The proportion of springwood to summerwood depends on wood species and climate.
Hotter, drier climates lead to slower-growing, stronger summerwood. Wetter, cooler climates result in faster-growing, weaker trees.
The pith is weaker due to more springwood, formed during the tree's initial establishment phase.

Hardwood vs. Softwood

Hardwood comes from broadleaf trees (deciduous trees like oaks, maples) with encapsulated seeds.
Softwood comes from conifer trees (evergreens) with non-encapsulated seeds (pine cones).
Softwood is more common in structural lumber; less expensive, coarser, with a less appealing grain.
Hardwood is used for flooring, trim, cabinets, furniture; more intricate grain patterns, finer woodworking, and wear resistance.

Examples of Hardwood and Softwood

Hardwood: Hickory, hard maple, birch, beech, oak, ash, black cherry, black walnut, butternut, aspen, basswood, balsa.
Softwood: Spruce, eastern white pine, southern yellow pine, larch, eastern red cedar, hemlock, redwood, Douglas fir.
Generally, hardwoods are harder than softwoods, but exceptions exist (e.g., balsa is a hardwood but not very hard).

Modulus of Elasticity

Modulus of elasticity measures a member's stiffness and overall strength of a species.
A higher number indicates stronger wood.
Example: Hickory has a higher modulus of elasticity than western red cedar, making it stronger.

Plain Sawn vs. Quarter Sawn Lumber

Cutting method affects wood properties due to its cellular geometry.
Quarter sawn lumber is more expensive. Plain Sawn lumber is the alternative.

Plain Sawn Lumber

Cuts are parallel to the grain, potentially exposing weaker springwood on the surface.
More likely to warp and less expensive due to minimal log repositioning during sawing.
Most of the wood is used, minimizing waste.

Quarter Sawn Lumber

Log is broken into quarters, and cuts are made from the outside towards the center.
Finer grain, better appearance, more dimensionally stable, and less likely to warp/shrink.
Floors wear better and siding weathers better when quarter sawn.
The process is more expensive due to increased wood waste and log rotation.

Moisture Content (MC) in Lumber

Dry wood is stronger and less prone to shrinking, warping, or twisting.
Freshly cut wood is green and contains a lot of water.
Moisture content is a percentage indicating water left in the wood.
MC 15 means 15% moisture; MC 19 means 19% moisture. Lower MC indicates higher structural quality.
Formula: MC = ((WetWeight - DryWeight) / DryWeight) * 100%

Drying Process

Water is initially stored within cells (free water), which dries first without changing structural qualities.
Fiber Saturation Point: Approximately 30% moisture content when free water is gone.
Bound water is then gradually removed, causing wood to shrink, stiffen, and gain strength. Dry wood is preferable.

Seasoning

The drying process is called seasoning. This can be achieved naturally by leaving wood in a lumber yard for months or through kiln drying for days.

Lumber Stamps

Lumber is stamped to indicate quality.
Example: Douglas fir, kiln-dried to 19% or less (MC 19 or S-Dry). Machine tested with a bending capacity of 2,400 psi, certified by Western Wood Products grading rules.

Acclimatization

Lumber acclimatizes at the job site to reach an equilibrium moisture content matching the environment.
Outdoor wood: Approximately 12% moisture.
Interior wood: Approximately 8% moisture (may vary by region: higher in the Gulf Coast, lower in the Intermountain West).
Wood flooring is left inside for 48 hours before installation to acclimatize.
Green lumber is not dried and requires caution due to significant changes after fastening.

Wood Shrinkage

Wood shrinks most tangentially due to cellular geometry.
Longitudinal shrinkage is minimal.
Radial shrinkage is significant as cells shrink in diameter.
Tangential shrinkage is even more pronounced due to a larger number of cells on the perimeter.
Checks: Cracks that occur on the perimeter of a log due to differential shrinkage.

Distortion in Lumber

Boards cut parallel to the grain are more prone to warping.
Quarter sawn wood warps less.
Wood from the circumference distorts more; wood cut perpendicular to the center is preferred.
The pith (center, containing springwood) is the least strong and dimensionally stable, and prone to checking.

Defects in Lumber

Bow: One type of lumber defect.
Crooking, bowing, twisting, and cupping result from differential shrinkage.
Crooks can be used advantageously as joists or rafters with the crown facing up for increased strength under load.
Decay/insect damage, wane (irregular edges from sawing too close to the perimeter), knots (where branches connected), and checks are all types of defects.

End Grain Exposure

End grain should not be exposed outdoors to prevent moisture absorption, splitting, rotting, and warping.
This applies to both dimension lumber and plywood; cover with metal end caps or other wood pieces.

Strength of Wood

Wood has usable tensile strength (less than steel but unlike concrete or masonry).
Per unit weight, defect-free wood can be as strong as or stronger than steel, but wood always has defects.
Wood is much stronger in compression parallel to the grain than across it (three or four times as strong).
Wood is weakest in shear parallel to the grain.

Wood Grading

Wood is graded for structural quality and appearance by humans or machines.
Number one structural framing is used for headers or long spans requiring high strength.
Utility light framing is for blocking where high strength is not needed.
Stud grade is for studs, including load-bearing walls.

Wood Veneers

Rotary sliced veneer is the least expensive.

Slicing Methods

Rotary slicing: Log is sliced thin by unraveling it like a toilet paper roll. Used for plywood.
Plain slicing: Better quality than rotary slicing.
Quarter slicing: Creates parallel grain on the veneer; used for fine woodworking. Most expensive and best-looking.

Sequencing Veneer

Veneer from the same log (flitch) is specified for uniform appearance.

Veneer Matching

Book match: Alternates pieces, flipping every other face for a symmetrical pattern.
Slip match: Joins slices in sequence without flipping.
Random match: Uses varying widths, colors, or grains.
Pleasing match: Matches by color.
End matching: Joins leaves to create longer panels.
Balance matching/Center matching: Symmetrical appearance from uniform pieces.
Mismatching: Non-symmetrical; uses pieces of unequal width.

Manufactured Wood Products

Glue laminated wood (Glulam)
Cross laminated timbers (CLTs)
Laminated strand lumber (LSL)
Oriented strand lumber (OSL)
Parallel strand lumber (PSL)
Wood I joists
Laminated veneer lumber (LVL)
Wood plastic composites (WPC)

Applications of Specific Wood Products

Wood decking and outdoor railings: Wood plastic composite.
Overlapping dimension lumber pieces adhered together: Glulam.
Large panels for floors, roofs, and walls: Cross laminated timbers (CLTs).
Web typically made from OSB: Wood I joists.
Strongest: Parallel strand lumber (PSL).
Least strong: Laminated strand lumber (LSL) and oriented strand lumber (OSL).
Least expensive: Laminated strand lumber (LSL) and oriented strand lumber (OSL).
More flexible, requires closer spaced supports: Wood plastic composite.

Glue Laminated Wood (Glulam)

Smaller wood strips joined with scarf or finger joints.
More expensive due to high strength per board foot.
Length limited by transportation concerns.
Treated versions for outdoor applications.
Can be reinforced with high tensile strength laminate strips for increased strength.

Cross Laminated Timber (CLT)

Alternating layers of solid timber at 90-degree angles.
Creates large, strong panels (up to 15 ft wide, 60 ft long, 16 inches thick).
Two-way structural action; can create walls, floors, or roofs.

Laminated Strand Lumber (LSL) & Oriented Strand Lumber (OSL)

Shredded wood strands glued and compressed into rectangular shapes.
Not super strong or expensive.
Used as rim boards or short span headers.
LSL has longer strands than OSL.

Parallel Strand Lumber (PSL)

Strips oriented in the same direction make a stronger product.
Heavy and expensive, but very strong.

Wood I Joists

Utilize the geometry of an I-beam for longer horizontal spans.
Flanges are made from dimension lumber/manufactured wood; webs often from OSB.
Used in roofs, floors, and joists.

Laminated Veneer Lumber (LVL)

Thick plywood made by laminating veneers.

Wood Plastic Composite (WPC)

Plastic, wood, and chemicals injection-molded.
Used for exterior decking/railings due to weather resistance.
More flexible and requires more support. Prone to staining.

Board Feet Calculation

Formula: (Width
omal * Length{nominal} * Length{feet}) / 12
Nominal dimensions are used for width and length, with the length calculation in feet.

Example: A 1x10x8 piece of lumber

(1 * 10 * 8) / 12 = 6.667
ewline board feet (6 and 2/3 board feet).

Example: A 2x6x12 piece of lumber

(2 * 6 * 12) / 12 = 12
ewline board feet (12 board feet).

Nominal vs. Actual Dimensions for Lumber

Nominal dimensions are different from actual dimensions.
A 2x4 is actually closer to 1.5 x 3.5 inches.

Common Dimension Conversions

Nominal 1x = 0.75 inches actual.
Nominal Five Quarter = 1 inches actual.
Nominal 2x = 1.5 inches actual.
Nominal 3x = 2.5 inches actual.
Nominal 4x = 3.5 inches actual.
Nominal 5x = 4.5 inches actual.
Nominal 6x = 5.5 inches actual.
Nominal 8x = 7.25 inches actual.
Nominal 10x = 9.25 inches actual.
Nominal 12x = 11.25 inches actual.
For dimensions over 12, subtract 0.75 inches from the nominal dimension to get the actual dimension.
Use inch marks for actual dimensions, not nominal dimensions (e.g., use "2 x 4" not "2 inches x 4 inches" for nominal dimensions).

Plywood

Typically uses an odd number of layers.
Outer faces have the same grain dimension to prevent cupping or warping.
Grain alternates directions to balance stress.
Intermediate layers alternate between horizontal and vertical grain.
Three, five, seven, or nine layers are typical.
Sheets are commonly 4x8.
Less shrinking relative to dimensional lumber.
Rotary sliced, glued together.
Composite panels have a veneer face.
Medium or high-density overlay resists weather; used for concrete formwork, cabinets, exterior siding, signs, and furniture.

Non-Veneer Wood Panels

Oriented Strand Board (OSB), particleboard, and fiberboard types exist.
The scarcity of high-quality logs for peeling has led to using plantation trees with less quality wood.
Small chips from this wood are used with glues/resins to form the products.

Oriented Strand Board (OSB)

Long strands of wood compressed and glued.
Generally strong but brittle, consisting of three or five layers.
Strands alternate directions in each layer.
Used for exterior sheathing.
Resin coatings available for wet conditions.

Particleboard

Smaller fibers than OSB.
Used under wood veneers or plastic laminates, commonly in furniture.
Not strong or moisture-resistant.

Fiberboard

Even smaller wood fibers combined with resin.
Typically used for interior applications.
More stable and can handle fasteners better than particleboard.
Medium-density fiberboard (MDF) and hardboard (Masonite) are types of fiberboard.
Used for furniture, molding, paneling, and clipboards.

Structural Wood Panel Ratings

Span Rating for Sub-Flooring

A panel rated as "32/16" used for subflooring has a maximum joist spacing of 16 inches.
The rating indicates the maximum rafter spacing for roofs (32 inches) and maximum joist spacing for floors (16 inches).
Ratings depend on thickness, glue strength, wood fiber strength, and fiber length.

Weather Exposure Ratings

Structural wood panels, such as plywood and OSB, rated as "Exterior" perform better when exposed to weather for long periods.
Exterior plywood uses veneers that better handle weather.
Exposure 1 can be exposed to some weather during construction but is not for long-term exposure.
Exposure 1 is fine for short exposures and interior applications.
Exposure 2 has been phased out.
Bond classifications include Exterior, Exposure 1, and Exposure 2.

Specific Wood Panel Types

Hardboard is a type of fiberboard (high-density fiberboard) used for exterior siding/paneling and furniture.
Insulating fiberboard sheathing is a low-density fiberboard coated with asphalt for water resistance used on building exteriors for thermal insulation.
Agri-fiber panels (wheatboard, riceboard, strawboard) are agricultural waste products glued together and can be structural and as thick as 8 inches.

Environmental Considerations

Manufactured wood products have higher embodied energy than solid lumber due to chemicals and petrochemicals in binders (resins and plastics).
The Forest Stewardship Council (FSC) certifies wood grown with sustainable practices.
Wood is the only renewable structural material.

Off-Gassing and Air Quality

Dimension lumber has minimal off-gassing, but manufactured lumber products (OSB, MDF, LVLs) off-gas due to binders (especially formaldehyde).
Low VOC (volatile organic compound) finishes improve air quality.
Off-gassing is most prevalent in carpets, paints, and binders in manufactured wood products.

Treatment for Rot and Insects

Types of Treatments

Creosote: Oil-based, can't be painted, and toxic. Mostly phased out.
Pentachlorophenol: Oily, can't be painted, but widely used on telephone/power poles.
Chromated Copper Arsenate (CCA): Very common, phased out in 2004 due to toxicity (arsenate is arsenic). Requires safety precautions during removal.
Alkaline Copper Quat (ACQ) and Copper Boron Azole (CBA): Less toxic alternatives to CCA used above ground and for ground contact approved for freshwater immersion. Cannot be used with steel or aluminum fasteners due to galvanic action; use stainless steel, heavily galvanized zinc-coated steel, or copper-alloy nails.
Micronized Copper: Copper applied as a solid powder, less corrosive to metals.
Carbon-Based PTI: New and less corrosive.
Borate Compounds (Sodium Borate (SBX) and Disodium Octoborate Tetrahydrate (DOT)): thought to be nontoxic and used above ground to treat termites.

Metal Compatibility

*Use Stainless steel with copper based ACQ and CBA treated wood for ground contact
*Use Hot dip galvanized ASTM with ACQ and CBA treated wood not in ground contact

Toxicity

Creosote and CCA are most toxic.

Paintability

Creosote and Pentachlorophenol cannot be painted.

Required Treated Wood

Wood joists are treated if less than 18 in above grade.
Wood beams/girders are treated if less than 12 in above grade.
Plates/sills/sleepers in direct contact with masonry/concrete are treated.
Wood framing less than 8 in from the soil must be treated.

Natural Decay and Termite Resistance

Black locust is naturally decay-resistant.
Other decay-resistant species: Red mulberry, Osage orange, Pacific yew, cypress, catalpa, cedar, chestnut, white oak, redwood, and black walnut.
Eastern red cedar is naturally termite-resistant.

Design Considerations

Moisture control: Overhangs, drip edges, proper detailing, ventilation, waterproofing, and maintenance prevent decay. When wood is fully submerged in water there is no longer a requirement for decay resistance. The submerged consideration is only in freshwater, saltwater is still harmful. Overhangs, drip edges, proper detailing, ventilation, waterproofing, and maintenance prevent decay.