Wood in Architecture: Properties and Classification

Wood in Architecture: Introduction

• Natural wood is an attractive building material, owing to its properties.

Advantages of Using Wood

• Natural warmth is achieved with wood, which insulates.
• Wood is workable and shaped easily with simple tools.
• Infinite variety exists in wood; no two trees or pieces are alike.
• A high strength-to-weight ratio makes wood suitable for bridges, boats, homes, and furniture.
• Flexibility allows huge trees bend even in heavy winds.
• Wood chars instead of melting or crumbling, providing fire protection.
• Natural color is exhibited, and wood receives many types of stains and finishes.

Wood Classification

• Softwoods come from conifers/evergreens with needles, e.g., pine.
• Hardwoods come from broad-leaved/deciduous trees; most Philippine timber is of this sort.
• Hardwood and softwood terms are misleading, as actual hardness varies.

Structure of Wood

• Softwoods vs Hardwoods is not an indication of physical hardness

Sapwood

• Sapwood is the softer, younger outer portion between the cambium layer and the heartwood of a tree.
• Sapwood is more permeable, less durable, and often lighter than heartwood.
• Sapwood is the log’s periphery, generally lighter in color than the center.
• Living cells in sapwood actively participate in the tree's life cycle.
• Sapwood is more susceptible to blueing fungi and insects due to higher organic matter content.

Heartwood

• Heartwood is the older, harder central portion of a tree.
• Various materials deposited in heartwood give it a darker color than sapwood.
• Heartwood is denser, less permeable, and more durable than sapwood.
• The central core, composed of inactive cells, acts as a mechanical support system.

Properties of Wood

• Hardness indicates the material’s level of compression.
• Flexibility refers to how much it will bend prior to breaking.
• Softwoods tend to be brittle
• Hardwoods tend to be flexible
• Strength
• Durability

Defects of Wood

• Decay is caused by fungi.
• Checks are lengthwise separations across annual rings due to irregular shrinkage during drying.
• Shakes involve cracks between and parallel to the annual rings.
• Knots are irregular growths interrupting the smooth grain curve, with fibers turning from their normal course.
• Pitch pockets feature well-defined openings between annual rings containing solid or liquid pitch.
• Wane refers to the lack of wood on the edge or corner of a piece
• Warping causes variations from the plane surface because of unequal shrinkage.

Forms of Warping

• Crook refers to an edge distortion, either convex or concave.
• Bow refers to a face distortion that can be convex or concave longitudinally.
• Cup refers to a face distortion that is either convex or concave across the board.
• Twist refers to a distortion where one corner is raised

Sawing Methods

• Plainsawing involves cutting lumber tangent to the annual rings or growth
• Annual rings are cut with 0°–45° angle in commercial practice
• Plainsawn Lumber results in a pleasing pattern for wall paneling.
• Quartersawing cuts wood radially to annual growth rings parallel to rays
• Annual rings will be 45°-90° angle in commercial practice.
• Quartersawn Lumber is desirable because it has less shrinkage.

Lumber Classification

• Lumber is construction wood classified by size in inches, e.g., a 2x4.
• Widths are usually available in even-numbered increments: 4, 6, 8, 10, and 12 inches.
• Lumber classifications: strips, board lumber, dimension lumber, timbers.
• Strips: Less than 2" thick, less than 8" wide.
• Board Lumber: Less than 2" thick, at least 8" wide.
• Dimension Lumber: More than 2", less than 5" in any dimension.
• Pieces: 5" or more on the smallest dimension

Lumber Information

• Dimension lumber and board lumber are common types of Lumber.
• The stated lumber size is the nominal size instead of the Lumber's actual finished size.
• Nominal size refers to dimension when initially cut.
• Lumber, once cut, dries and is planed on the sides resulting in a smaller real dimension.

Lumber Measurement

• Lumber sold from 6' - 20' in 2' increments, with special lengths over 20' costing more per board foot.
• Lumber is measured in board feet (1" thick, 12" wide/long).
• Equation to find the total board feet: Board Feet = (Thickness x Width x Length)/12 (inches/feet).

Seasoning Lumber

• Moisture influences wood behavior; after cutting, water is detrimental, attracting insects/fungi.
• Water in wood is either free inside cell cavities/intercellular spaces.
• Absorbed/hygroscopic refers to water held in the wood’s cell walls
• Surface water evaporates upon air exposure and is offset by moisture traveling from the interior.
• Free water passes before the absorbed water in the cell wall vaporizes.
• Seasoning refers to removing moisture from green wood.
• After all free water depletion, the "fiber-saturation point" is attained (30% moisture).
• Wood shrinkage at percentages lower than the fiber saturation point.
• Drying continues until the air's vapor pressure equals the wood surface vapor pressure
• Equilibrium moisture content (E.M.C.) in the Philippines: 12 to 16 percent moisture.

Seasoning Methods

• Air-drying involves exposing lumber to air.
• Kiln-drying uses warm, moist air/superheated steam.
• Ideal seasoning has interior wood moisture replace vaporized surface moisture.
• Unequal drying leads to shrinkage/stress, causing seasoning checks.

Seasoned Lumber Advantages

• Reduction in the wood’s liability to be attacked by the fungi as well as wood-boring insects.
• Reduction of weight
• Increased strength.
• Minimum shrinkage avoids opening of joints
• Reduced checking and warping
• Improvement of application for paint and preservatives protection.

Kiln-Drying Advantages Over Air-Drying

• Greater weight reduction
• Ability to control desired target moisture level.
• Reduced time necessary for drying.
• Killing insects or any present fungi.
• Setting resins present in resinous wood.
• Less degrade (quality loss from unequal shrinkage causing checks/loose knots/warping).

Deterioration of Lumber

• Molds, stains, and decay are caused by fungi whose growth depends on mild temperatures and dampness.
• Wood at 20% M.C. or less is safe; however, constantly wet or dry wood won't decay if moisture is absent.
• Use care where there is moisture collection

Insects

• Subterranean termites live in the ground building earthen tubes, consuming wood and/or cellulose
• Barriers against termites can involve using metal shields or termite proof constructing materials when building close to the soil

Termite Prevention and Control

• Soil can also be poisoned adjacent to the building.
• Consistent checks for tubes/signs of termites should be scheduled regularly.
• Signs of termites require destruction of tubes in addition to poisoning the surrounding soil

Non-Subterranean Termite Arrestment

• Arresting is best achieved by lumber with full-length termite proofing + attention to structural features/sanitation.
• Carpenter Ants and Power-Pest Beetles use wood for protection; if the wood is left undisturbed, significant damage still occurs.
• Lumber attacked by insects has channels cut across the grain converting it to powder
• Marine organisms, crustaceans and mollusks can be suppressed through heavy applications of creosote tar solutions

Lumber Preservation

• Wood treatment prevents/delays destruction from organisms and fire.
• Wood preservatives prevent decay/fungi/harmful insects/marine borers.

Oil-Type Wood Preservatives

• Coal-tar creosotes are black/brownish oil from distilling coal tar.
• Coal-tar creosotes provide significant toxicity but are also insoluble in water
• Coal-tar creosotes share the disadvantages of being highly odorous, unpaintable and easily ignitable
• Creosotes derived from wood, oil, and water gas have the benefits of creosote by being less effective
• Creosote solutions combine coal-tar/petroleum oils and 50-80% coal-tar and are also less effective.
• Pentachlorophenol consists of petroleum oils and 5% pentachlorophenol.
• Pentachlorophenol can be harmful but paintable and has no odor with less marine protection.
• Pentachlorophenol once popular is now exclusive to treatment for for utility poles and cross arms.

Water-Borne Wood Preservatives

• Chromated Zinc Chloride (CZC) provides protection without odor.
• Chromated Zinc Chloride’s disadvantage is not being able to be in the ground/water.
• Chromated Copper Arsenate (CCA), and Ammoniacal Copper Arsenate (ACA) are dissolved in the water for pressure treatment
• CCA/ACA are odorless products.
• Both bond in a fixation process for resistance and regularity with human surface-level exposure
• They offer good defense against decay and can make wood suitable for the ground

Wood Preservative Applications

• Creosote for railroad ties, utility poles, and marine pilings.
• Pentachlorophenol for arms, utility poles, and bridge timbers.
• ACA for pilings, utility poles, and marine timbers.
• CCA for posts, landscape timbers and residential construction as well as boardwalks fencing, bridge timbers, utility poles and structural laminated timbers

Applying Preservatives

• Pressure treatment involves placing wood in cylinders under the pressure.
• The Hot/Cold Bath Method includes hot bath preservatives for 1+ hour, then a cold bath.
• Dipping/immersing wood in hot preservative for a short time
• Brushing to apply
• Surface coverings involve materials that are applied to the compound and material

Two ways to increase fire resistance are:

• Cover the wood with fire protective compounds and materials
• Impregnate its composition with fire retardant chemicals
• Protective coatings are suitable for interior purposes, such as formulated ammonium phosphate, borax and sodium

Wood fire retardants

• The use of coating over the wood with a compound or material is a method used to treat lumber
• chemicals with low water solubility such as zinc borate, chlorinated paraffin and chlorinated rubber are also used for fire retardant coatings

Impregnating wood fire retardants

• Monobasic ammonium phosphate
• Dibasic ammonium phosphate
• Ammonium sulphate
• Borax
• Boric acid and zinc chloride

Commonly Pressure Treated Lumber and Plywood

• Wolmanized utilizes Wolman salts.
• Tanalized employs Tanalith G salts.
• Permanized uses B-S-25.
• Boliden Lumber and Plywood

Wood Composites

• Wood Composites are products from wood and most composites. These sheets are typically 1220mm (4') x 2440mm (8').

Plywood

• Thin plies or wood veneers glued together make plywood a common composite.
• The ply grain is at right angles to the next piece.
• Cross-layering results in a plywood that is very strong.
• Plywood coatings are good for interior use only.
• Fire retardant chemicals of the type used are ammonium phosphate, borax

Standard Plywood Sizes and Thicknesses

• Plywood sheets are usually either 3' x 6' (900mm x 1800mm) or 4' x 8' (1220mm x 2440mm)
• Standard thicknesses are 3/16" (4.5mm), 1/4" (6.0mm), 3/8" (10.0mm), 1/2" (12.0mm), 3/4" (19.0mm), 1" (25.0mm)

Plywood Advantages

• Strength is approximately equalized along its length and width.
• Greater resistance to checking and splitting
• Reduced dimensional change caused by moisture content

Types of Plywood

• Ordinary, Form, Marine, and Fancy Plywood made of Narra, Kalantas, Tanguile, Lauan, Dao, and Rosewood.

Hardboard

• Hardboard is a paneling material from reduced, refined wood chips pressed under heat in hydraulic pressure.
• The pulp is exploded to create brown fibers bonded together with glues and pressed between plates
• Heat and steam is applied leave fine, fluffy brown fibers
• Grainless glossy sheets result by pressing between steam-heated metal plates.
• Moisture is added to humidify against warping.
• Cheaper than plywood for low-strength applications.

Hardboard Characteristics

• Smooth surface on one side
• Screened on the other
• In reference to its inventor, hardboard is also known as “Masonite.”

Three Types of Hard Board

• Standard has a screening with easy flexibility.
• Standard is for interior use only, avoiding moisture.
• Panel is denser than Standard, but is not as dense as Tempered hard boards
• Tempered type impregnates the panel which improved moisture resistance - has additional water penetration making it ideal for exterior usage

Hardboard Sizes and Brand Names

• Usually 4'x8' panels (1220mm x 2440mm)
• Thicknesses of 1/8" (3mm), 3/16" (4.5mm), 1/4" (6mm) is usually obtainable plain
• The surfaces are usually textured or perforated.
• Brand names: LAWANIT, LAWANEX, and MASONITE.

Chipboard

• Chipboard combines wood particles with an adhesive under heat pressure.
• The results are a rigid board faced with veneer, or binding phenolic resin/urea formaldehyde glue. Vary amongst differing densities/grades

Chipboard Density

• Densities include high, Medium densities and Normal.
• Normal is flakey, Medium density is in between, while High Density useable for fire doors and worktops

Fiberboard

• Composite sheets made from corn/sugarcane stalks
• The strength is low with high insulation suitable for ceilings with 4'x8' paneling
• Thicknesses are 3/16", 1/4", 1/2" and ¾ " and includes Celotex, Canex, among others.

Medium Density Fiberboard

• Gluing wood fibers under heat/pressure makes up medium-density fiberboard.
• These properties make it ideal for many qualities.
• Also an ideal for wood alternative to wood and plywood/chipboard with smooth knots

Fiberboard

• Fiberboards MDF contains no internal voids, and produces fine-particled material lacking a recognizable wood” grain. MDF will produce better edges proving material is correctly machined

Gypsum boards

• Gypsum board is a smooth-paper enclosed gypsum core created for "dry-wall" construction, which usually is installed at the ceilings/or interior walls
• Commonly sold as Boral and Elephant Gypsum Boards
• Square edge is for clamp fixing and free suspension, while tapered edge is for panelling of smooth jointless ceilings

Types of Gypsum Board

• The materials are commonly seen in Backing, Core, Wall Gypsum as well as water resistant and type X boards,
• Wall Gypsum is for surface layering where its foil serves as a vapor thermal insulation.

Gypsum Board Types

• Backing Board serves as the bass later in multi-ply applications that require high strength
• Core Board Is generally Factory Laminated can save space when constructing a self supporting gypsum wall and will reduce layers needed when installed

Specialized Gypsum Board

• Type X contains glass fibres intended to be fire resistant
• Water Resistants contain a water repellants and/or water repellant paper

Other Gypsum Types

• Other types include: Sheathing for walls, Formboard for poured concrete roofs

Fibercement Board Composition

• Fibercement Boards commonly range in at 78% from Portland cement, cellulose fibers and is supplemented with Calcium carbonates.

Particle Board

• Board made using sawdust, cotton, or rice stalk as base and is pressed together.

Millwork

• Millwork involves finished lumber cut and processed at a mill.
• This includes doors, windows, mouldings, trim, and other ornamental details.

Types of Wood Mouldings

• Wood Mouldings are usually Crown, bed, cone, half/quarter, and astragal moulding. Wood Moulding shapes also include, but not limited too panels strips and or pictures