Annual ring (annual growth ring): The layer of wood growth added each growing season to the diameter of the tree. In temperate regions, with distinct growing seasons, annual rings of most species are distinct, some very much so due to difference in cells formed early and late in the growing season. Many tropical timbers have no growth rings.
Anti-stain treatment: Fungicide solution applied to timber at some sawmills, to minimise staining during transit and storage.
Bark pocket: An opening between annual growth rings that contains bark - appearing as dark streaks on quarter sawn and rounded areas on flat sawn stock.
Beam: A structural member loaded on its edge.
Bearing: The contact area over which one structural element, e.g. a joist, is supported on another, e.g. a wall plate.
Birdseye: Dimpling of the tangential surface of some hardwoods, notably sugar maple, which forms small circular features which are decorative.
Bleeding: Diffusion of resin, from such as a knot, through paint or varnish resulting in discoloration.
Blue stain: Blue-grey discoloration caused by mould-type fungi in damp timber (above 20%); also known as sapstain.
Bow: A curve along the face of a plank normally due to growth stress or poor stacking.
Box Beam: A built-up beam with solid wood flanges and wood panel product webs.
Boxed heart plank: A plank in which the pith (see pith) is enclosed; more liable to twist and fissure than other planks.
Brace, Lateral: A continuous member connected to a truss chord to maintain the vertical position of the truss and assembly of trusses.
Broadleaved trees: A grouping of trees (botanically known as angiosperms) with lanceolate or other wide leaves (e.g. oak, ash, mahogany), often deciduous, which provide hardwood timber.
Camber: An upward vertical displacement built into a truss or glued-laminated beam to offset deflection.
Cantilever: The part of a truss or structural member that extends beyond its support.
Case-hardening: Where the outer part of the wood has dried before the centre, and has become set in a stretched condition which causes stress between the outer and inner parts of the wood. The wood will likely distort if further sawn or machined.
CCA: Acronym for copper/chromium/arsenic wood preservative.
Cell: The minute structures of which wood is composed, including fibres, vessels and other elements.
Cellulose: The carbohydrate that is the principal constituent of wood. It has large, long-chain molecules which, when bonded together, provide a very strong framework to the wood cells.
Check: A separation of the fibres along the grain, forming a crack that does not extend through the timber.
Clear span: Horizontal distance between inner edges of supports.
Compression failure: Localised buckling of wood fibres, due to compression along the grain, caused by direct compression or bending; in planed timber may appear as fine wrinkles across the surface.
Compression wood: Dense, short-fibred wood occurring on leeward side of wind-stressed conifer trees; usually darker in colour; causes unequal shrinkage, distortion and reduced strength.
Concealed surface: As intended by BS 1186, a surface in joinery or trim which, after installation, will be concealed, not only by decoration.
Conifer trees: A grouping of trees (botanically known as gymnosperms) with needle or scale-like leaves (e.g. pine, spruce, cypress), most of which are evergreen, which provide softwood timber.
Cross-cut: A cut across the grain, to cut timber to length.
Cup: Curvature across the face of a plank.
Dead load: A permanent load resulting from the weight of the building materials or installed equipment.
Decay: The decomposition of wood resulting from the action of wood-rotting fungi in damp/wet conditions; results in loss of strength and weight, generally with a change in texture and colour.
Density: The mass of wood substance per unit volume; expressed as kilograms per cubic metre, at a specified moisture content, generally 12%; there is a strong positive correlation between density and strength.
Durability: The level of resistance to decay or insect attack of heartwood. The durability of timbers is given in years of life before deterioration, described as: Very durable = > 25 yrs Durable = 15 - 25 yrs Moderately durable = 10 -15 yrs Slightly durable = 5 - 10 yrs Perishable = < 5 yrs
Earlywood: Also known as springwood; the portion of the annual ring formed at the beginning of the growing season; generally of lower density and weaker than the latewood.(q.v.)
Edge: The narrow surface of a rectangular piece of timber.
Edge distance: The distance from the edge of the timber to the centre of the nearest fastening.
End distance: The distance measured at right angles from the end of the timber along its length.
Equilibrium moisture content: The moisture content (see moisture content) at which wood neither loses nor gains moisture when exposed to air at a constant relative humidity and temperature.
Face: The wide surface of a rectangular piece of timber; or any of the surfaces of a square piece of timber.
Fibre saturation point (fsp): The moisture content (m.c.) (see moisture content) of wood at which all free water is lost from cell cavities, and only water bound within the cell walls remains; generally between 25 and 30% moisture content; shrinkage occurs only as wood m.c. drops below fsp.
Figure: The pattern produced, on the surface of wood, by growth rings, rays and variations in grain structure.
Fingerjoint: An end joint made by cutting wedges or fingers into the ends of boards, meshing them together and bonding with adhesive.
Fissure: A generic term to include checks, splits and shakes.
Fire-resistance rating: The time, usually noted in minutes, that a material or structure will withstand the passage of flame when exposed to fire under specified conditions of test.
Fire retardant: A chemical preparation which reduces flammability or retards the spread of flame over a surface.
Flat-sawn timber: Timber sawn so that the growth rings are at an angle less than 45° to the face.
Glulam: Structural wood product made by bonding together laminations of dimension lumber.
Grain: Primarily, the direction of the main fibres of the wood; when qualified, may refer to their size, arrangement and/or appearance (see close, open, coarse grained).
Green timber: Freshly felled or undried timber with its moisture content above fibre saturation point (see fsp).
Hardness: The capacity of the wood to resist indentation.
Hardwood: Timber of broad-leaved trees; the term relates to the botanical grouping of the trees and not to the hardness of the wood (some hardwoods, e.g. balsa, are softer than softwoods).
Heartwood: Wood of the inner growth rings, extending from the pith to the sapwood; no longer participating in the life processes of the tree. The starches are depleted and often replaced with resins and other substances which may make the wood darker and more decay resistant.
Heel: Point on a truss where the top and bottom chords intersect.
Honeycomb: Internal splitting in a plank as a result of casehardening (q.v.).
Intumescence: The swelling of a fire-retardant coating when heated, resulting in a film providing a degree of resistance to surface spread of flame.
Joist: One of a series of parallel beams used to support floor or ceiling loads, which are themselves supported by bearing walls or other beams.
Knot: The portion of a branch that has been surrounded by wood in the subsequent growth of the stem. The shape of the knot on the surface of a plank will depend on the angle at which it is cut.
Knot area ratio (KAR): In a plank, the proportion of the cross-section at any point occupied by knots; used to calculate the visual stress grade of timber.
Latewood: Also known as summerwood; the portion of the annual ring formed in the later part of the growing season; generally of higher density and stronger than the early wood (q.v.).
Lignin: The second most abundant constituent of wood; a cementing substance that bonds adjoining cells and the cellulose framework.
Live load: Loading of a temporary nature such as wind, snow and construction loads.
Machine stress-rated timber: Timber that has been mechanically evaluated for stiffness from which its bending strength is automatically calculated resulting in the timber being assigned to a strength class.
Make good: A term usually applied to repairing wood by means of a plug, insert or filler.
Moisture content: The weight of water in a piece of wood expressed as a percentage of the weight of the wood when oven-dry.
Movement: The change in dimensions that accompanies normal fluctuations in relative humidity after wood is put in service; rated over a relative humidity (q.v.) change of 60 to 90% as follows: small = < 3% medium = 3 - 4.5% large = > 4.5%
Nominal size: A term whose definition may vary; normally refers to the size by which timber is known and sold, which is often different from the actual size of the timber; or to the size to which tolerances apply, but the tolerances may exceed those of EN 336.
Notional charring rate: The rate at which a timber member of a particular species over a defined size chars when exposed to a standard fire test.
Oven dry weight: The weight obtained by drying wood in an oven at 102°C ±3°C until no further loss in weight occurs.
Permeability: The capability of the wood to absorb preservative; often varies between sap and heartwood. The classification refers to heartwood only, as sapwood is generally permeable, and may vary with the preservative used and type of treatment.
Pith: The core of a tree stem, consisting of dark-coloured very soft tissue; it can show on the surface of planks, taken from the centre of the tree, as a dark line of easily indented tissue.
Pitch pocket: An opening between growth rings which contains, or has contained, resin.
Pores: Openings of vessels on the surface of cut timber, occurring only in hardwoods, seen as minute holes on end grain or scratches on side grain.
Post: A timber with the larger dimension not more than 50mm greater than the smaller dimension and usually graded for use as a column.
Quarter-sawn timber (edge grain): Timber sawn so that the growth rings are at an angle greater than 45° to the face.
Rays: Bands of soft tissue vertically aligned and radiating from the centre of the tree; insignificant in softwoods and variable in hardwoods - if broad can produce distinctive figure - e.g. silver grain in oak.
Relative humidity (RH): The ratio of the amount of water vapour present in air to the amount which the air would hold if saturated at the same temperature.
Sapwood: Wood of the outer growth rings, extending from the heartwood to the bark; contains living cells, with carbohydrate food reserves, and conducts the sap up the tree; generally considerably wetter than heartwood when freshly felled, and is perishable.
Seasoning or drying: The process of removing moisture from green wood to improve its serviceability. Seasoning often refers to drying in the atmosphere; kiln drying to accelerated drying under controlled conditions in a drying chamber or kiln.
Shake: A separation of the fibres along the grain, usually between the annual rings.
Softwood: Timber of conifer trees; the term relates to the botanical grouping of the trees and not to the hardness of the wood ( some softwoods, e.g. yew, are harder than some hardwoods).
Spiral grain: Growth of fibres in a spiral direction around the trunk of the tree; may cause twisting of timber during drying.
Spring: A curve along the edge of a plank; normally due to growth stress, e.g. compression wood (q.v.).
Stress: The applied force per unit area or volume; the primary stresses are tensile, compression and shear; a combination of all three occurs in bending.
Structural timber: Timber which has a determined strength and thus may be used in calculated structural designs.
Tension wood: Comparable to compression wood (q.v.) in conifers, it occurs in broad-leaved trees, on the upper side of leaning trees. Shorter gelatinous fibres causes unequal shrinkage, distortion and reduced strength.
Texture: The appearance of the timber produced by variations in the size of vessels and other cells, from fine (narrow vessels and rays) to coarse (wide vessels and rays).
Twist: Warping in which one corner of a plank twists out of the plane of the other three; associated with split and boxed-heart planks due to shrinkage of spiral-grained wood.
Visually stress-graded timber: Timber graded for strength based on visual assessment of limiting features (see KAR above).
Wane: The original rounded surface of a tree remaining (with or without bark) on timber after conversion.
Water repellent: A liquid that penetrates the wood and retards changes in moisture content while still allowing the wood to breathe; often incorporates a preservative, when it is known as a water-repellent preservative.
Workability: The degree of ease and smoothness with which wood can be sawn, planed and otherwise worked.