B

Balsa — Ochroma lagopus
Although it is one of the softest and lightest weight timbers commercially available with an average density of 160 kg per cubic metre, balsa is a hardwood. It is used for heat sound and vibration insulation and for modelmaking.

Beech, European — Fagus sylvatica
Grown in Europe. A white-ish to pale brown hardwood, used for furniture, interior joinery and flooring. May also be used for plywood.

Botanical names
The scientific naming of plants is based on a system adopted by a Swedish botanist, Carl Linnaeus and published in his 'Species Planetarium' in 1753. Each plant receives two Latin names; a generic, or genus name and a specific, or species name which is applicable to it alone. The species name may be followed by the name (often abbreviated) of the botanist who discovered it. The Latin names are correctly written in italics with a Capital letter for the generic name. Thus Pinus pinaster Ait is the botanical name of the maritime pine. Pinus indicates that it belongs to the genus of true pines, Pinus; pinaster the particular species of pine, and the botanist who published the original description of the tree was Aitchison, abbreviated to Ait.

Broad-leaved trees
Broad-leaved trees produce hardwood timber. Their seeds are in an enclosed case or ovary, such as an acorn or walnut. In temperate climates they are usually deciduous, ie they lose their leaves in winter.

C

Carbon dioxide
A naturally occurring gas in the atmosphere. However, carbon dioxide is released by burning fuels and increasing levels are thought to be responsible for the 'greenhouse effect' and to contribute to global warming. Plants take in carbon dioxide and, in a process known as photosynthesis, combine it with water to produce carbohydrates used as food for the growing tree. Therefore plants, and especially trees because of their size, 'lock up' large quantities of carbon dioxide by removing it from the atmosphere.

Cell
Microscopic units that make up the structure of wood and other plant tissues. The type and configuration of cells can be used in the identification of different timber species.

Cellulose
Cellulose and the related hemicellulose are complex chemicals made from sugars which provide strength and elasticity to timber.

Cold- and Warm-construction
The difference between 'cold' and 'warm' construction lies in the positioning of the insulation. In cold construction all, or part of, the timber structural elements are on the cold side of the structure (or the insulation is placed between the timber sections). In warm construction the timber structural elements are on the warm side of the insulation.

Composites
Structural timber composites are a family of engineered wood-based materials or components. Those currently available include: Glued laminated timber or glulam; Laminated Veneer Lumber or LVL; Parallel Strand Lumber or PSL; Laminated Strand Lumber or LSL. More information is given in the section on Timber Composites and in the TRADA Technology publication Structural Timber Composites.

Conifer
Coniferous, or cone bearing trees are known as conifers. usually evergreen, the trees have needle-like leaves and produce softwood timber.

Conversion
The process of cutting logs by sawing into usable section of timber, such as beams and planks

D

Density
Mass per unit of volume, measured in kilogrammes per cubic metre.

Dewpoint
The temperature at which condensation forms, it depends upon the saturation of the air. Warm air can hold more water vapour than cold. Thus when warm air is at 100% saturation, a reduction in temperature means that it is unable to support the volume of water vapour in suspension and liquid water condenses out.

Distortion
Change in the shape of a piece of timber or timber-based material, eg bowing, twisting or cupping, brought about by shrinkage as the timber dries.

Douglas fir — Psuedotsuga menziesii
Grown in North America and the UK. A light reddish-brown softwood, used for construction, interior and exterior joinery and in plywood.

Durability
In timber, durability relates to the resistance of timber or timber-based products to biological attack - by fungi, insects or marine borers. The classifications usually quoted refer to the resistance to fungal attack by the heartwood of a species only. The sapwood of most species is 'not durable' or 'slightly durable'; in situations where the timber is at risk the sapwood should be excluded or preservative treatment should be specified. Five natural durability to wood-destroying fungi classes are recognised in 'BS EN 350-1' Guide to the principles of testing and classification of the natural durability of wood:

Class 1: very durable
Class 2: durable
Class 3: moderately durable
Class 4: slightly durable
Class 5: not durable

Insect attack is generally of less significance in the UK. Information on the resistance of some timbers to insect and marine borer attack is given in 'BS EN 350-2' Guide to the natural durability and treatability of selected wood species of importance in Europe.

E

Earlywood
The less dense wood formed during the early stage of a growth season, eg the spring or rainy season, when the tree is growing quickly. Also sometimes called springwood. Earlywood and latewood together form the growth rings of a tree.

Edge and end spacing
Structural timber design codes define safe spacings between fasteners and from the edges and ends of the components being joined.

F

Figure
Ornamental markings seen on a cut surface of timber, formed by the structural features of the wood.

Five times rule
The 'five times rule' ensures that a timber construction will be free from potentially harmful condensation if the vapour resistance of the layers on the warm side is at least five times those on the cold side.

Foliage
Leaves

Fruits
For example, nuts, acorns and cones (which contain the seeds of conifers).

G

Glulam
Glulam or glued laminated timber is one of a range of structural timber composites. Glulam is manufactured by gluing together strength graded laminates to produce large section structural components which can be straight or curved. Beams are manufactured in stock sizes or are purpose-designed and manufactured.

Grain
The general direction of the fibres relative to the axis of the tree. Grain may be straight, spiral, interlocked or wavy.

Greenheart — Ocotea rodiaeai
Grown in Guyana. A dense yellow/olive green to brown hardwood used for heavy construction such as bridges, marine and freshwater construction.

H

Hardwood
Timber produced from broad-leaved trees.

Heartwood
The inner zone of a tree trunk or log that, when the tree was growing, had ceased to contain living cells and reserve materials, such as starch. The heartwood may be darker in colour than the outer sapwood though they are not clearly differentiated in all species. The heartwood is often more durable than sapwood.

I

Interlocking joint
Interlocking is in certain respects an elegant way of jointing timber. Each member is cut to fit against or into another; both to prevent displacement and to transfer forces. The joint must either be in compression or the joint must be pinned or keyed after assembly. The use of pins does not effectively resist tensile forces and thus tensile joints were kept to a minimum in historic construction.

Interstitial condensation
Interstitial condensation forms within structural elements, rather than on the surface.

Inverted construction
‘Warm’ construction, typically used for flat roofs where the insulation is located above the weatherproof membrane.

Iroko — Milicia excelsa
Grown in West Africa. A yellow-brown hardwood, used for interior and exterior joinery and constructional work.

K

Knots
The remains of branches in timber. A branch sawn off close to the trunk or shed naturally forms a sound or live knot. A broken branch stub which becomes surrounded by new growth produces a loose or dead knot in the timber.

L

Latewood
The denser wood formed during the later stages in a season, when growth slows down, eg summer and autumn or the dry season. Also sometimes called summerwood, earlywood and latewood together form the growth rings of a tree.

Lignin
Acts as a bonding agent in the cellular structure of timber.

M

Managed
Good forest management practices ensure that timber is produced on a sustainable basis, ensuring that new trees are planted to replace those harvested and that the volume of timber removed is replaced by new growth. Forestry policy in a particular country may be set by government or left to forest owners. A number of schemes whereby sustainability performance may be judged and monitored are currently being developed.

Further information is available from:
Forests Forever — A Campaign for Wood
4th Floor, Clareville House
26, 27 Oxendon Street
London SW1Y 4EL
Telephone: 0171 839 1891
Fax: 0171 930 0094

Mass law
'Mass law' suggests that sound insulation increases by 5dB per doubling of mass.

Meranti
A mixed species grouping of the Shorea genus. Grown in South East Asia. Commercially the timbers are grouped according to their colour and density:

Dark red meranti or dark red seraya and red lauan, is a group of medium to dark red brown hardwoods used for interior and exterior joinery and for plywood.

Yellow meranti or yellow seraya are yellow-brown hardwoods, used for interior joinery and plywood.

Light red meranti, light red seraya or white lauan, are pale pink to mid red hardwoods, used for interior joinery and plywood.

Moisture content
The amount of moisture in timber and wood-based products, usually expressed as a percentage of the oven dry mass. The moisture may be present as liquid in the cell voids as well as chemically bound into the cell walls. As timber dries, liquid water is removed from the cell voids, until the fibre saturation point is reached at about 28% moisture content. After this the bound water in the cell walls is removed and the timber starts to shrink. The equilibrium moisture content is the point at which the timber or wood-based product neither gains nor loses moisture when exposed to a constant condition of temperature and humidity. Moisture content of timber on site, in the yard or in buildings is generally measured using an electrical moisture meter. The electrical resistance between two probes hammered into the timber provide a reading, which may be calibrated for species or type of product from a chart provided. For a more accurate average moisture content measurement, a piece of timber is weighed, dried in an oven until a constant weight is reached. The moisture content is then calculated from the formula:

mc = (wt of wet wood ÷ wt of dry wood) x 100

Movement
The swelling and shrinkage of wood with changing moisture content. Movement in length is always negligible. Movement is greater parallel with the growth rings than at right angles to them. The degree of movement varies between species and for some uses can be a significant factor in the choice of timber.

O

Oak, European — Quercus robur and Quercus petraea
Grown in Europe. A yellowish-brown hardwood used for furniture, interior and exterior joinery, flooring, barrels and fencing.

Opepe — Nauclea diderrichii
Grown in West Africa. A yellow to orange-yellow hardwood, used for heavy construction, marine and freshwater uses and for exterior joinery and flooring.

P

Permeability
The ease with which liquids, such as preservatives or flame retardants can be impregnated into timber. Permeability varies with species, though the sapwood of all species is more permeable than the heartwood. Permeability ratings relate to the heartwood of the species.

Pointside
The piece of timber in a joint which receives the point of a nail or screw. The other section is known as the headside.

Preservative treatment
The treatment of timber with chemicals to improve its resistance to attack by biological organisms, such as fungi, insects and marine borers. The chemicals can be brushed or sprayed onto the surface of the timber but treatment is more effective if the chemicals are impregnated into the timber under vacuum and/or pressure in special treatment vessels.

R

Rays
Narrow ribbons of cells which conduct and store food in the tree. They run across the grain of timber.

Redwood, European — Pinus sylvestris
Grown in Scandinavia, the Baltic States and the Russian Commonwealth (the former USSR). Pinus sylvestris is also grown in the UK, where is is known as Scots pine or British pine. A pale yellowish-brown to red brown softwood, commonly used for construction, joinery and furniture.

S

Sandwich construction
A warm roof construction where the insulation is located above the roof deck but below the weatherproof membrane.

May also refer to composite panel products, known as sandwich panels where panels are built up from layers of different materials, for example plywood outer layers with a core of insulating foam, paper honeycomb cores for doors and timber spacers for stressed skin panels.

Sap
Liquid, mostly water, contained in cells in a tree or timber. Sap is the means by which dissolved food and salts are moved around the tree.

Sapele — Entandrophragma cylindricum
Grown in South East Asia. A medium reddish-brown hardwood with a marked stripe figure, used for interior joinery, furniture and flooring.

Sapwood
The outer zone of a tree trunk or log, which in the growing tree contains living cells and reserve materials such as starch. Sapwood is generally lighter in colour than the inner heartwood, though they are not clearly differentiated in all species. The sapwood is more vulnerable to attack by biological organisms such as fungi and insects but is also usually more permeable than the heartwood making it easier to treat with preservatives.

Sawn timber
Logs are sawn into rectangular sections of timber by sawing. The way in which the log is cut affects the behaviour and the grain pattern of the resulting sections. Logs sawn 'through and through' produce plain sawn sections where the growth rings meet the face at an angle of less than 45 degrees. Quarter sawn timber has been converted so that the growth rings meet the face at an angle of more than 45 degrees.

Shear
Shear occurs on the interface of two members which are being moved in din different directions by forces outside the joint. A pin, inserted through both members to hold them together will be torn apart, or sheared, if the forces are large enough.

Shrinkage
Reduction in dimension or volume of a material, due to a reduction in moisture content.

Softwood
Timber produced from coniferous trees. Most structural timber used in the UK is softwood.

Species
The botanical classification of trees and timber. The Latin species name defines a timber more accurately than common names which are sometimes used for more than one species of timber, or may vary between countries.

Standard names
The commercial names for timber recommended for use to avoid confusion. These are included in BS 7359 'Commercial timber, including sources of supply'.

Strength class
The strength of timber varies with species and is also affected by characteristics such as knots, slope of grain, splits etc. Each piece of timber used structurally therefore has to be graded, either by visual inspection or by machine. Species and grade combinations of similar strength properties are grouped together into strength classes which are defined in 'BS EN 338' Structural timber - Strength classes.

Strength grade
The strength of timber varies with species and is also affected by characteristics such as knots, slope of grain, splits etc. Each piece of timber used structurally therefore has to be strength graded, either by visual inspection or by machine. The timber will be marked with its grade and other information such as species, whether the timber was graded wet or dry, the company responsible for the grading and the certification body responsible for overseeing the grading operation.

T

Teak — Tectona grandis
Grown in Burma and Thailand and has been extensively planted elsewhere. A golden-brown hardwood, sometimes with dark markings. Used for furniture, interior and exterior joinery.

Texture
The structural character of timber as revealed by touch or reaction to cutting tools. The texture of timber is determined by the distribution and size of the various cell types.

Thermal resistivity
Thermal resistivity, termed r, is a measure of the resistance to heat flow of a unit thickness of a material. It is measured in mK/W. The greater the value of r, the greater the insulation provided.

Trunk
The trunk of a tree is sometimes also called the bole. After felling, the branches are removed, leaving the trunk - at this stage known as a log.

U

U values
U-values express the rate of thermal conduction across a complete building element. They are expressed in Watts per square metre per degree Kelvin. The lower the U value, the better the insulation.

Utile — Entandrophragma utile
Grown in West Africa. A reddish-brown hardwood, used for interior and exterior joinery, furniture and cabinet work.

W

Wane
The original rounded surface of a log, with or without bark, on any face or edge of sawn timber.

Western hemlock — Tsuga heterophylla
Grown in North America. A pale brown softwood, used for construction and joinery.

Western red cedar — Thuja plicata
Grown in North America. A reddish-brown softwood, used for roofing shingles, exterior cladding and greenhouses.

Whitewood, European — Picea abies and Abies alba
Grown in Europe, the Baltic States and the Russian Commonwealth (the former USSR). A commercial grouping of white to pale yellowish-brown softwoods, commonly used for construction, joinery and flooring.

Wood-based board
Wood-based board materials are manufactured from layers, particles or fibres of wood, glued or compressed together to produce a flat board. The most common examples include plywood, chipboard and various types of fibre building boards, including hardboard and MDF.