Assessing the suitability of a piece of timber for non structural uses, based on the appearance of its surface characteristics. (Appearance grading should not be confused with visual grading which only applies to strength grading of structural timber). There are three main approaches:
• Defects system – Each piece of timber is assessed against rules for the maximum allowable size or degree of each type of feature that is permitted within a grade. The grades describe the whole piece of timber including, in some cases, defects that will have to be removed by resawing.
• Cutting system – This system is based on the amount of timber free of defects, or with acceptable features, assessed as rectangular areas called cuttings. The grades are defined in terms of the minimum area of cuttings (rectangles clear of defects) that are allowed within a single piece.
• Fit-for-purpose – grading rules that describe timber characteristics affecting the performance of a plank or board in demanding product uses such as steam bending, or gun stocks. For example BS 3823 gives rules for ash timber intended for use as tool handles . Some of these grades, such as those for decking, are effectively strength grades.
Historically the defects system was used to grade timber cut to specified dimensions (scantlings) for a particular use, while the cutting system was used for pieces not dimensioned for a known market. Nowadays, in countries having large volumes of clear timber, the cutting system is more common, while the defects system tends to be used where the timber is more variable.
Most North American hardwood timber coming into Europe is graded to a cutting system developed by the NHLA (National Hardwood Lumber Association of North America) . These rules have been summarised in a well-illustrated booklet by the American Hardwood Export Council. The NHLA grades are frequently used in modified form in other parts of the world. In contrast, European Standard EN 975-1 is a hybrid of the defects and cutting system. In the case of oak it follows a defects system derived from French grading practice whereas the beech grades in the standard combine both systems; In practice, sawmillers who are aware of EN 975-1 usually work from the summary information on grading contained in two guides to French hardwoods as opposed to using the standard itself. No other species are covered in EN 975-1 although a standard for appearance grading of poplar is being prepared.
Sharp external angle of a piece of wood where two surfaces meet
The outer protective covering of a tree.
A small section of bark that is partly or wholly enclosed within the inner wood.
The size by which a piece of sawn timber is known or specified, at a stated moisture content, regardless of sawing tolerances or subsequent reductions by processing.
A squared timber with a minimum cross section of 100 x 100 mm.
See non inter-grown knot.
Face that, using a particular grading rule, is judged to be superior to the other face.
A piece of square- or waney-edged sawn timber 50mm or less in thickness.
The North American unit of lumber measurement. A board foot is equivalent to a piece of timber measuring one foot long, one foot wide, and one inch thick; in other words a volume of 144 cubic inches. There are thus 12 board feet to the cubic foot.
A stack of timber formed from a log that is sawn longitudinally by a series of successive parallel cuts with the resultant waney-edged pieces then assembled to recreate the original form of the log.
A piece that has been sawn so that it contains the core of the tree known as the pith.
See Tiger oak
A spectacular feature comprising the distorted growth rings of large numbers of small knots caused by groups of epicormic shoots.
One of the small, often microscopic, units that make up the structure of wood.
Hardwood timber with a mix of inter-grown knots, pin knots, heart shake, and colour variation.
A cluster of pin knots.
Short, narrow, separation of fibres along the grain; often the result of drying stresses. Checks in the ends of a piece of timber are particularly common.
A mix of colour caused by fungi, chemical reaction or other causes.
See log conversion
See log conversion
Grain that occurs in irregular curves
See appearance grading
See non inter-grown knot
An imperfection that lowers the timber quality
See appearance grading
Stain in timber, due to fungi, chemical reaction, or other causes, that may lower its merchantable value in some markets
Early stage of fungal decay recognisable by frequent discoloured spots, streaks or patches.
The process of bringing timber to a moisture content range that is suitable for an intended use.
Either of the narrower longitudinal opposite surfaces of a square- or waney-edged piece.
Small buds and branches which appear on the trunk of some timber species usually as a result of an increased exposure to sunlight. Eventually these shoots become engulfed by the enlarging trunk, which creates a decorative feature called a burr.
See natural durability
Timber sawn so that the pith is visible on a face or edge
Either of the wider longitudinal opposite surfaces of a square- or waney-edged piece
Physical, morphological, or growth characteristic of a piece, which could affect its use
Fibre saturation point (FSP)
A zone at which virtually all moisture has been removed from the cell cavities of timber but where the cell walls remain saturated. In most species it equates to a moisture content of 25 – 30%. Many timber properties change as the moisture content passes the FSP.
Ornamental markings on the cut surface of timber, formed by the structural features of the wood.
The size of a piece after machining, subject to machining tolerances
Decomposition of wood by fungi, resulting in softening, progressive loss of strength and density, and often a change in texture and colour.
A way of sorting pieces of timber into broadly similar groups, according to quality or mechanical performance, so that marketing can be rationalised and selection for a specific use is simplified; it is usually divided into structural and appearance grading.
Character of wood as revealed by touch or reaction to cutting tools. It is determined by the distribution and size of the various cells.
(1) Oak that is used as a construction timber before it has been dried. The oak dries out in service and care is needed to ensure that the resultant shrinkage does not cause unacceptable gaps or splitting. Oak is used green because it is cost effective and easy to work with hand tools.
(2) A very rare blue-green colouration in oak caused by the green wood-cup fungus Chlorosplenium aeruginascens. The coloured timber was formally used in the manufacture of marquetry boxes called Tunbridge Wear.
Layer of wood produced in one growing season.
Wood of broadleaved trees, that is, trees from the botanical group Dicotyledonae
The inner zone of wood that, in growing trees, has ceased to contain living cells. In some species, such as oak, the heartwood is darker than the sapwood while in other species such as sycamore it is not easily identifiable. Heartwood is often more durable than sapwood.
A radial shake originating at the centre of the log.
A unit of roundwood measurement where the cross-sectional area of a log is taken as the square of one quarter of its circumference. One hoppus foot is equivalent to 1.27 cubic feet. A hoppus foot is thus about 21% short of a cubic foot – the reduction helping to compensate the sawmiller the volume loss involved in converting roundwood to sawnwood. Nowadays hoppus measure is generally restricted to hardwoods.
An area of sapwood enclosed within the heartwood and showing as a light coloured patch. Included sapwood can be a problem in oak and sweet chestnut whenever sapwood free timber is required either for its appearance or for use out of doors where it may be exposed to fungal decay
A knot that, on the surface considered, is inter-grown with the surrounding wood over all or most of its perimeter. Also called a live knot.
A characteristic tunnel caused by one of several species of beetle larvae that burrow into wood.
A portion of a branch that became embedded in the wood as the tree grew around it.
A group of knots around which the wood fibres are deflected.
See inter-grown knot
The way in which a log is sawn into planks and boards determines the type of grain and figuring that will appear in the finished sawn timber, and may also affect its stability. There are three main types of log conversion used with hardwood timber:
• Plain sawn – a log sawn longitudinally with a succession of parallel cuts. Initially this yields planks or boards that are sawn tangentially to the growth rings. Later, as the cuts get near the centre of the log, the pieces will be quarter sawn (i.e. with the growth rings running near vertical to the face). Plain sawn timber is also known as 'crown cut' or 'through-and-through'.
• Rift sawn – a log sawn longitudinally through the middle and then at right-angles to the initial cut. Rift sawing yields sawn timber with grain and moisture movement characteristics between plain sawn and quarter sawn
• Quarter sawn – a method of radial sawing used to produce pieces where the growth rings run vertically or near vertically to the face of the plank or board. In practice a grain angle of over 45˚ is generally accepted as being quarter sawn. When the angle is close to 90˚ the pieces are very stable and, in the case of oak, have a distinctive silver ribbon figure running across the face. Quarter sawing is expensive and so it is seldom undertaken as part of a normal milling operation. Some mills will, however, select quarter sawn timber, to order, from logs that are plain or rift sawn.
Large volume or specialised joinery fabrication, often for public or commercial buildings.
A term sometimes used for an attractive brown colouration in sycamore.
A measurement technique, most commonly applied to waney-edged timber, where the final dimensions are quoted as if the area containing any defects had already been removed before measurement. This guide permits measuring out in both square- and waney-edged pieces.
The amount of moisture that is present in timber, usually expressed as a percentage of the oven dry mass. European Standard EN 942 gives recommended moisture contents for four typical situations. Modern houses are becoming increasingly dry and so the moisture content recommendations given in this standard are lower than earlier guidance. They are also different to the moisture content recommendations for structural timber, which are generally less demanding.
Ongoing moisture-induced change in across-the-grain dimension exhibited by timber after its initial shrinkage due to drying.
Classification of relative moisture movement
Resistance of timber to damage by wood destroying organisms such as fungi or insects. For example European Standard EN 350-2 gives a relative classification of the inherent resistance of wood to attack by wood destroying fungi. Note that all sapwood should be considered as being not durable
Non inter-grown knot
A knot that, on the surface considered, is detached from the surrounding wood over all or most of its perimeter. Non inter-grown knots can sometimes become loose and fall out. Also known as a dead knot or black knot.
This term should be interpreted according to normal trade usage, e.g. up to 10% of pieces in a parcel.
A piece of timber immediately after conversion and before any drying shrinkage has occurred.
An attractive dark brown stain in ash.
Knot cut more or less perpendicular to its long axis so that the exposed cross-section is approximately oval.
Moisture content of timber that has been dried in a ventilated oven at 103°C until there is no further fall in moisture content.
An inter-grown or non inter-grown knot with a maximum diameter of 5mm.
Pieces with intermittent pin knots, either singly or in clusters, giving a much sought after grain feature. (See also burr)
One plank or board.
Zone of soft tissue within the first growth ring.
See log conversion.
A piece that has been machined, on all faces and edges, resulting in a square or rectangular cross-section.
A piece of square- or waney-edged sawn timber more than 50mm in thickness.
Machining one or more surfaces of sawn timber to specified tolerances at an appropriate moisture content.
See log conversion.
Sawing of timber into smaller cross sections
See wavy grain.
A shake that follows the line of a growth ring.
See fungal decay.
A knot that is softer than the surrounding wood due to fungal decay
The outer zone of a tree underneath the bark that, when the tree is growing, contains living cells and conducts sap. Sapwood is frequently paler than the heartwood though is not clearly differentiated in all species. Sapwood has a low natural durability.
A longitudinal fissure in timber, irrespective of the extent of penetration
A divergence in the direction of the grain from the longitudinal axis of the piece
Wood of coniferous trees; that is, from the botanical group Coniferae
A knot that is free from decay and at least as hard as the surrounding wood. Some splits are generally permitted in sound knots but limits may be set in the supply agreement.
Irregular dark lines showing on a wood surface caused by individual wood inhabiting fungi forming sharp boundary zones to separate themselves from other decay organisms of the same or different species. If arrested in the early stages of decay these zones can be a decorative feature in some light coloured timbers, particularly beech. In other timbers such as birch, however, spalting only becomes visible in the later stages of decay by which time the timber is too degraded to be usable.
A knot cut approximately parallel to its axis so that the exposed section is elongated and emerges on the arris.
Grain that follows a spiralling course in one direction around a log.
Sawn timber of regular cross section, with wane, if permitted, not exceeding a specified limit.
Steam is sometimes used as a convenient heat source to enable some timber species such as ash to be bent into tight curves. In other cases steam is used to darken timbers such as beech and pear, and to make them easier to work.
Grain that is parallel or nearly parallel to the longitudinal axis of the piece.
Unsightly stain on light coloured timbers resulting from oxidative reactions between the piece of timber and the spacer (or sticker) used for separating the planks or boards as they dry. It is particularly common in beech, ash, and sycamore.
Assessing the load-bearing characteristics of a piece of timber. There are two main strength grading systems:
• Visual grading – assessing the load-bearing capacity of a piece of timber visually, using grading rules that define limits for rate of growth, and for strength reducing factors such as: knots, sloping grain, fissures, and fungal attack. Hardwood timber is almost always visually graded. Note that visual grading and appearance grading should not be confused - appearance grading only applies to grading for non-structural purposes.
• Machine grading – measuring the strength of timber using special grading machines, which, in most cases, exploit a correlation between the deflection of a piece of timber under load and particular mechanical properties. Because it is so accurate, machine grading is preferred to visual grading wherever possible. At present however its use is generally restricted to softwood timber less than 80mm thick. Larger section softwood timber and all hardwood species, except poplar, still have to be strength graded visually.
In the UK the current engineering design code for timber is BS 5268-2  though this is due to be replaced by Eurocode 5 . These codes require that timber used for load bearing purposes is strength graded. In practise, however, many building projects using structural hardwoods are approved via an engineers certificate as opposed to formal engineering calculation. In this case the timber quality is selected to a 'framing grade' agreed between the engineer and carpenter, with only a few key structural members being formally strength graded.
Where timber is strength graded it is assigned to specific 'strength classes', based on characteristic values for timber strength, stiffness, and density. In European Standard EN 338 the strength classes for softwood timber are prefixed by the letter C, while hardwood classes are assigned the letter D. The only exception is poplar, which, though it is a hardwood, is given a C class in the standard.
The current British Standard for visual grading of hardwoods is BS 5756 , which gives four strength grades for temperate hardwoods. These correspond to specific strength classes in EN 338. BS 5756 is sometimes criticised because there is a large gap between the two highest and the two lowest grades, and because it does not distinguish between heartwood and sapwood. Thus for specific purposes, such as building conservation, some companies using structural oak still work to the grades in CP 112 , which was withdrawn in 1980. Whatever visual grading system is used it is important that the grader is properly trained, the grades chosen are specified by a structural engineer, and that the system follows the requirements for visual grading in European Standard EN 518
An attractive dark brown stripy colouration in the heartwood of oak caused by the beef steak fungus Fistulina hepatica. Unlike most wood inhabiting fungi, F. hepatica does not cause significant breakdown of the timber until a late stage of colonisation. Very rarely the whole cross section of the heartwood is affected, this is known as brown oak. Similar stripy markings are occasionally seen in other hardwoods particularly sweet chestnut.
See strength grading.
Original rounded surface of a log, with or without the bark, which occurs on any face or edge.
Sawn timber having parallel faces and with one or both edges left unsawn so that the bark or irregular surface is retained. Also called un-edged.
Grain occurring in fairly uniform waves. A decorative feature, particularly in sycamore. Occasionally found in other species, particularly ash. Also known as ripple.
Distortion of a piece of timber during the process of log conversion, drying, or storage.
Face that, using a particular grading rule, is judged to be inferior to the other face.
The glossary is an extract from 'Making the Grade: A guide to appearance grading in UK grown hardwood timber' by Ivor Davis and Guy Watt.