Timber defects

A defect in wood may be defined as any irregularity that affects its normal strength, durability, appearance or value for use.

Natural defects

Knots are caused as limbs (branches) form on the tree. There is a distinct change in grain direction, which often presents seasoning and working problems. There are two types of knot-live and dead. Live knots occur when the trunk (bole) grows to a larger diameter with age and gradually grows over a live limb. When converted to boards, the limb shows as a live knot.

Live knots

While live knots generally weaken the timber and spoil its appearance, in some timbers such as cypress pine these knots remain sound and tight and are not classed as defects; in fact, they can be used to enhance aesthetics on otherwise plain timber.

A dead knot is produced when a branch is removed and ceases to grow. As the trunk grows its diameter increases, eventually covering the limb completely, including its bark. When this timber is converted to boards it shows the knot, including its bark, which can be loose and fall out.

A dead knot showing bark

Burls, or burrs, are large abnormal bulges that form on the trunk and limbs of the tree

Burl on a eucalyptus

They are believed to be caused by an injury, such as a bump from a falling limb, or by insect attack. Turned bowls and veneers cut from burls are generally highly prized, as the wavy or uneven fibre alignment usually presents an attractive figure.

A turned burl bowl

Gum veins and pockets may occur naturally or be caused by injury to the tree. Timber is weakened and disfigured by their presence

Gum veins and pockets in Mountain Ash

Brittle heart and heart rot are common defects found near the pith or medulla of the tree. The wood is pulpy, affected by decay, or of such a brittle nature that it is structurally weak.

Shakes are caused by separation of the adjoining layers of wood either from the centre of the tree along the medullary rays or between the growth rings. Shakes arise in the standing timber or during felling, not during the seasoning process. However, they usually extend as the timber dries, following the split lines originally established. Heart shakes, which occur at the central region of the tree, are believed to be caused by swaying stresses or growth stresses.

Grain direction is classed as a defect only if it weakens the timber or makes it too difficult to work; and although there are many variations of cross-grain (such as spiral, diagonal, wavy, interlocked and curly) only the first two are generally classed as defects. When the fibre alignment runs at an angle to the axis of a piece of timber, the timber is said to be cross-grained . Interlocked grain is caused by the fibres of the adjacent layers of wood being spirally inclined in opposite directions . This produces a distinctive figure called 'ribbon grain' or 'fiddleback' (see the section on grain within the section 'Properties of wood'). This is sometimes difficult to plane. Queensland maple and walnut can show interlocked grain, but it is not classed as a defect if it does not weaken the timber.

Seasoning and handling defects

Checks or splits are ruptures in wood along the grain that develop during the seasoning process. They are usually caused by drying that is too rapid or uneven resulting in uneven shrinkage at the ends of the timber. Seasoning checks are of two types, end checks and surface checks, and once they develop they remain as a permanent weakness in the timber, even though they may appear to have closed up.

Collapse results from a flattening of the cell structure due to excessive or uneven drying. It is usually evidenced by severe and irregular shrinkage, but in most cases it may be corrected by steaming in a reconditioner.

Case hardening is a condition in which the outer layers of timber become abnormally hard as a result of drying that has occurred too rapidly. Uneven stressing results as seasoning progresses, and unless reconditioning is carried out the timber will be prone to cut or warp when being worked.

Compression wood is indicated by the presence of eccentric growth rings. Compared with normal wood it is heavier and weaker and exhibits unusually high and irregular shrinkage along the grain.

Warping is the broad term that is used to describe any distortion from the true plane that may occur in a piece of wood during or after seasoning (e.g. bow, spring, cup, twist). 'Bow' may be described as the longitudinal curvature of the wide face of a board caused by uneven shrinkage during seasoning. 'Spring' is the curvature of the edge of a piece of timber, with the face remaining flat. 'Cup' is a simple warp across the face of the timber and is commonly associated with back-sawn wood. The curvature, which is caused by uneven shrinkage, is generally away from the heart and can be minimised by careful stacking and seasoning. 'Twist' or 'wind' is a spiral warp along the length of a piece of timber and may be caused by sloping grain or poor stacking when drying.

Waney edge is the absence of the wood on the edge or corner, showing the bark or sapwood

Cut pine showing waney edge

Defects due to insect attack

Borers and termites are responsible for considerable damage to trees, logs, freshly sawn timber and even wood that has been seasoned for many years.

Borers

The furniture beetle (Anobium) is the most common member of the furniture borer group and is found in most parts of the world. It mainly infests the sapwood of softwoods (conifers), although damage to the heartwood is not uncommon. Attacks in native eucalypts are rare. Larvae (grubs) may feed and tunnel in the timber for a number of years before entering pupal or resting stage near the surface of the timber. The newly developed adult beetle, which emerges a few weeks later, then bores an escape tunnel to the surface.

Furniture beetle infestations are usually noticed in old timber (from seven to 40 years), partly because of the insects' comparatively long life cycle (from two to six years) and also because it is possible for several generations to emerge before attack is detected.

Powder post or Lyctus beetles are borers commonly found attacking the sapwood of certain hardwoods. The main damage is caused by the larvae, which feed on the starch in the sapwood. The attack is confined to the sapwood because the heartwood (truewood) does not contain starch. Timber that contains susceptible sapwood is normally attacked within the first year after milling, and the infestation continues until all of the food supply (starch) has been exhausted. As the eggs are laid in the vessels or pores of the wood, the Lyctus beetle cannot attack softwoods (non-pored woods) or hardwoods with minute pores. The complete life cycle may be as short as four to five months, but normally there is only one emergence a year.

Pin-hole borers (Ambrosia) attack only green timber. The adult beetles attack almost all species of trees, and frequently tunnel deeply into the wood of weakened trees, freshly felled logs, and even milled timber that still has a high moisture content. The tunnels or holes are typically long and straight across the grain, so that a pin may be inserted for some considerable depth. Fungi are introduced into the tunnel by the beetle, and the young larvae feed on the ambrosial fungus . The fungus is dependent upon a generous moisture supply and will not grow in dry wood. Therefore, the pin-hole borer can cause no further damage in seasoned timber. The wood surrounding the borer holes is generally discoloured by the presence of the fungi; and although pin-hole damage does not generally weaken the timber structurally, where appearance is important it lowers the market value of the timber.

Termites

Termites (also often called white ants, even though they are not ants) destroy large quantities of timber annually in Australia, attacking poles, posts, sleepers and wooden structures of all kinds. They live in colonies, and individuals cannot exist without communications with the colony. In some species the nest consists of galleries in timber; others build a mound-shaped nest above the ground; yet others construct a subterranean nest. Practically all of the termite damage to seasoned timber is caused by the subterranean species, and timber in contact with soil is most readily attacked.

Termite damage is often difficult to detect in a building, owing to the insects' habit of seeking dark and badly ventilated situations. Unless the termites' galleries leading from the ground are detected, damage is not usually discovered until the timber has been almost completely hollowed out. Once discovered, the damage is easy to recognise, as the termites work inside the boards, along the grain, eating out large galleries or runways. The galleries are often lined with disintegrated wood and clay mixture, and the shell of timber left is often decayed.

Prevention and control

Termite species that cause serious damage must have contact with the ground or access to a continuous source of moisture. Therefore, prevention of damage involves measures that will make it impossible for termites to make a concealed entry into a building from the surrounding soil, and measures to eliminate dampness within the building:

Physical barriers. Foundations are preferably constructed from an impenetrable material such as brick, steel or concrete slab, so that termite tunnels may be visible on them. All foundation walls and piers should be fitted with termite shields made from durable material such as crushed stone, steel capping or stainless steel mesh.

Soil treatment. The soil around foundations and piers may be 'puddled' with termicides. Long-life chemicals are no longer registered for this use, therefore modern chemicals must be re-applied to maintain protection. This may involve the use of hand sprayed chemicals or reticulation systems, which allow re-application through a system of dedicated pipework hidden in the construction.

Ventilation. Adequate underfloor ventilation is very important, as termites seek moist, shaded and sheltered positions.

Drainage. The site must be adequately drained to prevent water collecting under the building.

Use of treated timber. Naturally durable timbers, such as cypress pine, or timbers that have been impregnated with suitable preservatives are also important deterrents to termite infestation.

The building Code of Australia (BCA) requires all new building work to conform to Australian Standard AS3660. l-2000 Termite management.

Life cycle of the termite

Every termite colony is characterised by the presence of a king and queen-the founders of the colony.

The queen produces large numbers of eggs that give rise to the various castes of termites; namely, workers, soldiers and reproductive alates (winged types). The workers in our common pest species are small, wingless, white-bodied, blind insects with well-developed jaws for gnawing wood. The workers construct the nest, collect food, and feed and care for the rest of the colony. The soldiers, whose function it is to protect the nest from invaders, are also blind, but are provided with longer jaws than the workers. The reproducers are males and females provided with wings, which enable them to fly from the nest to form new colonies.

Two peculiar habits of the termites enable us to control their activities:

They groom or clean each other by removing particles of dust and moisture.
They eat their own dead.

Additional Resources

1.1.4.1 Timber Defects - natural.pdf

1.1.4.2 Timber Defects - felling milling.pdf

1.1.4.3 Timber Defects - seasoning handling.pdf

1.1.4.4 Timber Defects - fungal.pdf

1.1.4.6 Timber Defects - insects.pdf

1.1.4.5 Timber Defects - weathering.pdf

3.3 Timber defects - Worksheet

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