Defects and Decay in Timber

Timber is subject to many defects, which should, as far as possible, be cut out during its conversion. These defects can be divided into 2 groups;

Seasoning defects; these are a result of poor seasoning of timber. They include the following;

I. Bowing; this is a curvature along the face of a board; and often occurs where insufficient piling sticks are used during seasoning.

II. Springing; this is a curvature along the edge of the board where the face remains flat. It’s often caused through bad conversion of curved grain.

III. Winding; this is a twisting of a board and often occurs in timber, which is not converted parallel to the pith of the tree.

IV. Cupping; this is a curvature across the width of the board and is due to the fact that timber shrinks more tangentially (less than 45◦) than it does radially (more than 45◦).

V. Shaking; these are splits which develop along the grain of a piece of timber particularly as it ends, and is the result of the surface or ends of the timber drying out too fast during seasoning.

VI. Collapse; this is also known as wash boarding and is caused by cells collapsing through being kiln dried too rapidly.

VII. Case hardening; this is also as a result of too much rapid kiln drying. In this case, the outside of the board is dry but moisture is trapped in the center cells of the timber. This defect is not apparent until the board is re-sawn when it will tend to twist.

Natural defects; these are a result of the natural growth of the trees. They include;

I. Heart shakes; these are splits along the heart of a tree and are probably due to over-maturity.

II. Star shakes; these are a number of heart shakes, which form an approximate star.

III. Cup shakes; this is a separation between the annual rings and is normally the result of a lack of nutrient. Also said to be caused by rising say freezing during early spring cold spells.

IV. Radial shakes; these are splits along the outside log, which are caused by rapid drying of the outside of the log between it is converted.

V. Waney edge; this is where the bark is left on left on the edge of converted timber and is the result of too economical a conversion.

VI. Knots; these are end sections of branches where they grow out of the trunk. These can be a serious defect in timber especially when they are dead knots (knots which have become loose in their sockets or show signs of decay). And this causes difficulties when finishing. However, contrary to the above; knots are sometimes used to provide a decorated feature e.g. knotty pine cladding.

VII. Upsets; also known as thunder shake and is a fracture of the timber fibres across the grain. They are mainly caused by the severed jarring the tree receives when being fallen but also: can be caused by the tree being struck by lightning sometime during growth. This is a serious defect which is most common in mahogany; isn’t apparent until the timber has been planed.

VIII. Sloping grain; this is where the grain doesn’t run parallel to the edge of the board and is often caused by bad conversion. And when the sloping grain is pronounced, the defect is called short graining. This seriously affects the strength of the timber and shouldn’t be used for structural work.

IX. Sap stain; often occurs in felled logs while still in the forest. Can also occur in damp timber that has been improperly stacked close together without piling sticks or with sufficient air circulation. The strain occurs as a result of a harmless fungus feeding on the contents of the sapwood cells but no structural damage is caused. It’s only considered as a defect in timber that is to have a polish of clear varnish finish.

Decay of timber; this is majorly caused by one or both of the following;

·         An attack by wood-destroying fungi

·         An attack by wood-boring insects

Dry rot (weeping fungus); this is the most common wood-destroying fungus known as Merulius lacrymans that is mainly found in damp conditions. It’s also more serious and more difficult to eradicate than other fungus. It attacks the cellulose found mainly in sapwood causing the affected timber to:

·Loose strength and weight

·Develop cracks, both with and across the grain

·Become so dry and powdery that it can be easily crumbled in the land

The initial factors that will lead to an attack of dry rot in timber are: damp timber i.e. with moisture content above 20%, bad or non-existent ventilation i.e. where there’s no circulation of air. Ways of prevention include the following;

· Always keep all timber dry (even before fixing into the building)

· Always ensure good ventilation

· Always use well-seasoned timber

· Always use preservative-treated timbers in un favorable or vulnerable positions

Wet rot (celler rot); this is another common type of wood-destroying fungus known as Coniophora cerebella that is mainly found in wet conditions e.g. neglected external joinery, ends of rafters, under leaking sinks or baths, under impervious (water proof) floor coverings. Ways of prevention isn’t complicated since wet rot doesn’t normally involve such drastic treatment to eliminate as dry rot as it doesn’t spread to adjoining dry timber. What is required is to cure the source of wetness and where the decay has become extensive, or where the structural timber is affected, some replacement will be necessary.

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