Defects in Timber

1. Defects in Timber • Timber develops many growth characteristics during or after milling. These may be considered defects if they are undesirable visual attributes or structurally affect the timber. • A blemish in the timber may not affect it structurally but will render the piece unusable for joinery. A piece of timber may have a defect in it that reduces it’s structural adequacy but enhances it’s aesthetic qualities. • There are three main groups of Timber Defects:- • Natural • Seasoning • Conversion

2. Natural Defects • Natural Defects are described as features developed in the living tree, or soon after it is felled which may detract from the usefulness of the timber. • Often it is more economical to remove defects such as dead knots and replace them with plugs. Today building up long lengths from shorter glued pieces is becoming more common. Building up members by lamination also tends to to more economical use of timber.

3. Examples of Natural Defects • Brittle Heart – found in the centre of trees and results in breaks with a brittle or ‘carrotty’ fracture • Sapwood – Most softwood timbers come from immature trees having a high proportion of sapwood. The boards having a small section of sapwood along one edge or face are susceptible to fungi and insect attack and must be treated with preservatives. • Wide Growth Rings – these indicate rapid growth resulting in thin walled fibres, with the consequent loss in density and strength. • Gum Pockets – Cavities (pockets) in the timber filled with gum. It may affect the structural quality of the timber, Appearance may be the biggest problem.

4. Examples of Natural Defects • Gum Veins – A lengthwise ribbon / vein filled with gum which is not breached with wood tissue. It will affect the structural ability of the timber and the appearance. • Shakes – Partial or complete separation (splitting) between adjoining layers of the wood fibres. Depending on the shape and location they can be classified as Cup, Heart or Star shake. Shakes can also be formed during seasoning if the timber is subject to heat or cold. • Knots – Parts of branches which become enclosed in a growing tree. They are very common in timbers such as Douglas Fir, Cypress Pine and Radiata Pine, but are comparatively rare in mature Australian hardwoods. The knots can be ‘sound’ and ‘unsound’. ‘Unsound’ knots may fall out and reduce the strength of the timber while the sound knots may enhance the attractiveness of the timber.

5. Grain Defects • Grain Defects – these are natural grain features but are referred to as defects because they cause working difficulties and affect the overall strength of the timber. Apart from some growth factors they are generally caused by the log not being sawn parallel to the bark.

6. Examples of Grain Defects • Cross Grain – Caused when fibres have varying inclination to the edges of the timber. • Diagonal Grain – When fibres run at a constant angle or slope to the axis of the timber • Interlocking Grain – When fibres of adjacent layers in the growth ring are inclined at different angles to the axis. Boards show ribbon or stripe effect.

7. Examples of Grain Defects • Spiral Grain – Occurs when fibres have taken a simple spiral course in the growing tree as if the tree has twisted. • Wavy Grain – A wavy undulating arrangement of the fibres • Wane or Waney – The absence of wood on the edge or corner showing the bark or surface of the sapwood, (due to inproper conversion).

8. Seasoning Defects. • These are defects caused during the drying of the timber. Extreme care must be taken to control the rate and degree of drying. More than half the weight of many freshly cut timbers consist of moister or sap. Seasoning is simply the drying-out , so that timber is conditioned (or pre-shrunk) before use. • Seasoning reduces the moister content of the wood to that of the relative humidity of the atmosphere, i.e. equilibrium moister content. Shrinkage is inevitable during drying and efforts are made to control the shrinkage without undue distortion and splitting.

9. Examples of Seasoning Defects • Split – A longitudinal crack extending through the log or timber from one surface to another, usually at the end of the piece. • Check – The separation of the wood along the grain, usually in the direction of thr rays. • Collapse – The flattening or distortion of the cerlls showing excessive or uneven shrinkage. It can sometimes seriously affect the structural soundness of the timber. • Case hardening – The exterior surface or ‘case’ of the timber dries out before the interior or ‘core’ commences to shrink. • Sap Stain – A stain caused by certain fungi which feed on the content of the cells of the sapwood. The most common is a bluish stain which penetrates deeply into the wood of light coloured timbers, many pines are susceptable.

10. More Examples of Seasoning Defects • Warp – Any variation of as piece of timber from a flat surface. This tenancy is caused by the way a piece of timber is cut, the presence of other defects, and natural stresses in the tree from which the timber is cut. a) Bow or Camber – a curvature along the wide surface in the direction of the length of the board. In floor joists and bearers this can be eliminated by using blocking. b) Cup – a concave curvature across the face of the board c) Spring – a curvature along the edge of the board, not affecting the face. d) Twist – also known as ‘wind’ is a spiral distortion along the length of the board caused by twisting of the fibres.

11. How the Natural Environment Affects Timber Durability • The natural environment can degrade timber where brought about by: • Decay e.g. damp and micro-organisms causing rot • Insect damage e.g. termites and Bettles (borers) • Weathering damage e.g. cyclic wetting/drying and ultra violet light cause discolouration and cracking

12. Defects caused by insects • Termites are the most serious danger to structural timbers as far as insect attack is concerned. They attack the sapwood and the heartwood of most timbers, but prefer the sapwood and the pith. • Some Australian timbers, because of the cellular structure, are resistant to termite attack,for example • Cypress Pine • Jarrah • Grey Ironbark • Red Gum • Tallowwood • Turpentine

13. Termites are of two wide species • Subterranean species, which live in the ground • The dry wood species, (Arboreal) which live in timber above the ground • Subterranean or ground dwelling termites can often be found in pieces of timber that are left on the ground. • Ground dwelling termites can often be found in pieces of timber that are left on the ground. These termites cause the most damage to buildings by preceding from their colony through formed subterranean galleries to the timber. The colony can be enlarged to encase a timber member. • Smaller tunnels may radiate out from one member to another e.g. bearer to joist. This enables the termites to find more cellulose material ( which is contained in the sapwood).

14. Subterranean species and The dry wood species, (Arboreal) continued • The dry wood species are wood dwelling and able to live in seasoned timber with a low moisture content. Colonies are situated in and between timber members such as in a wall frame or tree branch. • Subterranean termites travel inside tunnels, as they avoid light and prefer darkness. Metal ant caps used on top of piers and walls will not prevent termite infestation but they can make it obvious as they build out over the angled metal. Creosote is a method of deterring termites but one must ensure that the timber is coated on the ends. • Treatment can be carried out after completion but it is of course easier to do it during construction. Common methods today are by mechanical prevention such as Granitgard, Termimesh and TERMISHIELS. Spraying is no longer carried out ( especially not with organochemicals).

15. Borers

16. Timber Decay and Prevention

17. Characterizing Natural Environments Affecting Durability • Decay and insect attack are the main areas of concern. Timber exposure is characterised according to different hazard levels (Source AS1604): H1 - Inside and above ground; only exposed to insects other than termites H2 - Inside and above ground; exposed to moderate decay, borers and termites H3 - Outside above ground ; exposed to moderate decay, borers and termites H4 - Outside in ground; exposed to severe decay, borers and termites H5 - Outside in ground or fresh water; exposed to very severe decay, borers and termites H6 - Marine water exposure; exposed to marine wood borers and decay • Steps must be taken to ensure the right type of timber is selected to meet the needs of the different hazard levels

18. Examples of Timber Usage in H1 and H2 Environments • H1 applies to framing, flooring, furniture and interior joinery but only where the timber being used is exposed to attack from insects other than termites e.g. timbers affected by Lyctid borers • H2 also applies to framing, flooring etc. but is inclusive of exposure to termites and moderate decay

19. Examples of Timber Usage in H3 Environments H3 applies to weatherboards, fascia, window joinery, exterior framing and decking

20. Examples of Timber Usage in H4 Environments H4 applies to fencing, greenhouses, pergolas and landscaping timbers

21. Examples of Timber Usage in H5 Environments H5 applies to retaining walls, house stumps, building poles, cooling tower fill

22. Example of Timber usage in H6 Environment H6 applies to marine piles, jetty cross bracing, landing steps, boat hulls

23. Choosing the Timber to Suit the Environment • For low hazard environments non-durable timbers suffice • For intermediate to high hazard environments use: • Timber species that are naturally durable and/or, • Timbers that are chemically treated to improve natural durability and/or • Design the building to protect the timber from the environment

24. Naturally Durable Timbers • Grade 1 e.g. Turpentine • Grade 2 • Grade 3 • Grade 4 e.g. Radiata pine (untreated) • All timbers in Australia can be classified according to their natural durability for both in ground and above ground applications (refer AS5604) Most durable Least durable

25. Treated Timbers • Timber can be chemically treated to improve durability • Chemicals act as a fungicide and/or insecticide • Chemicals only absorb into the outer layer of the timber so care must be taken to patch or protect cut ends in areas of concern • Levels of chemical treatment vary to suit levels of hazard e.g. H1 ………..H6. Such timbers are branded to assist choosing a timber to suit the environment. treated area

26. Designing the Building to Protect the Timber Replaceable and sloping capping rail • The durability of timber can be improved by designing the “building” to help: • Use claddings, roofs, replaceable cappings, paint systems and sloping surfaces • Ensure timber posts are elevated above water e.g. using post brackets • Detail connections to avoid water entrapment in joints • Ensure subfloor construction incorporates an inspection zone to detect termite attack and allows appropriate levels of ventilation  