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APPENDIX
D The factors affecting the life of wood elements under the severe conditions likely to be experienced in cemeteries are:—
MECHANICAL DAMAGE Damage from vandalism, the operation of gravedigging equipment and mowers etc., is largely dependent on the degree of supervision possible which in most instances would be virtually negligible. Some timbers which might be chosen for durability against weathering and decay (e.g. Californian redwood, western red cedar) could be very soft and easily damaged. WEATHERING Wood is by nature absorbent of moisture and the surface layers readily take up dew and rain, with consequent expansion of the wood substance. Then the sun heats up the surface and the air humidity is reduced, resulting in contraction so the surface layers are continually buffeted by dimensional change. A multitude of fine surface cracks will often form and thus assist the erosion of the surface, a process speeded up by the softening of those surface layers by fungal organisms encouraged to develop by continuing dampness. End grain is more susceptible to breakdown than side grain because of its much greater absorbency so the provision of some inhibitor of water entry (e.g. metal caps on the tops of fence posts, coatings of bitumen or paint on other end grain) can be helpful. DECAY Decay (or ‘rot’ as it is commonly called) is the breakdown of the constituents of wood by various fungi when the moisture content of the wood is favourable to their growth. Wood kept reasonably dry (say, below 20% moisture content), or when saturated with water, is usually safe from attack. An illustration of the conditions favouring attack can often be observed on old fence posts removed from the ground: most of the decay will be within the zone 300mm above to 300mm below the groundline in the zone where the moisture content of the wood will be in the range of say 20% to 50%. The wood of the tree’s stem may be subdivided into sapwood and heartwood. The sapwood is the usually paler coloured wood just under the bark and often about 25 to 35mm wide. It is the conductor of the life processes of the tree and usually contains a lot of sugars and starches which enhance its attractiveness to the decay organisms. The sapwood of all species is liable to decay readily. Heartwood is non-living tissue; when it is converted from sapwood the connections between cells become blocked with materials with varying degrees of toxicity to fungi, depending on the tree species. The blocking of the cells also makes the heartwood much less absorbent of moisture. While density is a useful guide to the comparative durability of species, there are many exceptions (e.g. cedar, redwood). The very dense Australian eucalypts such as ironbark, grey gum, tallowwood and white mahogany have excellent durability but the colder climate ash—type eucalypts are only of moderate durability. The presence of sapwood is advantageous when preservatives are to be impregnated into the wood because of its greater permeability; otherwise, all sapwood should be removed from components which are to be exposed to the elements or ground contact. It should be noted that it is very difficult to obtain penetration of preservatives into the heartwood of most species except under very specialised and costly conditions. INSECT ATTACK Termites and borers cause significant damage to wood but termites are by far the greater hazard, especially the subterranean termites which cause millions of dollars damage each year to buildings throughout Australia, except except for the colder districts of Tasmania and south—eastern Victoria. Because of their subterranean habits their presence often goes unnoticed until considerable damage has been done and only an outside shell of untouched wood remains. Wood in ground contact can be protected by treating the adjacent soil with solutions of the termiticides chlordane or dieldrin. Such work should be carried out by qualified operators, with care taken to ensure that children and animals are kept away from the treated soil. The presence of borer holes on a piece of timber does not mean that it is under attack. Some borers, such as the common pin—hole borer die soon after the log is sawn up and reinfestation does not occur. The only type liable to cause some concern in cemetery wood components is the lyctid borer which attacks only the sapwood of some hardwoods; this attack usually occurs within the first year or two of service. Since only a small section of the component is likely to be affected the attack can usually be ignored; if extensive, replacement of the affected component is preferable to attempts at chemical treatment. FIRE If cemeteries are allowed to become overgrown with vegetation the resulting fire hazard presents a great risk to wood components. Timber of large section area does not burn readily since its low thermal conductivity slows the penetration of heat. However, most wood components will be of small section and ignite readily. Most commercial fire retardants are water soluble and thus only suitable for treating internal members. For further information see Bootle, K. 1983 Wood in Australia McGraw Hill |
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