Attack mechanisms

4.3.1 Mould

Moulds do not affect the structural performance of timber and wood-based composites, but their occurrence in indoor environments may pose health threats. Mould on timber usually occurs if the surrounding air temperature exceeds 20°C combined with relative humidity in excess of 80% for more than brief periods. At lower air temperatures, only very high humidity levels will cause mould on timber.

4.3.2 Decay

At temperatures between 0°C and 60°C decay fungi attacks can progress rapidly when the timber has a moisture content above the fibre saturation point (^≥ 30%) [72]. Therefore, such conditions should never be deliberately created internally or externally. The existence of mois-ture conditions in timber that sustain decay fungi are always the result of bad initial design, poor workmanship, inappropriate building renovation methods, unintended uses of buildings, and/or improper building maintenance. Excessive moisture in timber is often associated with

4.3 ATTACK MECHANISMS  49

capillary action (also known as wicking) and/or water entrapment with water coming from the ground, rain, or leaks within buildings. Thus, decay prevention must focus on practices th at keep timber dry.

Figure 4.2 illustrates mechanisms by which moisture typically enters structural and non-struc-tural elements of traditional timber structures. These examples illustrate that in all cases the problems result from poor or shoddy practices. Site investigations of buildings always reveal that best practices were not adopted during at least one stage in either design or maintenance.

Under proper practices, wetting of timbers should never occur except under rare or accidental events such as damage to glazing, spillage, or leaks.

This SED does not recommend impregnation of timber with chemical preservatives as a primary means of mitigating risk of decay in critical structural elements, because such treatments have not been proven reliable over the long term. Instead, the recommended practice is to employ durable design strategies at system and element levels that eliminate the hazards. In practice, this recommendation translates into designing the building superstructure (and foundation, where applicable) systems so that water is drained away from timber members and to ensure that intentional spaces are ventilated. Elements should be designed to have geometries that do not entrap moisture. This does not mean that exposing non-structural or non-critical struc-tural elements to risk of decay is not an acceptable design decision. However, any replaceable

(a) (b) (c)

(d) (e)

Wood based Material

Decay pocket Gap

Rain

Impermeable skin

Rain

Decay pocket Check 

Decay pocket Rain

Wood sucks up water from soil via footing

Sandstone

foundation Moist

soil Dry air

Masonry wet by rain

Wood absorbs moisture from masonry

Fig. 4.2: Avoidable mechanisms for moisture accumulation in traditional timber low-rise con-struction: (a) direct wetting by rain; (b) water ingress via surface check; (c) water ingress via construction joint; (d) moisture wicking via stone foundation; (e) moisture wicking via masonry contact (courtesy of CSIRO)

elements that are allowed to decay should not be directly attached to critical structural elements, should be separated from them by impermeable barriers (e.g. vapour membranes), and should be intentionally selected as sacrificial.

Models exist for estimating service lifespans of elements based on measures such as the time taken for non-structural elements to develop a certain depth of surface decay under a particular external service use, or the time for a certain depth of surface decay of non-critical structural elements in contact with the ground [73]. These should be used where appropriate.

4.3.3 Termites

Termite attacks are possible in all ecosystems where the mean annual temperature is greater than 10°C, with speed of travel and ferocity of attacks tending to accelerate with any increase in temperature. Boundaries of termite infestations can advance at around 1 or 2 km per year.

Therefore, maps showing areas of infestation are only rough indicators of whether a termite hazard might exist within the design life of a specific building (Fig. 4.1c). Accurate estimates of possible termite arrival dates can be made using termite attack models and up-to-date infestation data as discussed by Leicesteret al. [74].

Although unlikely, termite attack may exist where the mean external temperature is below 10°C.

Termites have spread northward in both North America and Europe, because they are sometimes transported along with traded commodities (e.g. in packing cases). For example, viable colonies exist in the core areas of several Canadian cities because metro systems and other underground spaces such as pedestrian walkways provide year-round conduits for termite movement under sufficiently warm conditions [75]. A rarer possibility also exists that termites get blown by wind onto the roofs of high-rise buildings during their alatory phase. They then establish colonies where rain water has penetrated building envelopes, as has been observed in Hawaii and else-where [76].

4.3.4 Corrosion

Corrosion attacks unprotected metal fasteners inserted into timber in two ways (Fig. 4.3). In the first, airborne salt attacks exposed parts of fasteners. In the second, the acidity of any impreg-nated chemicals for biological or fire protection and natural acidity of timber itself become agents of attack on the embedded parts of fasteners. In the case of joints with bolts fitted in tolerance holes, the corrosion rate of shanks is enhanced by any salt and water that accumulate within oversized holes (Fig. 4.3c). This type of attack may be mitigated by protection mea-sures such as covering a shank with a plastic sleeve, or using tight-fitting dowels instead of loose-fitting bolts. Notably, using tight-fitting dowels, instead of bolts in tolerance holes also has structural benefits in terms of greater stiffness and strength. Practical models are available for estimation of rates at which exposed and embedded parts of steel fasteners in timber will corrode [69,77,78].

The corrosion rate of embedded fasteners tends to be quite low, unless the timber has been treated with a mineral-based chemical, such as alkaline copper quaternary (ACQ), or if the timber has high moisture content [79]. For the case of atmospheric corrosion, the rate for mild steel can be very high for buildings located within 1 km of saltwater (Fig. 4.4). Corrosion rates