6 Wood Discoloration

7.4 Protection 147
(Willeitner and Schwab 1981; Erler 2002; Willeitner 2000, 2003; Goodell et al.
2003; Böttcher 2005; Borsch-Laaks 2005; Schmidt 2005) by:
– organizational protection (e.g., short and appropriate wood storage),
– use of durable wood species (natural methods),
– keeping away water by structural wood protection measures by design:
appropriate surface and weather protection, use of vapor barriers, avoidance
of condensation due to thermal insulations, salient roof to protect timber
from rain, drawing off of rain, barrier to avoid direct contact between wood
and adjacent material, or inside the wall against raise of moisture from the
ground,
– chemical wood preservation,
– wood modifications that increase dimensional stability of wood, reduce
uptake of moisture, or make it hard to digest,
– use of antagonisms.
The moisture conditions in wood are of decisive importance for the development
of wood fungi (Chap. 3.3). Table 7.5 shows the hazard classes of wood
[to be replaced by “use classes” according to prEN 335-1 (2004) respectively
ISO] that depend on wood use and timber moisture according to the German
standard DIN 68800, parts 2 and 3 (1990, 1996), the corresponding potential
application of durable timber, and the minimum requirements of chemical
preservation measures.
Natural durability means the wood-own resistance against bacteria, wooddecay
fungi, beetles, termites and marine borers, which will differ for a timber
species against the various organisms. Wood durability is based on the presence
of accessory compounds, whereby it concerns numerous compounds from
different chemical classes (Fengel and Wegener 1989; Obst 1998). They are produced
in the living tree during transition from the sapwood to the heartwood
and are deposited in the heartwood (Taylor et al. 2002). Thus only the heartwood
exhibits natural durability, while the sapwood of all wood species is only
little or not durable. The European standard EN 350-2 (1994) uses a five-class
system to group 128 timbers according to their durability against fungi. Wood
with high durability against fungi (durability class 1, very durable) is e.g.,
greenheart (durable also against termites and marine organisms). European
oak is durable (class 2), walnut is moderately durable (class 3), Norway spruce
is slightly durable (class 4), and European beech not durable (class 5) (also Augusta
and Rapp 2003, 2005; Willeitner 2005a). Natural durability of some bamboo
species against four decay fungi was investigated by Remadevi et al. (2005).
The influence of the felling time on resistance is controversially discussed.
It has to be considered that fresh winter-felled wood is less susceptible to
damage due to other moisture, drying, and climatic conditions than wood
felled in the summer. There are however no differences if the wood is carefully
dried (Willeitner 2005a).
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