STEP A
Objectives
To set out guidelines relating to the use of timber in a range of climatic conditions and limiting need for preservative treatment
compromising load-bearing integrity and durability of timber or wood-based product.
Prerequisite
A Durability - Preservative treatment
Summary
This lecture begins with an examination of the various conditions which Rvour biological attack by fungi or wood-boring insects (including termites). Particular attention is given to the influence of geographical zone and its corresponding climate. Practical examples are provided which identify building details to avoid, and of construction which are recommended.
Introduction
Timber is susceptible to biological whereas metal components may corrode.
Biological attack is of two types:
This occurs in timber which has a high moisture content, generally between 20%
and 30%. Fungi need the presence of water and oxygen to develop and the optimum moisture content varies according to the particular fungal species.
presence of fungal attack can seriously reduce the load-bearing capacity of timber structures. The loss of strength may be variable, depending on type of fungus and extent of attack. Significant strength loss may be present, even in cases where the appearance of timber largely unchanged.
Where possible, design of the building should minimise the situations in which timbers are to levels which allow fungal decay, fungi, such as sepiaria, can survive through dry periods and continue attack in which are subject to intermittent wetting. Therefore, design should provide conditions which:
- prevent wetting of timber wherever possible;
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ensure rapid drainage and ventilation of the where it is to reproduction. Termites are particularly aggressive to timber, are only active in of Europe. Their presence and activity decreases towards theEU
Northern parts of Europe. presence of central heating within buildings may the activity and development insects by moderate during the parts of year. Insect larvae die if subjected to low temperatures. The development of cracks or splits in preservative treated timber which penetrate the treated layer may provide sites for egg laying or the initiation of attack, significantly reducing the value the treatment.
natural durability of different limber species i n relation insect attack is variable. In most timbers, heartwood is normally durable, but heartwood different species shows various levels of resistance to termite attack. The sapwood region or may not be depending upon the timber species and insect type. The sapwood of all timbers is considered susceptible to termite attack. EN 350-2 durability of solid wood - 2: Guide to natural durability and of selected wood species of importance Europe" gives an indication of the durability to insect attack for timber species (Table In plywood construction, the natural durability be by the presence of chemicals associated with synthetic resin
name Termite to the variation of timber performance with moisture content:
is characterised by a moisture content in components corresponding to a of and a relative humidity of the surrounding air only exceeding 65% for a few weeks per year (maximum in the timber).
2 is characterised by a moisture content in components
to a of 20 and a relative humidity of the surrounding air. only exceeding 85% for a few weeks per year (maximum 20% in the timber).
3 involves conditions leading to higher contents than in service class 2.
In EN five hazard classes are defined with respect the risk of biological attacks:
- Programme
class I , situation in which timber wood-based product is under cover, from the weather not exposed wetting;
2, situation in which timber or wood-based product is under cover and protected weather but where high environmental lead to occasional but not persistent wetting;
3, situation in or wood-based product is not covered and contact ground. It is either continually exposed to the weather or is protected from weather but subject to wetting;
4, situation in which or wood-based is in contact
the or- fresh water is exposed to
5, situation which timber or wood-based is permanently exposed to salt water.
The examination of these classes shows that service and hazard classes I to conditions as service and hazard classes 2. The service class 3, however, relies on preservative treatment to laminations before gluing and assembly, as subsequent planing necessary to produce a would remove part of the effectively treated outer zones. heartwood regions, where penetration is this expose portions of untreated core. Suitable supplementary
is necessary.
Designing
for
durabilityMany factors come into play concerning the durability of timber. should be installed close to the estimated moisture content, appropriate to building, so is only necessary to limit variation during year.
which is installed at too content or is directly exposed to weathering or climate conditions produce wide variations in air humidity is likely to show cracks or splits by shrinkage. These can expose unprotected timber in preservative material, allowing water and spores to enter- or- insect eggs to be laid beyond the zone.
designer to consider moisture variation induced by:
- water liquid state,
Examples of some situations which result in high moisture contents in timber include:
- Moisture will easily penetrate timber placed in warm, air, for example in poorly ventilated attics where ventilation shafts emerge.
- Joints between timber elements or between timber masonry constitute an area end grain may be exposed to air if occurs after drying.
- Condensation can result in timber becoming cot. Condensation can be controlled by insulation combined a vapour barrier. Drainage should be provided where condensation is likely to occur, at the base of glass walls.
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Direct wetting will occur in in ground contact, in door window and in areas, where wind driven snow may collect. Consideration should be given to the risk of direct wetting in rooms where water is laid on, such as showers, bathroom, where there be an overflow or splashing.-
Water may become trapped waterproof barriers in walls, in the ground or in mechanical joints, preventing its natural elimination by evaporation.Arrangements be made to avoid the accumulation of water close to metal plates.
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Wetting are increased during storage on site and building erection prior rooting. Timber stocks should be covered arid only be left on the site forthe necessary for erection, one at most,
especially in bad weather.