Natural durability

The biological resistance or natural resistance is very different for different types of wood. Sapwood - that is, the outer portion of the stem - always has low resistance. The heartwood contains extractives that discourage fungus and reduces the wood's tendency to absorb water.

Estimation of life of different types of wood exposed in the open air
WoodExposed in the openIn the open air under roofConstantly dry
Alder, birch, poplar 3-20-40 years 3-20-40 years <400-500 years
Elm 60-80-100 years 80-130-180 years <1500 years
Ash 15-40-60 years 30-60-100 years 300-800 years
Book 10-35-60 years 30-60-100 years 300-800 years
Oak 50-85-120 years 100-150-200 years 300-800 years
Pine 40-60-85 years 90-100-120 years 120-1000 years
Spruce 40-55-70 years 50-60-75 years 120-900 years
Larch 40-65-90 years 90-120-150 years <1800 years

The table presents the estimated life of the wood exposed in the open air.As the tree grows new cells are formed into the sapwood and the older wood is converted into heartwood. The transition from sapwood to heartwood is for many types of wood visible on the log cross-section as a color difference. Usually have a lighter sapwood and heartwood a darker color.

The species-specific chemical substances stored in the heartwood called extractives. The extractive gives each species characteristics such as color and natural resistance. Wood extractive consists predominantly of resin acids, fatty acids, carbohydrates and minerals (ash). Some woods have a high concentration and a certain composition of extractives that provides a good natural resistance to biodegradation.

Resistence in soil contact heartwood

Different species have different natural resistance to microbial degradation. For all types of wood, the heartwood usually has better resistance than sapwood. At Princes Risborough Laboratory in the UK field tests of a large number of species have been carried out. Rods of heartwood of dimensions 50 x 50 mm have been stuck into the ground, then the natural resistance in contact with soil have been classified.

The heartwood is composed of "inactive" wood cells. The openings between the fibers which are in the sapwood and allowing water transport are here closed, and no longer work as a transport. Heartwood is usually quite resistant to water transport, especially in the tangential and radial surfaces while in the end grain can make some absorption. Sapwood usually sucks up far more water than the heartwood.

The decomposition of wood

Newly cut wood exposed outdoors grays relatively quickly. Sun light UV radiation breaks down mainly lignin in wood surface. This entails, in addition to the growing gray color, the lignin that binds together the wood cells in the wood surface disappears. The result is a wood surface with loose fibers.

How much loose fibres there are on the surface of a wooden structure has an impact of course on how surface treatment and maintenance can be carried out and how good the result is.

Resistance to chemicals

Wood is relatively resistant to chemicals, to acids with pH above 2 and to alkalis at pH below 10. Wood is therefore used in aggressive environments where other materials are broken down. It is common to warehouses for beer and salt are constructed with wood framing materials.

Wood can be used for piping above ground or in ground or water. Tubes of wood are particularly common in process industries where other tubes would not withstand the rigors of various chemicals.

Microorganisms

Wood is a renewable building material with a variety of uses. With the trees in nature is part of the natural cycle, the woodwork in our constructions is re-infected by micro-organisms. For an attack to occur, however, it is required that the right environmental conditions occur.

Wood's resistance to microbial attack differs between timber types. There are also differences between sapwood and heartwood. If a building is hit by a rot or mold damage, it is always a sign of underlying moisture problems.

Mold and fungi can grow on and in moist building materials, and it is mainly fungi which shortens the life of wooden structures.

Moulds grow on cellulose-based material, as well as other building materials can be affected by fouling, such as plasterboard, insulation materials, concrete and painted surfaces.

Resistance of the ground contact of the heartwood of various woods
Expected duration of ground contact, yearsWood
<5 Alder, birch
10 Spruce, pine
15 Larch
15-25 Oak
<25 Teak
Three standards, EN 350, part 1-2 and EN 460 classifies the natural resistance to rot fungus. EN 350-2 is of particular interest as it presents a classification of the natural durability of the heartwood of a hundred commercially valuable species.

Natural resistance to rot fungus. Classification of the heartwood of some species according to EN 350-2.
WoodDurability Class
Larch 3-4
Spruce 4
Pine 3-4
Douglas fir (European) 1-2
Iroko 1-2
Oak 2
Teak 1-3
Types of wood with the best natural resistance are in class 1. Class 5 is volatile.

Different values of relative humidity (RH) needed for a mold damage to emerge at ambient temperature
Material GroupCritical moisture condition [% RH]
Soiled materials 75-80
Wood and wood-based materials 75-80
Plasterboard with cardboard 80-85
Mineral wool insulation 90-95
Foam Insulation (EPS) 90-95
Concrete 90-95

 
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