Atmospheric impacts - weather.
The sun’s ultraviolet rays - along with the elements (rain, snow etc.) - consume the lignin in the wood. The lignin is the supporting material of the wood. As a result the wood surface becomes yellow and then grey. It is no longer water repellent and the surface treatment is no longer effective.
There is a common element or key characteristic in attacks on wood: humidity. There is a strong relationship between the environment in which the wood is used for construction purposes and the attacks by harmful biological agents. The European Committee for Standardization (CEN), through European Standard EN 335, has defined 5 risk classes based on the humidity to which wood is exposed under various application conditions (see page 13). Other factors considered in the risk classes include whether wood is in contact with the soil, clean water or sea water, or its exposure to weather conditions (indoor, covered and protected, or outdoor). Sometimes different risk classes overlap, particularly classes 2 (occasional humidity) and 3 (frequent humidity).
When in doubt, the highest class should be chosen. The higher the risk, the greater the need to increase the wood’s natural resistance through impregnation treatments. Illustration shows the application of an impregnation product on wood. The active ingredients are locked up in the impregnation film.
Wood swells and shrinks. Cracks appear. Attacks by decay fungi and blueing occur due to high moisture content.
Rain brings about an increase in the moisture content of wood. It also favours cracks and fungi development.
Wood discolours and becomes grey due to the impact of the sun’s UV-rays. UV rays do not penetrate wood deeply. They focus on the surface, as lignin is degraded, fibre adhesion is impaired, and the wood turns greyish. Degradation is a slow process.
Cracks appear, when wood is exposed to frost or heavy heat, as changes in temperature causes changes in the wood’s dimensions.
Infra-red rays heat up the area exposed, causing cracks along the grain and resins to rise. Their action is extremely harmful, since sooner or later cracks appear in the wood surface.
Wood’s appearance degrades.
The effect of sun rays on wood mainly occurs through ultraviolet and infrared radiation.
Changes in temperature cause changes in the wood’s dimensions.
Wood is a hygroscopic material, which means that it may release or absorb humidity from its environment. During this process, the wood’s dimensions change. When wood absorbs humidity, it swells and increases its dimensions. If wood releases humidity, it shrinks. The degree of swelling or shrinking varies considerably depending on the wood species. In general, these changes are more significant in high density woods.