Atmospheric impacts - weather

Ultraviolet rays

The sun’s ultraviolet rays, along with the elements (rain, snow, etc.), break down the lignin in the wood. 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.

Humidity

There is a common element in attacks on wood: humidity. There is a strong relationship between the environment in which wood is used for construction purposes and attacks by harmful biological agents. The European Committee for Standardization (CEN), through European Standard EN 335, has defined five risk classes based on the level of humidity to which wood is exposed under various application conditions (see page 13). Other factors considered in the risk classes include whether the wood is in contact with soil, clean water, or seawater, and its exposure to weather conditions (indoors, covered and protected, or outdoors). 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. The illustration shows the application of an impregnation product on wood. The active ingredients are locked into the impregnation film.

Humidity/Water

Wood swells and shrinks. Cracks appear. Attacks by decay fungi and blue stain occur due to high moisture content.

Rain

Rain increases the moisture content of wood. It also promotes cracking and fungal development.

UV rays

Wood discolours and turns grey due to the sun’s UV rays. UV rays do not penetrate wood deeply. They act on the surface, degrading the lignin, weakening fibre adhesion, and turning the wood grey. This degradation is a slow process.

Temperature changes

Cracks appear when wood is exposed to frost or intense heat, as changes in temperature cause changes in the wood’s dimensions.

Infrared rays

Infrared rays heat the exposed area, causing cracks along the grain and resins to rise. Their action is extremely harmful, as cracks eventually appear on the wood surface.

Pollution

The appearance of wood deteriorates due to pollution.

The sun

The effect of sunlight on wood mainly occurs through ultraviolet and infrared radiation.

Temperature

Changes in temperature cause changes in the dimensions of the wood.

Ambient humidity

Wood is a hygroscopic material, which means it can absorb or release humidity from its environment. During this process, the wood’s dimensions change. When wood absorbs humidity, it swells and increases in size. When it 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.