Blue mould affects the properties of wood. Under what conditions can affected wood be used in construction

Sometimes wood in building material stores, on building sites or in furniture factories has areas of bluish discolouration. The stains are not just on the surface but go deep into the joist or plank. This is blue mould or blue stain, a disease that can appear in wood even before it is cut. Being different from normal, healthy wood, the question arises as to whether it can be used. If the defect is mainly an aesthetic and finishing problem for furniture, the question for construction wood is its strength. Is the compressive strength of wood affected by the presence of blue mould? Can blue moulded wood be used in the structural strength of a house? The answers to these questions can be found below.

What causes blue stain on wood

Wood blight is caused by the fungus Ophiostomatales of the family Sordariomycetes, a spore-forming fungus. The mycelial filaments are brown, but as they develop in the wood they turn bluish. The fungus can occur in wood before felling, between felling and planing, or during natural drying in the pile. When it appears in the log, the stain is more extensive, sometimes covering the entire sapwood area. In planed timber it is more localised and is found mainly where the plank rests on the boards.

The development of the fungus is favoured by the presence of oxygen and water. In the standing tree it only occurs if the bark is damaged. The amount of water in the tree is high, which leads to a low amount of air and therefore oxygen. However, if a wound appears in the bark, the area dehydrates and allows air to enter, favouring the appearance and development of the fungus. Another factor that allows development before cutting is the presence of bark beetle. The bark beetle perforates the bark, but is also a means of transport for spores.

In felled trees, the long time between felling and cutting favours the appearance of blue mould. The fungus develops at a water content in wood above 30%. If the log is not felled, the water is released more slowly and the fungus can develop. It is therefore recommended to cut the log into planks as soon as possible after felling. However, if the cutting has to be delayed, it is advisable to keep the logs in water or to sprinkle them.

In the case of wood laid natural drying, The leaks prevent water from escaping into the support areas of the planks and there is a risk of blue mould growing there. Because it develops when the water in the wood is above 30%, dry wood is not affected by blue mould, even if the outside humidity is very high. Only if it sits directly in water for a long period and the humidity rises above this limit is there a risk of infestation.

The fungus grows fastest at 25°C, but it also grows at lower temperatures.

The difference between blue and black mould on exterior wood

Even though it's called mold, wood bleaching is very different from black mould that appears on houses or wooden objects standing outside. Blue mould needs moisture to be in the wood, it grows inside the wood and is not dangerous to human health. During processing, infested wood does not release spores or other substances harmful to humans or animals.

Unlike blue stain, black mould can appear on the surface of wood even when it is finished. Spores are easily carried by the wind from side to side and all it takes is to find damp, shady places and they grow easily. Development is favoured by the humidity of the outside air. Spores released by black mould are dangerous and have a negative impact on human health.

How blue mould affects wood

Blue mould does not affect or imperceptibly affects the cell wall of the wood and therefore its strength does not change. However, water permeability is affected, so the infested wood absorbs more water. If a wood attacked by blue mould is kept in water or rained on, it will absorb water faster than a healthy wood.

Because blue mould does not affect the wood cell, it is assumed that it does not affect the equilibrium humidity of it. This is why affected wood does not absorb moisture from the atmosphere (water vapour), even though it has the ability to absorb water more quickly.

Bleaching affects the resistance of wood to rot. An infested wood not only absorbs water faster, but also dries out much more slowly than a healthy one. The fungus is water-loving and creates the conditions for it to stay in the wood as long as possible. Unevenly scattered spots in the wood are also gateways for water to enter, creating the conditions for water retention and wood rot.

When infested wood can and cannot be used

In view of the above it can be said that wood affected by blue stain can be used in construction if it is protected from damp. Its compressive strength is not affected so it can be used for the manufacture of frames for interior wallsfor beams or support trusses or other such works in places where there is no risk of water. It is not recommended for use in roof construction, for roof battens or roof support posts.

Blue stained wood shall not be used in the construction of windows, exterior doors, pergolas, gazebos, landscaping or for cladding houses. Even if it is coated with water protection treatments, it will be less resistant than sound wood. The slightest crack in the finish will cause water to penetrate and the wood will absorb it very quickly. Water will stagnate inside, mould will develop and eventually the wood will rot.

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