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  • Writer's pictureBenny Kuriakose

Timber Decay and Ways to Prevent Fungal Attack on Timber

The biological invasion of specific species of fungi within the timber causes decay in timber. The conditions required to prevent the growth of fungi are to avoid oxygen, moisture and nutrients, with moisture being the most important factor. The timber rotting fungi can remain inactive for years, waiting for these ideal conditions to emerge. Even though oxygen and nutrients are available, the fungus will lie idle if moisture is not present in the timber.

Table of Contents:

1. Causes of Timber Decay

Any one of a number of wood–destroying fungi may attack timber that is persistently wet and has a moisture content of over 20%. Although in a new house, the moisture content of timber may for a while be as high as 18 percent, this is soon reduced in normal circumstances to some 12–14 percent, so that in fact, a sound, dry building has nothing to fear.

Most Fungi require oxygen for its survival and growth. Timber may last a long time at the base of a lake or the sea, due to inadequate air at bottom for decay organisms to thrive. The test comes when the timber is exposed to alternating dry and wet conditions – for example as a fence post.

Moisture & oxygen are required for the growth of fungi in timber
Moisture & oxygen are required for the growth of fungi in timber

It may seem contradictory that ventilation is one of the essentials for preventing the development of fungal decay. Even in the most stagnant and poorly ventilated environments, there would be enough oxygen to satisfy the requirement of fungal growth. However, ventilation is used to dry and maintain dry timber in structures.

It is not necessary that all construction timber should be given a preservative treatment. In the case of most buildings, it is impossible to treat the existing timber members because of the high costs and the effectiveness of the application. The main thrust should be to create conditions that are not conducive to the growth of insects and fungi. If chemical treatment had been the norm for the timber, then there would not have been a single timber framed building in Kerala.

One of the timber framed historic building which is typical of Kerala traditional architecture.
One of the timber framed historic buildings which is typical of Kerala traditional architecture.

2. How does Fungi attack Timber?

Fungal decay starts from “spores”. They are the equivalent of plant seeds, very small and can float in the air. They are scattered by air, water, animals or man. They occur in such large numbers and when conditions are suitable for fungal germination on timber, the timber decay starts.

3. Types of Rot

There are many types of fungi in India that attacks timber. There is not much literature available on the different species of wood fungus. Also, the timber which attack in cold climates in the Himalayas and other places is very different from those fungi which are found in tropical areas. Among the wood rotting fungi, more literature is available on wet rot and dry rot. There is wood staining fungi that merely discolour timber. They do not weaken the timber, but staining does reduce the value of the joinery timber, especially if they are to be varnished or polished.

Rafters of a timber roof which had been attacked by timber fungi.
Rafters of a timber roof which had been attacked by timber fungi.

4. Dry Rot in Timber

The explanation dry rot is inferred from the fact that the affected timber seems dry and powdered. Timber which is attacked by dry rot fungi turns dark brown shrinks and dries into a broken powdery dry mass that appears to have been burned by fire. The wrinkled surface, the odour and the exit holes associated with infestation by insects are the other systems of an attack of dry rot. The possibility of attacks in hidden areas must be suspected wherever the conditions might be favourable.

Essential conditions for the propagation of dry rot fungi are dampness and minimal ventilation aided by warmth. The optimum temperature for the growth of dry rot in timber is 23° C . Over 26° C and below 3° C the fungus is inactive. Dry rot fungus is killed at 40° C sustained for 15 minutes. The optimum moisture content in wood for the growth of dry rot is around 30%.

The penetrating qualities and persistence of the growths that are in such a way that it is unwise to assume that any course of treatment aimed at eradication is final and complete. The affected timbers must be destroyed by burning, if not wholly then for at least three feet away from the visible evidence of the attack.

Fungal growth which has spread to the plaster nearby.
Fungal growth which has spread to the plaster nearby.

5. Wet Rot in Timber

Wet rot is confined to timber that is in damp and ill-ventilated situations such as ground floors without damp–proof courses and roofs. The wet rot causes the wood to darken and fracture longitudinally, and there is normally little or no noticeable fungal growth on the surface of the wood. The optimum moisture content in wood for the growth of wet rot is around 40%.

6. Differences Between Dry Rot and Wet Rot

7. Wood Decay Fungi Identification

Dry rot in its early stages is difficult to distinguish from other timber rots without the benefit of laboratory analysis. It is important to figure out how far the dry rot has gone. All timber work in the area of any outbreaks should be thoroughly examined to determine the amount of decay and the moisture content of the wood.

8. How to prevent Fungal Attack on Timber?

The first step in timber fungus treatment is to identify the source of moisture and, if feasible, eradicate it. To restore rotting structural timber, remove the visible decaying zone as well as any adjacent areas that has been damaged to ensure that all active timber rotting fungus have been removed. This should be replaced with a sound, dry lumber. As a precaution, spray fungicides on the remaining wood. In the case of wet rot, no cutting back beyond the sign of decay is required. When it is not possible to eliminate the source of moisture entirely, the replacement lumber should be pressure treated with a timber preservative before installation.

Simple repairs such as these will often save lot of money by preventing damage and expense from timber-destroying fungi.

Provision for the ventilation below the timber floor is partially blocked with rubbish and hence the floor is liable to fungal attack.
Provision for the ventilation below the timber floor is partially blocked with rubbish and hence the floor is liable to fungal attack.

Proper ventilation should be obtained in those areas which are critical. Vents should be given at top and bottom in the case of roofs and at opposite ends in the case of existing ground floors. In many cases, the openings would have been filled up over the years when various alterations to the existing building had been made.

Ventilation should be provided for the roof so that the attic space is well ventilated.
Ventilation should be provided for the roof so that the attic space is well ventilated.

It is essential to emphasize that applying fungicides or insecticides to fungus-infected timber or soil will not prevent the deterioration of the timber. Only by eliminating the moisture source can timber decay be completely controlled. Therefore, the application of chemicals by pest management professionals is of minor importance in timber preservation and fungus control work.

9. How to fix Timber Fungal Decay in Carvings and Mouldings ?

When timbers have historic significance or cannot be removed without causing harm to vital historic fabric, higher risks may be accepted. For example, in the case of a Buddhist monastery in Leh, the paintings on the ceiling will be of immense value so that the maximum of the existing timbers will have to be retained. In such cases, timber preservative treatments that penetrate throughout the affected part of the timber have to be used.

10. Conclusion

Even after the timber decay treatment has been completed, monitoring the conditions in buildings are very important. The inspection of the affected parts should continue at regular intervals even after the entire thing has been treated. Early detection of timber rot and water penetration into the buildings will help the building to be in good condition.


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