Timber is said to be decayed when it is deteriorated to the extent that it losses its value as an engineering material. Timber generally deteriorates in strength when subjected to excessive defects or diseases or attack by certain categories of insects. The insects damage the appearance and reduce the strength in search of their food and shelter.

Following insects generally attack the timber.

(a) Pin hole borers – They attack the standing tree or recently felled timber.

(b) Powder – post beetles - They attack timber while it is stacked for seasoning, sale or awaiting use or has been used for years.

(c) Termites or white ants - They attack timber used for structural purposes in buildings, bridges, poles, sleepers, fence, posts, etc.

The magnitude of destruction caused depends upon following situations favouring decay:

(i) Alternate dry and wet conditions.

(ii) Improper stacking or storage of timber.

(iii) Improper seasoning or unseasoned timber.

(iv) Use of unseasoned timber treated with preservatives.

(v) Use of seasoned timber without treating with preservatives.

(vi) Presence of moisture, sap and different types of insects.

(vii) Development of defects during growth, conversion, improper use, etc.

(viii) Improper positioning of structural members of timber.

(ix) Development of timber diseases.

(x) Effect of natural agencies.

The decay of timber due to attack of insects can be prevented by adequate ventilation, exclusion of dampness and impregnation of properly seasoned timber with wood preservatives.

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Timber
Structure of a Timber Tree
Properties or Characteristics of Timber
Defects of Timber
Diseases of Timber - Dry and Wet Rot
Decay of Timber
Seasoning of Timber
Methods of Seasoning of Timber
Preservation of Timber