The defects which have been mentioned above are all of such a kind that they can be readily detected in the timber before it has been put in position in a structure, and, therefore, the use of the timber so affected may be avoided, but dry rot, while it is probably the most common and the most dangerous defect of them all, may start and spread rapidly in timber which appears to be absolutely sound when it is put in place. Dry rot is a disease which fastens itself upon the wood and spreads from one part of it to another, causing it to lose its strength and cohesive power and even to decay altogether. It may be readily seen that this process can lead to most serious results when it takes place in timber which is depended upon to carry heavy loads. Large beams and posts have been known to fail and thereby cause considerable damage solely because of dry rot, and others have been so weakened by the ravages of this disease that they have yielded when subjected to slight fires which would have had very little effect upon them if they had been sound.

The timber in which dry rot is most to be feared is that which is kept alternately wet and dry, while that which is always either entirely submerged in water or absolutely dry appears to be able to last indefinitely without a sign of the disease. For this reason wood piles should always be cut off below the water level. Decay takes place very rapidly when the wood is in a confined position where the gases can not escape. The ends of beams buried in brickwork and the ends of posts fitting into iron caps and bases are examples of such cases, and special precautions should be taken to allow the air to circulate freely around such woodwork wherever this is possible. Woodwork which is in contact with wet or damp materials, such as wet concrete or masonry in which the mortar has not dried out thoroughly, is peculiarly liable to dry rot. Wood flooring laid on top of newly-placed concrete slabs and immediately covered with some other substance has been known to rot very quickly. It is also noticeable that this form of decay seems to be hastened by warmth and is more common in the southern climates than in the northern. It may be prevented by introducing into the timber certain salts such as the salts of mercury, also by heating the wood to a temperature above 150° F. and keeping it at that temperature. As precautionary measures, all wood should be thoroughly seasoned before being painted, as good ventilation as possible should be provided for it, and it should be kept from contact with anything from which it can absorb moisture. Posts should have a hole about one and one-half inches in diameter bored through them from end to end, and other holes near each end bored through them crosswise, so as to provide for the free circulation of air in the interior of the post.