To preserve wood from decay it should be kept constantly dry and well ventilated; clear of the influence of damp earth or damp walls, and free from contact with mortar, which hastens decomposition. Wood kept constantly submerged is often weakened and rendered brittle, but some timbers are very durable in this state. Wood that is constantly dry is very durable, but also becomes brittle in time, though not for a great number of years. When timber is exposed to alternate moisture and dryness it soonest decays. The grneral causes of decay are (1) presence of sap, (2) exposure to alternate wet and dryness, or (3) to moisture accompanied by heat and want of ventilation.

"Rot" in timber is decomposition or putrefaction, generally occasioned by damp, and which proceeds by the emission of gases, chiefly carbonic acid and hydrogen; 2 kinds of rot are distinguished - "dry" and "wet." Their chief difference seems to be that wet-rot occurs where the gases evolved can escape; by it, the tissues of the wood, especially the sappy portions, are decomposed. Dry-rot, on the contrary, occurs in confined places, where the gases cannot get away, but enter into new combinations, forming fungi which feed upon and destroy the timber. Wet-rot may take place while the tree is standing; dry-rot occurs only when the wood is dead.

"Dry-rot" is generally caused by want of ventilation; confined air, without much moisture, encourages the growth of the fungus, which eats into the timber, renders it brittle, and so reduces the cohesion of the fibres that they are reduced to powder. It generally commences in the sapwood. Excess of moisture prevents the growth of the fungus, but moderate warmth, combined with damp and want of air, accelerates it. In the first stage of rottenness, the timber swells and changes colour, is often covered with fungus or mouldiness, and emits a musty smell. The principal parts of buildings in which it is found are - warm cellars, under unventilated wooden floors, or in basements particularly in kitchens or rooms where there are constant fires. All kinds of stoves increase the disease if moisture be present. The ends of timbers built into walls are nearly sure to be affected by dry-rot, unless they are protected by iron shoes, lead, or zinc. The same result is produced by fixing joinery and other woodwork to walls before they are dry. Oilcloth, kamptulicon, and other impervious floorcloths, by preventing access of air and retaining dampness, cause decay in the boards they cover carpets do the same to a certain extent.

Painting or tarring cut or unseasoned timber has a like effect.

Sometimes the roots of large trees near a house penetrate below the floors and cause dry-rot. It is said that if two kinds of wood - as, for example, oak and fir - are placed so as to touch end to end, the harder will decay at the point of junction. There is this particular danger about dry-rot, that the germs of the fungi producing it are carried easily, and in all directions, in a building where it once displays itself, without necessity for actual contact between the affected and the sound wood.

"Wet-rot" occurs in the growing tree, and in other positions where the timber may become saturated with rain. If the wood can be thoroughly dried by seasoning, and the access of further moisture can be prevented by painting or sheltering, wet-rot can be prevented. The communication of the disease only takes place by actual contact. To detect dry-rot, in the absence of any outward fungus, or other sign, the best way is to bore into the timber with a gimlet or auger. A log apparently sound, as far as external appearances go, may be full of dry-rot inside, which can be detected by the appearance of the dust extracted by the gimlet, or more especially by its smell. If a piece of sound timber be lightly struck with a key or scratched at one end, the sound can be distinctly heard by a person placing his ear against the other end, even if the balk be 50 ft. long; but if the timber be decayed, the sound will be very faint, or altogether prevented from passing along. Imported timber, especially fir, is often found to be suffering from incipient dry-rot upon arrival. This may have originated in the wood of the ship itself, or from the timber having been improperly stacked, or shipped in a wet state, or subjected to stagnant, moist, warm air during the voyage.

Sometimes the rot appears only in the form of reddish spots, which, upon being scratched, show that the fibres have been reduced to powder. After a long voyage, however, the timber will often be covered with white fibres of fungus. Canadian yellow pine is very often found in this state. The best way of checking the evil is to sweep the fungus off, and restack the timber in such a way that the air can circulate freely round each piece.