DRY ROT, a term applied to the decay of wood caused by various kinds of fungi. Frequently the term is applied solely to such decay as takes place in timber that has been worked or is in actual use : the present article deals solely with dry rot in this restricted sense. The fungi feed upon the wood, and thus cause it to become lighter in weight, weaker, more brittle and less elas tic, so that when struck the wood emits a muffled sound and readily snaps. In more advanced stages of rot the wood is apt to warp, and may show cracks ("cross-shakes") at right angles to the grain and can greedily absorb liquids, e.g., water, and, com paratively early, the wood may show discolourations and lose any characteristic scent.
The fungi causing dry rot belong to the great class that includes the familiar toadstools and mushroom. They are composed of very slender, hollow, jointed, closed tubes (hyphae) which grow in length and emit branches. These tubes permeate the wood, and outside this may also produce loose mould-like coatings, or by interweaving can give rise to denser sheets, nets, strings or more massive fruit-bodies shaped like cakes, brackets or mush rooms.
The fruit-bodies, which are easily visible to the naked eye, eject myriads of microscopic infectious germs, termed spores. But spores are likewise produced by certain species of these fungi quite apart from the fruit-bodies.
Infection of wood may also take place by means other than by spores. Certain species of fungi, including the most malignant causal agent of dry rot in houses in Europe, Merulius lacrymans (domesticus), not only grow inside the wood but can more rapidly spread over its surface, and advance over non-wooden surfaces, and, in the form of strings penetrate or perforate brick walls, attacking wood that they reach. Contrasting with such contagious species are other merely infectious species that usually live only in the interior of the wood until they thrust their f ruit bodies outside it ; such internal decay is often difficult to detect, as the superficial wood may be quite sound and normal in appear ance.
Wood may be protected against dry rot by one or both of two methods: (I) sanitation; (2) antiseptic (fungicidal) treatment.
An adequate supply of water is required by the fungus, which owes the main part of its weight to water and cannot take in food unless this be dissolved in water. So far as dry rot in tem perate regions is concerned, in the dry air of deep coal pits and of warmed rooms wood may last for decades or centuries; whereas dry rot is common in moist, shallower coal-pits, in moist cellars and damp ground-floors; especially can it appear near escapes of water from water pipes or steam pipes, and in stuffy, feebly ventilated places for instance under floors covered with linoleum. In a house fungus causing dry rot may be luxuriant on the hidden faces of floor-boards, panels and skirting-boards, but be entirely lacking on the visible faces of these in contact with drier air of rooms ; so that dry rot may be widespread through a building although no fungus be visible until the wood-work is disturbed.
Fungi causing dry rot cannot attack wood that is either too wet or too dry. Some species demand much moisture : among such are Coniophora cerebella (whose slender black threads are com mon on the surface of damp wood in houses) and a number of fungi that do not grow over the surface of the wood. These species are probably largely responsible for "wet rot," a popular term apparently applied to discoloured wood showing to the naked eye no fungus inside or outside the wood, no external cross-shakes, but giving evidence of weakness and possibly former or present wetness (which caused the death and disap pearance of any fungus formerly outside the wood) .
Other fungi, including Merulius lacrymans, produce consider able amounts of water and, transporting this, can thus moisten distant wood ; thus they can feed on drier wood.
Where dry rot in a building is caused by fungi demanding much moisture, it may be arrested by merely cutting off the excessive supply of water ; this will not suffice when Merulius lacrymans is the causal agent. Moreover cutting off the excess of moisture does not necessarily cause death of the fungus inside the wood : when wood attacked by Merulius or when certain internal feeders have been thoroughly dried for months, the fungus inside the wood may awaken into activity if the wood be remoistened.
Arrangements to secure a house against excess of dampness include: erection on a light pervious soil, provision of efficient dampcourses; care against absorption or condensation of water by and on the walls ; under the ground floor, adequate ventilation and provision of a water-tight coating over the concrete; obvia tion of leakage or overflow from rain-water pipes and gutters outside the house, and water pipes and steam pipes indoors; proper construction of window casements to allow water to flow properly away from them ; renewal of paint on external wood work ; and adequate maintenance of the roofing.
Fungi causing dry rot are active only within certain ranges of temperature ; but inasmuch as the temperatures prevailing in all parts of a building, from cellar to roof, in temperate regions at times enable these fungi to be active, temperature from the prac tical point of view intervenes only as a means of eradication. Low temperatures (for instance the freezing point of water) at least in the case of Merulius lacrymans do not suffice to kill fungus or spores. Higher temperatures (over 5o° C.) are much more effect ive, so that infected wood can be sterilized by heat, especially by steam, at temperatures below the boiling point of water. A painter's lamp suffices to sterilize infected surfaces, but an oxy acetylene flame, or some equivalent, is required for rapid steri lization of walls permeated with fungi causing dry rot.
Among organic wood-preservatives the most familiar is so called coal-tar "creosote oil," which is very widely used out of doors to preserve railway-sleepers, paving blocks, telegraph-poles, etc. Its scent is too powerful and persistent to permit of its use indoors, so that in houses, etc., "creosote" is often replaced by somewhat similar preservatives from which the more volatile and strong-scented oils have been removed. Extremely powerful organic fungicides suitable for use on wood are dinitrophenol and sodium dinitrophenate, to which are added other substances, such as sodium fluoride, designed to render the mixture non explosive. On the other hand carbolic acid and formalin are too evanescent ; and many other disinfectants used to destroy bacteria causing disease are too weak in action on fungi to act as timber preservatives.
Among aqueous solutions of inorganic salts used to preserve wood are copper sulphate (which attacks iron), zinc chloride (which easily washes out and when too warm or strong destroys wood), corrosive sublimate (very poisonous), sodium fluoride, and acid (commercial) magnesium silico-fluoride (which attacks metals and glass). Of these inorganic salts the last two may be generally regarded as the best for use in houses.
It is generally true that with increasing depth of penetration of the preservative into the wood the greater is the durability conferred, but the more costly is the process of treatment. Out of doors the preservative (usually "creosote") is driven deep into thick pieces of timber (such as sleepers, poles or paving blocks) by pneumatic pressure or by immersion in hot tanks. Whereas wood-work in buildings is usually merely coated with the preservative, reliance being more economically based upon sani tation including proper methods of construction. (P. GM.)