To preserve timber from rot or decay it should be kept constantly dry and well ventilated. It should be clear of the influence of damp earth or damp walls, and free from contact with mortar, which hastens decomposition.
Timber that is constantly dry is very durable. However, it also becomes brittle in time, though not for a great number of years.
1 Patentee's Circular.
"When timber is exposed to alternate moisture and dryness it soon decays." 1
Rot in timber is decomposition or putrefaction, generally occasioned by damp, and which proceeds by the emission of gases, chiefly carbonic acid and hydrogen.
There are two kinds of rot generally known to practical men - dry rot and wet rot.
The chief difference between them 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.
Tredgold says that wet rot may take place while the tree is standing, whereas dry rot takes place 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.
An excess of moisture prevents the growth of the fungus, but moderate warmth, combined with damp and want of air, accelerates it.
"When the fungus first appears on the sides and ends of timbers it covers the surface with a fine delicate vegetation called by shipwrights a mildew.
"These fine shoots afterwards collect together, and the appearance may then be compared to hoar-frost, and increases rapidly, assuming gradually a more compact form, like the external coat of a mushroom, but spreads alike over wood, brickwork, stone, and plastering in the form of leaves, being larger or smaller, most probably, in proportion to the nutriment the wood affords. The colours of the fungus are various, sometimes white, greyish white with violet, often yellowish brown, or a deep shade of fine rich brown." 2
The positions in which dry rot occurs are, as already mentioned, those where the timber is exposed to warmth and damp stagnant air.
The principal parts of buildings in which it is found are -
In warm cellars, under unventilated wooden floors, or in basements, particularly in kitchens or rooms where there are constant fires. "All kinds of stoves are sure to 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.
2 Britton On Dry Rot.
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 the same effect
Sometimes the roots of large trees near a house penetrate below the floors and cause dry rot.
It is said that if two different kinds of wood - as, for example, oak and fir - are placed so as to touch end to end, the harder of the two will decay at the point of junction.
"There is this particular danger about the dry rot - viz., 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."
"Before dry rot has time to destroy the principal timbers in a building it penetrates behind the skirtings, dadoes, and wainscotings, drawing in the edges of the boards and splitting them both horizontally and vertically. When the fungus is taken off they exhibit an appearance similar, both in back and front, to wood that has been charred; a slight pressure with the hand will break them asunder, even though affected with the rot but a short time, and in taking down the wainscot the fibrous and thin-coated fungus will generally be seen closely attached to the decayed wood. In timber of moderate length the fungus becomes larger and more distinctive in consequence of the matter congenial to its growth affording a more plentiful supply." 1
Wet Rot occurs, as before mentioned, 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 the timber, then wet rot can be prevented.
"The communication of the disease resulting from the putrefactive fermentation or the wet rot only takes place by actual contact," not by the dissemination of the germs of fungi as with dry rot.
In the absence of any outward fungus, or other visible sign, the best way is to bore into the timber with a gimlet or augur. 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 feet long; but if the timber be decayed, the sound will be very faint, or altogether prevented from passing along the balk.
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 the timber, and restack it in such a way that the air can circulate freely round each piece.1