A treatise on the origin, progress, prevention, and cure of dry rot in timber

by Thomas Allen Britton

Wood, when it is cut into small pieces, very soon acquires its utmost degree of dryness. Dr. Watson, Bishop of Llandaff, in the month of March, cut a piece from the middle of a large ash tree that had been felled about six weeks, and weighed it; its weight was 317 grains. In seven days it lost 62 grains, or nearly one-fifth of its weight. It was weighed again in August of the same year, but had not lost any more of its weight; hence it had become perfectly dry in the short space of seven days. He also found that the sap-wood of oak lost more weight in drying than the heart-wood, in the proportion of 10 to 7.

The time that is required to season or dry a piece of timber obviously depends upon its magnitude; as a general rule, large timbers will not continue good so long as small ones, as sufficient time is rarely given for a thorough seasoning. The time required to dry a piece of timber, all other things being alike, will depend on the quantity of surface exposed to the action of the air; therefore, while the quantity of timber remains the same, the larger the surface, the sooner it will dry. Also, if the quantity of surface remains the same, the time of drying will be proportioned to the quantity of matter; as the greater the quantity of matter under the same surface, the longer it will be in drying.

As drying proceeds most rapidly in small pieces, it is therefore important to reduce the timber to its proper scantlings or size for use; for however dry a piece of timber may be, when it is cut to a smaller scantling it will shrink and lose weight, being always less dry in the centre than at the surface; and the more rapidly the drying has been carried on, the greater will be the difference. Nevertheless, in the first stage of seasoning it is best that it should proceed slowly; otherwise, the external pores shrink so close as not to permit the full evaporation of the internal moisture, and the piece would split from unequal shrinking; and lastly, it should be reduced to the proper scantling, as already observed, some time before it is to be framed. Various tables have been given by writers on timber, the result of algebraical calculations, of the times of seasoning and drying for different woods of different lengths, breadths, and thicknesses, in the open air; but as wood even of the same description and quality varies so much, this matter is best left to those who are well acquainted with timber. It may, however, be stated that the time required for drying under cover is shorter than in the open air, in the proportion of 5 to 7.

The English shipwright considers that three years are required to thoroughly season timber. The timbers for ships are usually cut out to their shape and dimensions for about a year before they are framed together, and they are commonly left a year longer in the skeleton shape to complete the seasoning, as in that condition they are more favourably situate as regards exposure to the air than when they are closely covered in with planking.

It is worthy of mention that all the harder woods require increased care in the seasoning, which is often badly begun by exposure to the sun or hot winds in their native climates: their greater impenetrability to the air the more disposes them to crack, and their comparative scarcity and expense are also powerful arguments on the score of precaution. Oak timber requires to be very carefully seasoned, as it is generally used in buildings for the best description of work, and should unseasoned oak be used for “panelling,” any shrinkage will be fatal to the work. Mr. George Marshall, timber merchant (see the Builder, January 20, 1872), with respect to seasoning oak timber, observes: “I should select oak trees known to be old and hearty, with clean, straight butts, from 15 inches to 20 inches in diameter. I should then have the bark taken off as they stand, and leave them thus till the winter; the sap will then partially dry out, and make the wood a rich brown colour. As soon as they are cut down, have them sawn up at once into the lengths you require the panelling, 6 inches or 8 inches wide and 1 inch to 1½ inch thick. Be careful to cut all the heart shakes, by having one cut through the centre of the log before edging the boards to the required width. With regard to the drying process, stack the boards in a shed with a good draught through it, and load them down, with slips between each board, to prevent warping. If this be done they will be found to dry well and speedily, and they will not require to be exposed to the weather.”

Sir. Robert Phillips, on seasoning oak for panelling, states: “If the tree is large enough for the purpose, cut it into four, in sections, by drawing a vertical and horizontal line across the end, meeting in the centre. If too small for this, cut it into 4½ inch or 6-inch plank, as soon as possible after felling, and then stack on end out in the open: do not lay on the ground, but stand it as nearly vertical on its end as possible, and keep it wet during the first three months. If the weather is dry, well wet it with water poured on the top, and allowed to run down. Let the ends stand on a piece of quartering, to keep it out of the dirt, or it will be stained some distance up. After standing thus for some six months, after putting it in a dry place for some time, cut it into the scantlings you require, always bearing in mind that oak will, after this seasoning, shrink at least half an inch to a foot, in width and thickness. They should then be stacked and stripped, and covered with spare boards, and weighted on the top, for at least six months—as much more as possible—in a covered shed, with plenty of air, occasionally turned over and shifted, till they are dry enough to make dust when planed, and not turn the shaving black. They will then be fit for use.

“I should advise for the panels to be cut feather-edged boards, in radial lines from the centre of the tree: it will be a waste of material, but will repay in the beauty of the wood, and the way it will stand without warping. Most of the panels of our old cathedrals were rent (not sawn) in this way, and stand admirably. The butt of the tree should be taken, the top being used for a rougher purpose.”

Mr. George Marshall and Mr. Robert Phillips might have mentioned that the oak trees should be of the Quercus Robur species, and not the Quercus Sessiliflora. They are easily distinguished when growing by the following peculiarities: The acorn-stalks of the Robur are long; the acorns grow singly, or seldom two on the same footstalk; the leaves are short. The acorn-stalks of the Sessiliflora are short; the acorns grow in clusters of two or three, close to the stem of the branch; the leaves are long.

WATER SEASONING.

When there is not time for gradual drying, the best method, perhaps, that can be adopted, especially for sappy timber, and if strength is not principally required, is immediately on felling to immerse it in running water; and after allowing it to remain there about a fortnight, to set it in the wind to dry. Some persons prefer this method of seasoning timber, as they say it prevents cleaving, and strips and seasons better afterwards. This process has been adopted with good results by placing the boards end on at the head of a mill race for fourteen or twenty days, at most, and then setting the boards upright, and subject to the action of the sun and wind; though it is questionable whether the sun will not do them more harm than good. As they stand, turn them daily, and when perfectly dry—which process will take about one month—it is considered they will be found to floor better than timber after many years of dry seasoning. The sap-wood of oak is said to be improved by this method, being much less subject to be worm-eaten; and providing it is placed in fresh running water, Mr. G. A. Rogers, the celebrated wood carver, is of opinion that the colour of the oak is improved. The more tender woods, such as alder and the like, are less subject to the worm when water seasoned. Beech is said to be much benefited by immersion. It should be remembered that the timber should be altogether under water (chained down beneath its surface), as partial immersion is very destructive. Du Hamel considers “that where strength is required, wood ought not to be put in water.” Timber should never be kept floating in ponds or docks, as in London; but it should be stacked, as at Liverpool and Gloucester. Timber that has been lying for months in ponds or docks is sometimes cut up, and in six or seven days fixed in a building; consequently, the usual result takes place, viz. dry rot. After having been swelled by soaking much beyond its former bulk, the baulk of timber is put on the saw-pit, and cut into scantlings, and framed while in this wet state, therefore it cannot be surprising that the dry rot soon appears as a natural consequence.