From the book Timber Decay in Buildings: The Conservation Approach to Treatment, by Brian Ridout, 2013
- There are two camps: remedial timber treatment industry vs. environmental control (drying etc.)
- The epithet ‘preservative’, although doubtless a powerful marketing device, is an unfortunate generic name to apply to biocides for use on timber, because it implies that decay will inevitably occur unless the timber is given some form of treatment. Yet timber is easily preserved by a dry environment.
- Current legislation requires that precautionary treatments be justified
- Some organisations: BRE & British Wood Preserving and Damp Proofing Association
Chapter 1: Origins and Durability of Building Timber
- Wood is not the uniform material it appears to be
Between Softwood and Hardwood
- The terms ‘softwood’ and ‘hardwood’ are used extensively within the timber trade, and frequently lead to confusion. Softwood refers to the conifers, the needle-leafed or cone-bearing trees (for example pine and cedar), some of which provide quite hard timber. Hardwood is used to describe timber from the so-called broad-leafed trees (for example oak and mahogany) and includes species whose wood is in fact very soft. Nonetheless, the distinction between softwood from cone-bearing trees and hardwood from broad-leafed trees remains extremely useful.
- The softwood trees are botanically known as gymnosperms (from the Greek: naked-seeded) because the seeds develop exposed on the surface of a cone scale. The gymnosperms living today are the representatives of a group that extends back in time for more than 300 million years (Sporne, 1965). Modern softwood trees are mostly restricted to regions where the climate is harsh and the soil is poor in nutrients. Their ability to survive in these areas derives to a large extent from an ability to restrict water loss, by the possession of a water-conducting system controlled by valves, and by narrow waxy needles that restrict vapour loss.
- The hardwood trees are known botanically as angiosperms, or hidden-seeded plants, because the seeds develop enclosed in an ovary which eventually becomes a seed capsule. They are a more recent addition to the flora, and do not appear in the fossil record until about 100 million years ago (Sporne, 1974). The majority of the hardwoods have broad leaves to maximize light absorption, and an open water conducting system. They tend to favour environments where conditions are suitable for prolonged vigorous growth, although many are able to tolerate poor soils and harsh weather.
- These differences in efficiency of water transportation and habitat have a considerable effect on the structure of their timbers.
- At the molecular level, softwood trees and hardwood trees are similar, even though the exact make-up of the lignin and hemicellulose components may differ. At the structural level, however, there are differences that relate to environment and growth form. Softwood trees have leaves and wood structure that reduce water loss in harsh environments (Rundel and Yoder, 1998). But these environments, particularly the colder environments, impose restraints on form. Damage from high winds and snowfall is easier to avoid if the tree has a straight trunk and an open-branched structure (Figure 1.6): wind resistance is reduced and snow falls off as the branches bend down under its weight. The tree can maintain sufficient growth by maximizing conduction within the tracheids during the early vigorous growing season, when the tracheid walls are thin (earlywood cells), and maximizing strength by thickening the tracheid walls during the latter part of the year (latewood cells). The two types of tissue together produce a more or less distinctive annual ring. Storage requirements are low because the tree can maintain a longer growing period by minimizing water loss, and the soil is of poor quality.
- Hardwood trees have a rather different lifestyle. They favour milder climates where water and nutrients are more plentiful. These conditions mean that the tree can sustain a bulky growth form if competition will allow, producing a wide crown of leaves for photosynthesis. If growth is affected by direct competition from other trees, as in an oak woodland, then long straight trunks will be produced, pushing the leafy head towards the light (Figure 1.7). In contrast, parkland growth allows plenty of space, and produces oaks with a lower and more spreading form because light is readily available (Figure 1.8). In economic terms, the forest growth produces a greater volume of better quality timber. Modern forestry techniques aim to maximize growth by giving space, while providing enough competition to maintain quality. A thicker stem and wider crown require stronger wood, and the larger species tend to have denser timber and a high volume of structural fibres. Tree roots cannot function at soil temperatures below 4 ° C so water vapour lost from the leaves during cold periods cannot be replaced from the soil. Hardwood trees, with thin-cuticled broad leaves that maximize photosynthesis, must either drop their leaves when the ground is cold, and become dormant, or risk desiccation and the loss of leaves (and the nutrients they contain) as a result of frost damage. If leaf fall is part of the tree’s natural strategy then nutrients can be withdrawn, and the leaf sealed from the stem. The leaf then dies, changes colour, and falls off. Loss of leaves in winter also reduces wind resistance and snow deposition, thus reducing mechanical stress to the tree during adverse weather.
- Substantial storage facilities are required, and many hardwood trees have wide rays which add considerably to the decorative qualities of timber such as oak by producing a patterned grain. When the favourable season returns the stored material must be rapidly mobilized and photosynthesis maximized. This is aided by an open and direct water-conducting system.
- The start of heartwood production varies. In European redwood in southern Norway it has been shown to commence around the pith after 20 years (Uusvaara, 1974) but the starting point apparently depends upon latitude. Bruun and Wilberg (1964) demonstrated heartwood production after 30– 40 years in Finnish redwood, whereas Hägglund (1935) showed that it commenced after about 25 years in southern Sweden and 70 years in northern Sweden.
- (In the living tree) The woody tissue of the tree is protected from attack by the living sapwood, which is mostly too wet to be susceptible to colonization by pathogens (Hudson, 1986), and is also able to respond in a variety of ways to infection. Sap rots do sometimes occur in living trees if the sapwood is damaged, but the decay is usually localized.
- If a wound occurs in the sapwood zone of a living tree then the tissue can respond by compartmentalizing the damage and any consequent infection (Shigo, 1983). The wound is healed by the stimulation of growth hormone production, which in turn stimulates the production of callus tissue over the surface. Damaged and diseased sapwood is isolated by the production and deposition of tyloses, gums, resins and other toxic materials in wall-like zones which box in the injury (Shain, 1979). If the wound penetrates to the inert heartwood, then the barrier will be incomplete and decay may occur.
- It is the dead heartwood in the living tree which is vulnerable to a greater or lesser degree, depending on its natural durability. This difference in durability between the sapwood and heartwood is, as described presently, reversed when the tree dies. Most fungi appear to enter the heartwood zone via small dead branches, sapwood wounds, broken tops and roots. Decay may be categorized as a butt rot if it is at the base of the tree, or a heart rot if it is further up the stem. The molecular and structural organization of wood is not the only influence on durability. Age and extractives also have a bearing on durability and resistance to decay.
- The period of juvenile growth varies in length and the density/ strength of the timber increases away from the centre. Eventually, the cells reach a maximum length with nearly vertical microfibrils in the S2 layer, and the tree may be considered to be mature. At about this time (usually about 10– 20 growth rings from the centre) the cycle of earlywood and latewood is fully established in those species where this occurs (Krahmer, 1986).
- A tree may live for a very long time, but eventually vigorous growth slows and the tree becomes senile. Increase in trunk height ceases first; increase in diameter may continue for many years. Growth rings become narrow, and latewood production declines as lignin production decreases, so that the timber produced is brittle. Accompanying these changes are changes within the heartwood, which progress outwards from the pith. These changes, which frequently include the breakdown of extractives and the formation of minute compression fractures, increase the timber’s susceptibility to decay. The centre of the tree is usually invaded by fungi which slowly destroy the core, leaving a hollowed trunk.
Sorption of Water by Timber
- Distortion caused by shrinkage and swelling is generally recoverable to a large extent, provided that no plastic deformation of the cell walls has occurred. Permanent distortion may, however, arise if there is lateral restraint, perhaps caused by panel surrounds or floorboard fixings.
- The shrinkage of timber from green to air dry was important in traditional timber-framed construction. Joint pegs provide an interesting example. Oak was usually worked more or less green and would therefore shrink. If the joint pegs behaved in a similar fashion then the joints would loosen, but if the pegs were made from dry timber, which would expand as it absorbed moisture, the joints would be tightened.
- Dimensional changes which are a response to diurnal or seasonal humidity fluctuations are generally called ‘movement’, and are of importance in good quality joinery and carpentry. If timber with a high movement value is used, or timbers with different movement values are mixed, or timber is worked at a substantially higher moisture content than it will achieve in its intended environment, then loose joints and undesirable gaps are likely to occur. Tables that group commercially available timbers by their movement values are published by the Building Research Establishment and other similar organizations. Values are usually quoted for dimensional change from fibre saturation down to 12% moisture content.
- Sometimes changes in the building environment make undesirable movement inevitable. This type of damage frequently occurs when central heating is installed in rooms with window linings and dado panelling. Heat from radiators placed in front of the windows may cause localized lowering of humidities to 25– 35% and timber moisture contents to 6– 8%. The resulting movement of the timber will be readily visible, and is frequently mistaken for decay. If some concealed decay is present, perhaps as a result of water penetration 200 years earlier, substantial buckling and distortion may occur. This is a major reason why active dry rot is sometimes reported from dry buildings after restoration works.
- Responses to moisture vary according to the age of the timber.
- Decay commences in softwood timber by the breakdown of pit membranes, thus increasing porosity. This process is sometimes encouraged, particularly for species which are difficult to treat with preservatives, by storage in water. Increased porosity can, however, lead to excessive and uneven uptake of preservative, which may cause surface bleeding, and to difficulties with the application of glues and coatings. Decay by brown and white rot fungi will substantially alter moisture uptake. It will be remembered that water molecules are held by the cellulose/ hemicellulose within the timber and it is these structural polymers that are destroyed by brown rots. Brown rots therefore cause a sharp drop in equilibrium moisture contents, particularly during the early stages of decay. White rots remove lignin and the proportion of cellulose exposed increases. Timber decayed by white rots therefore tends to have a higher equilibrium moisture content once about 60% weight loss has been exceeded. Moisture meter readings should not normally be taken from decayed sections of timber.
- The moisture level in Guangzhou is high – there is high level of air moisture and also seasonal monsoons. How does this affect timber structures there?
Chapter 3: Post-Harvest Changes and Decay
Effects of Moisture Contents
- It has been shown that it is difficult for most decay organisms to exploit the living tree (see Section 1.5). The nutrient-rich sapwood, the most vulnerable part of the actively growing tree, is too wet for colonization by most organisms, and the living tissue can produce growths that check the spread of pathogens. Decay fungi therefore mostly enter via dead or damaged tissue, particularly branch wounds and roots. The fungus still has to overcome the natural resistance of the tree, and also any gums, resins or phenolic compounds produced at the site of wounds, which inhibit spore germination and compartmentalize fungal growth. It is easier for fungi to attack the juvenile wood at the centre, where the concentration of antiseptic extractives and the moisture content are lower. The most common forms of decay in standing trees are therefore butt or heart rots.
- Because of its moisture content, sapwood may weigh twice as much when green as it does after oven drying. Heartwood is usually drier in softwoods (and in some hardwoods); in some pines, for example, moisture levels are about 120% of the dry weight in the sapwood and 35% in the heartwood. This situation changes when the tree is felled. The moisture content is slowly reduced as the timber dries, to about 17– 20% in the wood yard and perhaps about 15% in a cool building. In a heated building the moisture content may drop to less than 9%. As the water content drops and the air content increases, a variety of changes, including decay, may occur. Decay remains a possibility until the timber moisture content is too low for the relevant organisms to survive.
- The progressive moisture loss, from living tree through felled log, conversion and finally incorporation into a building, presents a wide range of conditions which different decay organisms may exploit, although the extractives may remain an insurmountable obstacle in some timber species. Sapwood, which was a resistant material in the living tree, becomes highly susceptible to decay as it dries, because of its elevated nutrient content.
- Wood decay fungi and insects vary widely in the moisture contents they can tolerate. Most require relatively high timber moisture levels in order to thrive. Some groups can cope with drier conditions, but as moisture content decreases, so does the variety of decay organisms. Moisture levels below about 18% provide a harsh environment for all except a few specialized insects, which probably derive a proportion of their water requirements from the breakdown of cellulose. In temperate climates, most of these specialists are beetles; in warmer regions termites have a major economic impact. In general, fungi in buildings will not damage timber with a moisture content below about 22%, whereas colonies of most wood-boring beetles will not thrive at moisture levels below about 12%. A few may continue their attack at moisture levels as low as 8%. These differences enable the classification of biological hazards (see Section 3.5).
- It is important to remember that damp timber provides an environment which will ultimately be colonized and decayed by a succession of organisms. The types of organisms that take part in this progression and the speed of attack depend on parameters which include moisture content, temperature and the different chemicals present in the wood. Preservative treatments may inhibit or destroy some organisms, but preservatives are really only changing the environment, and sooner or later a suitably tolerant sequence of colonizers will reach the damp timber and commence its destruction. These colonizers need not themselves be decay organisms. Benign microfungi, for example, may modify the toxins so that decay fungi may develop. Wood is only immune from decay if it is kept dry.
- Decay organisms may be classified by their ability to degrade the components of the timber, and they will range from cell content feeders in sapwood, to those capable of destroying the entire cell wall of sapwood and heartwood.
- So basically how moisture works in timber is that the green timber comes with lots of water, then during the conversion process, the timber is dried out to a certain percentage of water content. If the percentage is not low enough, organisms might start to attack it. Preservative treatments might help but really still depends on the drying. These organisms eat away at the wood mass and increase its porosity (decreases density). Timber that is always submerged in water does not have the same issue because due to the absence of air, not many organisms can attack at the timber. (is that true?) So it is really a combination of heat, moisture and air that creates decay in timber.
Nutrient availability after conversion: the potential for decay
- Wood is not an easy food source for decay organisms to exploit. Timber conversion may increase the problem for decay organisms because soluble nutrients tend to be carried to the surface as the timber dries and large amounts of this surface zone are lost when the timber is planed and worked.
Damage to timber can be caused by
- Fungus (moulds and stains or soft rot)
- Mould and stain fungi (fungi imperfecti) are mostly cell-content feeders that exploit the nutrients in sapwood. They usually do little significant damage (Viitanen and Ritschkoff, 1991b), although the bulk of pigmented hyphae produced by stain fungi may cause the discoloration of damp timber. They do, however, have a primary part to play in the natural cycle of decay by increasing porosity and detoxifying some natural fungicides. Colonization by moulds is facilitated by some types of bacteria that break down the pit membranes within the sapwood (Blanchette et al., 1990). Stain fungi are more independent than other moulds, and travel from cell to cell by boring fine holes in the walls. Viitanen and Ritschkoff (1991b) grew a range of common moulds on redwood and spruce. They concluded that the lowest air relative humidity for growth on sapwood was 80%, and that growth was very slow at this humidity level.
- Soft rot damage progresses slowly as a surface decay in wet timber, and the wood has a fine surface checking, similar in appearance to brown rot damage, when it dries. The fungi can, however, tolerate a wide range of environmental conditions, and dry timber which is intermittently wetted may eventually be destroyed (Savory, 1955). Savory also showed that the brash surface frequently found on timbers where there were no other indications of fungal decay, could usually be attributed to soft rot fungi. Significant rot damage is more common in timber exposed in soil or aquatic environments than it is in buildings. This restriction has been ascribed to a lack of additional
- The primary division of decay into white rots and brown rots, according to the colour of the visible damage, was proposed by Hartig in 1874. The fungi that cause white rot (Basidiomycete; Figure 3.3) grow within cell cavities and attack all constituents of the cell wall from the lumen inwards. Some white rot fungi commence by attacking hemicelluloses and lignin, whereas others utilize all cell-wall components at the same rate. Decay commences with the depolymerization of the hemicellulose. The resulting decay appears as a mass of white fibres, and weight loss may eventually exceed 95% (Zabel and Morrell, 1992). White rot decay is predominantly associated with hardwoods (Hudson, 1986). The reason seems to be that most of the white rot fungi have difficulty in attacking the lignin unit which predominates in softwood lignin (see Figure 1.2). White rot fungi in buildings tend to thrive in wetter sites than brown rot fungi; thus they are frequently found in external window sills and below substantial roof leaks. This view is perhaps supported by laboratory decay tests, which showed that white rots required more water than brown rots to achieve an optimal wood-weight loss (Highley and Scheffer, 1970). Examples are oak rot (see Section 8.5.2) and some others of the ‘pore’ fungi (Polyporaceae).
- Surface degradation caused by mechanical damage
- The combined effects of light, wind and water movement produce stresses which result in small surface checks and cracks. Surface material loosened in this fashion is eventually lost and erosion takes place. This process is mostly very slow, published estimates ranging from 1 to 7 mm per century (Kühne et al., 1972; Browne, 1960). Erosion varies with the type of tissue: the softer earlywood is likely to be preferentially removed, leaving behind ridges of latewood (Feist and Mraz, 1978). A severe artificial form of this type of damage will occur when softwood is cleaned by grit blasting (see Section 12.3).
- Both alkaline and acid attack may occur in damp timber if there are partially embedded iron fastenings (Figure 3.9). The resulting electrochemical reaction is driven by differential oxygen availability (Baker, 1974). In essence, the exposed portion of the fastening acts as a cathode while the concealed section acts as an anode. Electron flow from the anode to the cathode produces corrosion products at the anode which may be hydrolyzed to free acid. A progressive and concomitant increase in alkalinity at the cathode produces an increasingly alkaline environment so that surface degradation also occurs. Blue/ black staining frequently spreads from the cathode, particularly in oak, as soluble iron salts react with tannins to form iron tannate.
Acidity and Corrosion of metals by timber
- Fresh oak tends to contain particularly high levels of acetic acid. This can often cause metal corrosion, so fixings in oak must be corrosion resistant.
- It is worth noting that kiln drying tends to raise acidity
Drying and wetting: A historical perspective on timber decay within buildings changes in moisture content after felling
- Timber is not a homogeneous solid like a metal which contracts and expands equally in all directions. It has different internal arrangements along the grain and across it, so that it shrinks and swells unevenly as it loses or gains moisture. Green mature timber shrinks little along its length, more across the radial face and much more across the tangential face. Tangential shrinkage is usually 1.5– 2 times as great as radial shrinkage. Unequal shrinkage causes stresses within the log that lead to splits along the weakest tissue, which is the rays. Sawn timber subjected to uneven drying will warp, the severity and type of distortion being dependent on the manner in which the log was cut.
- Sawn building timbers therefore have to be dried to a level that does not suit the majority of decay organisms, but in a controlled fashion so that distortions are minimized. This process is known as seasoning, a term that probably derives from the practice of leaving oak logs and ships’ frames for several seasons in order to rid them of saps liable to ‘ferment’ (Bowden, 1815, pp. 85– 93).
- Nowadays seasoning is usually undertaken in a more controlled fashion and its purpose is to make the timber as stable as possible with regard to both distortion and decay. Seasoned timber is also lighter (therefore less costly to transport), stronger, holds nails better and is easier to machine, paint and glue. It also usually accepts chemical preservatives readily because the wood cells are empty of liquid. Ideally the less durable timbers should be converted as soon after felling as practicable because they are susceptible to a wide variety of decay organisms.
- If the bark is completely removed from the log before or after felling, then uneven drying and the release of stresses may cause severe permanent shakes to develop from the outside inwards. This reduces the amount of usable timber that can be cut from the log. However, if the bark is left on the log to retard drying, a different set of problems occurs; these include end splitting and staining as well as attack by wood-boring beetles and fungi.
- Different timbers vary considerably in their anatomical structure and in their physical properties, and there is a wide variation also within any one species. It follows, therefore, that although the drying of all wood, regardless of its species and size, is governed by the same physical laws, there are great differences in the drying conditions that can safely be imposed. Water seasoning is worth a mention here even though it is not a method of drying, because it is an old-established practice. If logs remain waterlogged then starch is removed and with it the risk of powder post beetle damage when the timber dries. Fungi are also inhibited but bacterial attack may increase the porosity of the sapwood. This is not necessarily a disadvantage because it may aid the penetration of preservatives.
- Air drying (Figure 11.1) is probably the oldest technique available. The timbers must be stacked in a way that allows adequate, even air flow and prevents distortion. Correct stacking is of considerable importance, as the diameter of the felled trees decreases and the risk of distortion, particularly cupping, in the resulting smaller section timber increases (Wengert and Denig, 1995). Planks and other timbers are normally stacked horizontally with thin spacing timbers of uniform size, known as stickers, between them. Stickers are placed so as to form vertical lines throughout the stack, usually at about 300 mm centres, in order to avoid unequal stresses. Stickers have to be kept thoroughly dry in order to avoid staining in susceptible timbers (Wengert, 1990). The rate of drying may be controlled to some extent by the thickness of the stickers and their distance apart. Heavy weights may be placed evenly on top of the stack to minimize distortion. If stacks are too large for adequate ventilation then the air becomes saturated and drying ceases. Maximum dimensions have been given as 4 m wide and 5 m high with a 1 m minimum wide passageway between stacks. A suitable cover is constructed over the top to protect the stack from rain. The edges of the covering should extend beyond the sides of the pile to prevent rainwater from dripping onto the timber (Desch and Dinwoodie, 1981; Rietz and Page, 1975). The cover may be a shed, with or without fan-assisted ventilation (Figure 11.2).
- If carefully piled, the timber may dry to a moisture content of about 17– 20%. The time taken depends on numerous factors, including type and thickness of timber, season of the year (see Section 12.7), weather, and dry conditions within the yard. The latter is important because if the area around the stack is cluttered and damp then drying will be impaired. Good foundations (i.e. a flat surface that is sufficiently firm to take the weight of the timber) are also required to prevent deflection (Food and Agriculture Organisation, 1986). If conditions are satisfactory, 25 mm-thick redwood should air dry in about 3 months. Hardwoods such as beech and oak of similar dimension may take 7 or 8 months, and it is usual to add one year for each extra 25 mm of thickness (Desch and Dinwoodie, 1981). End shakes are the most common form of defect during drying; they occur because of fast evaporation from end grain. They may be avoided by using a sealant, or by using some form of end covering.
- Natural seasoning does not produce dry enough timbers for many purposes and requires a large amount of yard space for a long time. The timber industry in many parts of the world is having to become more efficient and competitive. As the cost of logs and the demand for dry timber rise, storage time and drying degrade have to be kept to a minimum if profits are to be maintained. Kiln drying (Figure 11.3) is therefore frequently more economical, but a range of defects may still occur. There are two conventional types of ventilated kiln. In the first the conditions are uniform and the timber remains stationary within the kiln for a set period of time while being dried by a current of hot, moist air. The second type of kiln is long and conditions become progressively hotter and less humid along its length. The timber is slowly moved from one end to the other as drying proceeds. The second type of kiln is more difficult to control, and is normally only used for softwoods, which can safely withstand faster drying than hardwoods. These kilns may assist in avoiding a high proportion of kiln-induced defects which have been shown to be caused by variations in moisture content in both the timber and the kiln. Defects are mostly produced by localized variations in conditions within the kiln. Figure 11.2 Air drying under cover in a drying shed. Timber is stacked in the kiln in the same manner as for air drying, but great care has to be taken because the risk of defects occurring is far higher under the more severe kiln-drying regime. It is essential to ensure that loads are composed of similar species and sizes. Different species of timber are dried according to schedules that have been developed by trial and error in order to hasten drying and reduce defects. However, the satisfactory kiln drying of timber depends ultimately upon the degree to which the individual pieces of wood are uniform in structure, upon careful monitoring and upon the skill of the kiln operator. It has not been unusual for up to 25% of pieces passing through a kiln to be subsequently downgraded due to kiln-induced defects (Hart, 1990). The cost implications of this may be substantial (data from Southern Pine Lumber Mills). Figure 11.3 Drying kilns at Henry Venables timber yard. Moisture movement from the core of the timber to the surface is a relatively slow process. If drying proceeds too rapidly because the air is too dry, then the outside of the timber may dry and set while the inside remains damp. The result is known as ‘case hardening’ (Figure 11.4), and differential drying when the plank is resawn will cause severe cupping (Stumbo, 1990). If case hardening continues too far then shrinkage of the tissue at the centre will be restrained by the hardened shell. The result is a mass of small shakes and defects at the centre which are invisible from the surface of the timbers. This defect is known as honeycombing (Figure 11.5).
- If the timber is very green and evaporation of moisture occurs too rapidly, moisture is forced from the timbers faster than air can be drawn in to replace it. In this case the diminishing volume of water causes the cell walls to cave in, especially in the spring wood, and the whole timber shrinks and warps. This is known appropriately as collapse (Figure 11.6). Most of these defects can be corrected by changing the drying schedule, provided that they are detected early by careful monitoring, and that they do not derive from abnormalities in the wood (Lamb, 1990). Collapse may be avoided by predrying. Most modern kilns today dry a charge of softwood every 24 h although some operate on a 12 h drying cycle (McConnell, 1990). Hardwoods must be dried at a much slower rate if defects are to be minimized, and 25 mm thick oak dried from green will take about 5– 6 weeks. This period may be halved if the timber is air dried first. One problem with conventional kilns is that the water collected from the timber may contain up to 3% of organic extractives, some of which may be hazardous (Singer et al., 1995). Waste disposal must therefore be taken into consideration. These condensates may also cause significant corrosion damage to the kiln (Little and Moschler, 1994).
(Above) Honeycomb shakes are separations of the fibres caused by drying stresses. These occur after case hardening when the interior of the timber dries.
- Several other methods of drying have been tried with varying success (an excellent overview is provided by Milota and Wengert, 1995). Drying by dehumidification seems likely to become of increasing importance, particularly for drying pretreated timber, and drying with vacuum kilns is becoming an accepted practice in many parts of Europe. Commercial vacuum kilns can produce a vacuum which halves the boiling point of the water within the timber; the moisture can therefore be driven out at a lower temperature than in a conventional kiln. Timber is frequently dried to a moisture content specified for a particular end use, and its moisture content when supplied should preferably be within about 2% of that specified. Dimensional change for every percentage variation in moisture content may be small, but may nevertheless affect gluing because a glue layer may only be 0.23 mm thick (Taylor, 1994). Care must be taken after drying to ensure that the moisture content remains stable, and that it is not altered by inappropriate transportation, on-site storage, or other building works. The average moisture contents that modern softwood timbers attain in dry buildings are generally about 12– 15% in domestic roofs, and perhaps 12– 14% in floor timbers (above ground floor) and joinery. Heating systems may reduce moisture levels to about 6– 9%, when cracking or joint separation may occur, whereas suspended ground floor timbers may have moisture contents in excess of about 18% – high enough to allow insect attack if sapwood is present.
- Recent amendments to BS4978 and BS5268 (Part 2) 1977 concerning dry stress-graded structural timbers, under 100 mm thick and for interior use, require that they be graded at a maximum average moisture content of 20%. If these timbers are subsequently pretreated with a water-based preservative then they must be redried to the same level. Timbers must then be marked as ‘Dry’ or ‘KD’ (and ‘Wet’ if undried, and therefore for outdoor use). Kiln drying will make a substantial difference to the cost of timber for repairs. Kiln-dried oak costs about three times as much as green (Venebles, 1993)
Figure 11.6 Collapse is a kilning defect that occurs in some timbers if a rapid loss of moisture causes cells to deflate.
- Large-dimension softwoods also present a problem, and sizes over about 100 × 300 × 6000 mm are likely to be green and prone to shrinkage.