Timber

10.1

Cross-section of a tree trunk.

10.1

Cross-section of a tree trunk.

10.2

Logs of pine with large amounts of heart-wood.

10.2

Logs of pine with large amounts of heart-wood.

There is no doubt that the earliest building material in common use was wood, as branches, bushes, etc. (Wright, 2005). Through subsequent ages, people continued to reproduce such light shelters, both permanent and transportable (cf wigwams, yurts, bowers ...). However, solid log houses appeared in Northern Europe during the Neolithic Period and eventually a complete mastery of joinery and carpentry permitted massive construction with accurately shaped timber.

Trees are mainly composed of long cells stretched vertically, forming wood fibres. Across the trunk are pith divisions, forming rectangular cells. This structure gives timber elasticity and strength. Cells vary in form from timber to timber, but they all contain carbon, oxygen, hydrogen and nitrogen as their main chemical constituents. They also contain small amounts of minerals, which are left in the ashes if the tree is burned. A healthy tree consists mainly of cellulose, lignin and other organic substances such as proteins, sugar, resin and water.

A cross-section of a tree trunk consists of bark, bast, sapwood, heartwood and pith (Figure 10.1). The annual growth rings in a tree are visible because summerwood is darker than springwood. The number of rings gives the age of the tree, and the width of the rings indicates the growth conditions and, therefore, the quality. In coniferous trees, narrow rings usually indicate better quality than wide rings. In deciduous trees wide rings indicate better quality timber.

On the island of Madagascar there are 1000 species of tree. In Northern Europe there are only approximately 35 species, of which about two-thirds can be used for construction (Table 10.8). Despite this, usually only two coniferous trees are used (spruce and pine), and increasingly large areas supporting deciduous trees have been given over to industrial cultivation of spruce and pine forests. There is also a tendency to replace pine

Table 10.8 Wood species used as timber

Species

Properties1

Areas of use

Alder, common (Alnus glutinosa)

Not particularly durable in air, very durable under water, soft, light, brittle, twists easily, easy to work

Piles, gutters, plywood, internal cladding

Apple (Malus pumile)

Hard, homogeneous, hard wearing, low resistance to moisture

Wooden screws, dowels, thresholds

Ash, common (Fraxinus excelsior)

Hard, dense, tough, elastic, low resistance to moisture, easy to bend under steam

Flooring, plywood, internal panelling, stairs, internal structural details

Aspen (Populus tremula)

Moisture resistant but strongest when dry, does not twist

Flooring, plywood, suspended ceiling, smaller structures, cladding, piping for water and gutters, piles

Beech, common (Fagus sylvatica)

Hard, strong, medium resistance to moisture, twists easily, no smell, easy to work

Flooring, balustrades, smaller structures, plywood, internal panelling, tar, vinegar

Birch (Betula pubescens and Betula pendula)

Tough, strong, elastic, low resistance to moisture, twists easily, easy to work

Flooring, stairs, internal panelling, plywood, chipboard, bark for damp proofing, smaller structures

Douglas Fir

(Preudotsuga menziesii)

Durable, easy to work, limited moisture movements, difficult to impregnate

Structures, flooring, cladding

Grey Alder (Alnus incama)

Not particularly durable, light and brittle, easy to work

Internal panelling, veneer

Hazel, common (Corylus avallana)

Strong and elastic, not particularly durable

Wattle walling in timber framework

Holly (Helix aquifolium)

Hard, homogeneous, hard wearing

Plywood

Juniper, common (Juniperus communis)

Tough, firm and very durable, difficult to split but easy to work

Cladding plugs

Larch, European (Larix decidua)

Tough, strong and durable, good moisture resistance, easy to work, cannot be painted

Structures, flooring, doors, windows, roofing

Lime (Tilia cordata)

Tough, medium strong, slightly elastic, easy to work

Smaller structures (used for log buildings in the Carpathians), internal panelling, plywood, fibre for woven wallpaper and rope

Maple (Acer platanoides)

Hard, dense, tough, elastic, flexible, hard wearing, low resistance to moisture, easy to work

Flooring, balustrades, stairs, plugs

Oak, English (Quercus robur)

Dense, heavy, hard, hard wearing, elastic and durable, tendency to twist, quite difficult to work, moisture resistant

Structures, flooring, windows, doors, thresholds, plugs, cladding, roofing

Plane (Platanus hybrida)

Medium resistance to moisture, medium workability

Plywood

Plum (Prunus domestica)

Splits easily when dried

Plywood

Rowan or Mountain Ash (Sorbus aucuparia)

Heavy, hard, tough, durable, hard wearing, easy to work

Wattle cladding on external walls

Table 10.8 (Continued)

Scots Pine (Pinus sylvestris)

Soft, elastic, strong, durable, easy to cleave and work, denser and more resin than in spruce, difficult to glue and paint, can be pressure impregnated

Structures, flooring, cladding, windows, doors, tar, roofing, foundations below ground level, plugs

Sitka Spruce (Picea sitchensis)

Low resistance to moisture, medium workability

Joinery, cladding

Spruce, European (Picea abies and Abies alba)

Soft, elastic and medium hard wearing, sensitive to moisture, easy to glue and paint, difficult to pressure impregnate

Structures, cladding, laminated timber, fibreboard

Sycamore (Acer pseudoplatan)

Medium resistance to moisture, large moisture movements, easy to work

Flooring

Walnut (Juglans regia)

Moisture resistant, medium moisture movement, easy to work

Plywood

Western Red Cedar (Thuja Plicita)

Very durable, limited shrinkage, low strength, high resistance to mould, easy to work

Exterior cladding, roofing

White Willow (Salix alloa)

Tough, elastic, easy to cleave

Veneer, wattle cladding on external walls

Wild Cherry (Prunnus avium)

Stable, hard wearing

Flooring

Wych Elm (Ulmus glabra)

Strong, tough, elastic, durable, moisture resistant, not particularly easy to work

Flooring, balustrades, piles, stairs, panelling, internal structural details

1 Varies according to place of orgin and the conditions of growth.

1 Varies according to place of orgin and the conditions of growth.

with spruce, as it produces less waste and is more practical to handle in the sawmill.

Many deciduous timbers have qualities that should encourage their more widespread use in building. In certain areas they are superior to spruce and pine, because of their higher resistance to moisture and greater strength. Ash, for example, is 60 to 70% stronger than spruce. Buildings using only accessible deciduous trees, and using materials according to their strength, could reduce the amount of structural timber needed by 25% (Bunkholt, 1988).

In India, 300 different types of timber were analysed to assess how useful they could be in building. Factors such as weight, strength, durability and shrinkage were investigated. Timber varieties were then graded according to their properties. By doing this a whole new range became available for use in building, including types previously classified as non-resources or firewood.

Timber is often used in its untreated form. Resistance to rot and fungus is most often increased by applying biocides on the surfaces or by pressure treatment where the whole cross-section of the timber is impregnated. There are also less hazardous ways of improving the durability, e.g. heat treatment and chemical modification, see Chapter 19.

Shredded wood in various ways, often from waste and off-cuts, is also common in various building boards such as chipboard. Sawdust and wood shavings are also used as loose filling for thermal insulation. Defibrated wood is shavings treated with humidity so that the binding agents in the wood are softened, then pulped to a mass with fibres of

1.5 mm thickness or more. This increases the specific surface area by 100 000 times and this is the raw material for the production of cellulose as well as for acoustic and thermal insulation in loose form, as matting and boards. Matting and boards often include glues. Harder board products are often produced at high temperatures, and by applying high pressure the natural binders within the wood melt and act as the glue. Fibrous wood mass has also become common more recently as filler and reinforcing in plastic materials.

Latex from rubber trees has been widely used as a water repellant in exterior quality building boards. Tar from conifers is used for surface treatment of timber. And all timber contains useful materials that can be used to produce solvents, bioplastics, etc.

10.2.1 Forestry

Modern forestry is often managed as a monoculture of coniferous trees, mainly spruce. This is especially the case when producing timber for the cellulose industry. Such monoculture forestry leads to acidic soil, and reduces the pH level in the rivers; these forests are, ecologically speaking, near-deserts; few ecological systems can function. They also can cause increased soil erosion due to comprehensive drainage systems that quickly channel rainfall into rivers and streams, and therefore threaten many species of plants and animals.

Forestry can, however, be run on ecological principles. The key lies in the natural regeneration of a varied forest. This requires sowing seeds of a wide variety of local tree species, including deciduous trees that prevent acidity; and careful harvesting so that younger trees and other plants are preserved. There is clear evidence that timber from these mixed forests is of a higher quality than that cultivated in monocultures (Thornquist, 1990). When felled, the bark is kept in the forest, providing nutrition on the forest floor including nitrogen from coniferous needles that make the addition of artificial nitrogen fertilizer unnecessary.

People used to be far more careful when selecting trees for felling. They chose mature trees: conifers more than 80-years-old, most deciduous trees between 30 and 60-years-old. Beech and oak should be well over 100-years-old to be ready for felling. The definition of a mature pine is that pith and heartwood form at least half of the cross-section of the trunk. In both spruce and pine the heartwood begins to form around the age of 30 to 40 years.

In view of climate change it is becoming important to maximize the capacity of different trees to bind carbon. For example, in spruce, the uptake of carbon slows down considerably earlier than in pine (Liski etal., 2001). One should thus operate with a shorter rotation cycle for spruce than pine - which is in accordance with the conventional view of the maturing times of these species.

The best quality conifers grow in lean soil. Heartwood timber shrinks less than other timber and is more durable, making it well suited to the construction of doors, windows or external details. The demands of quality are lower for the production of cellulose, internal panelling, etc.

In order to make the best of heartwood timber, it used to be prepared while the tree was still standing. This process, called

10.3

A traditional method of cultivating special qualities in timber.

10.3

A traditional method of cultivating special qualities in timber.

self-impregnation, is known in many cultures, from the British Isles to Japan. The most common method is to lop off the top of the tree and remove a few strips of bark from the bottom to the top. Three or four of the highest branches are left to draw up resin. After two to seven years the whole trunk is filled. There is little growth during these years, but a very high timber quality is produced. The method is especially effective with pine, which contains three times as much resin as spruce.

When timber for felling was first categorized, people would find suitable special features for its use, such as diagonal ties and bracing in post and lintel construction or framework construction. Crooked trees and round growths on the roots of trees proved particularly interesting. The tree could also be worked with and formed while it was still growing to achieve certain particular effects (Figure 10.3). Traditional English framework structures are, in many cases, based on intentionally bent timber. A 'bulge' occurs when a coniferous tree that was bent straightens up, the bulge occurring on the underside of the bend. Timber at this point is close knit and strong and has been used for exposed items such as thresholds. It can be considerably stronger than laminated timber.

With today's production techniques, hand picking timber has become uneconomical. Even the quality of timber is given little attention, apart from the desire for straight trunks with few knots. Focus has been on volume not quality. However, there is a growing awareness of issues of both sustainable forestry and timber durability, so there may perhaps be a revival of small, more specialized industries in this field. In Sweden research is now being undertaken to evaluate the possibilities of differentiating qualities of timber in modern forestry, in order to return to a situation where the best quality timber is used in the most exposed situations.

Felling. Both deciduous and coniferous trees intended for construction purposes should be felled in winter when the quantity of sap is at its lowest and the state of swelling and acidity are at their most favourable. Timber felled during spring and summer is more susceptible to mould. Another advantage of winter felling is that the sawn timber dries out more slowly and is therefore less likely to split. Many felling traditions were related to the phases of the moon. Coniferous trees were felled at full moon, as it was assumed that the resins were well drawn out of the roots and into the trunk.

It has been assumed that the large amount of mould damage in newer Swedish timber buildings, especially in windows and external panelling, relates to the fact that the timber was felled during the summer-a usual practice in Sweden during the 1960s (Thornquist, 1990).

Storage. Although newly felled timber should be treated as soon as possible, it is usually some time before this can be done. The timber should be ideally stored in water, where there is hardly any oxygen. This reduces the risk of mould and insect damage. Timber stored in water during the summer months, however, often becomes porous through the action of anaerobic bacteria that eat the contents of the cells and pore membranes. This can dramatically reduce resistance to rotting, as water is more easily absorbed.

Splitting. The trunk is transported to the site where it is to be milled. Splitting should take place while it is still damp. For log construction and certain other forms of building the log is used whole, occasionally with its sides trimmed slightly flat with an axe. Pine has a longer lifespan if it is split in this way along two sides, because the hardness and amount of resin increases towards the centre of the trunk. Spruce should not be chopped along its sides, because the outside wood is stronger and heavier than the wood in the middle of the tree.

The oldest way of splitting a trunk is by cleaving through the core of the tree. The halves can be used as logs almost as they are, or they can be trimmed to a rectangular cross-section. They can otherwise be cleft radially, giving planks or 'shakes' with a slightly triangular profile. In technical terms, cutting timber trunks radially (in towards the centre) gives the most stable timber sections; however it is more time consuming and is seldom done in modern processes.

With the invention of the vertically adjustable saw during the sixteenth century, splitting timber by saw became the dominant technique. This method was particularly effective for cutting logs into panelling. For the last couple of centuries, the even more efficient circular saw has been used. For this method, there has to be a rotational force, usually produced by electricity. Rotational energy can also be produced directly by local wind or water turbines, as was the case in most former timber mills. In this way, the loss of energy in conversion to and transport of electricity is eliminated, and energy consumption can be strongly reduced. One should note that sawmills create a lot of dust in the working environment. Dust from western red cedar contains allergenic thujaplicins, and dust from oak and beech is carcinogenic.

There are different techniques of sawing logs: sawing through and through, boxing the heart, true quarter cutting and quarter cutting (see Figure 10.4). Boxing the heart works well with the circular saw and is almost the only method used today.

The wedge is more sensitive with wood than the axe, and the axe is more sensitive than the saw. By using a wedge, the cells are kept whole when the wood is split. Saws cut through the cell walls. This is critical for the timber's absorption of water, which in turn governs the risk of attack by mould or insects. In spruce, which when whole has an impermeable membrane between the pores, this is particularly important.

Timber from deciduous trees often has high inner tensions. To avoid twisting in the sawn timber, it is preferable to keep to smaller dimensions. On average, a sawmill will convert about half of a typical trunk to construction lumber, whilst the rest is by-products and waste, often going to production of paper, energy or chemicals.

Drying. Some researchers say that the drying routines for freshly sawn timber are more important for its durability than the time of felling (Raknes, 1987).

In order to use sawn timber, 70-90% of the original moisture must be dried out, depending upon the end use. The sawn timber is stacked horizontally with plenty of air movement around it, and is dried under pressure. The stack can be placed outdoors or in special drying rooms. The outdoor method is more reliable for drying winter-felled trees during spring, as artificial drying produces some

10.4

(a) Different methods of dividing up timber; (b) Qualities of panelling and planks.

10.4

(a) Different methods of dividing up timber; (b) Qualities of panelling and planks.

problems. Certain types of mould tolerate the temperatures used in this technique, and develop quickly on the surface of the wood during drying, emitting spores that can cause allergies. It has also been noted that the easily soluble sugars that usually evaporate during the slow drying process are still present in artificially dried timber, and become a perfect breeding ground for mould. It is also possible that the natural resins in the timber do not harden properly. This could be, for example, why there often are considerable emissions of natural formaldehyde in buildings made purely of timber. Formaldehyde is an unwelcome substance in an indoor climate, and can cause irritation in the ears, nose and throat, allergies, etc. Another reason for drying timber outside is the lower energy consumption, which for an ordinary load of timber rises by 300% when dried artificially.

Drying outside is best carried out in the spring. The number of months required for drying can be roughly estimated by multiplying the thickness of the timber in centimetres with 3.2 for spruce and 4.5 for pine. Normal planks take about three months, deciduous trees take longer. When building with logs it is best to fell, notch and use the timber while it is still moist. Logs with large dimensions have a long drying time - it can take years. A log building will therefore shrink between 5 and 10 cm on each floor. When the moisture content has decreased to 15-20%, windows and panelling can be installed.

10.5

Solar drying of timber. Source: Hall, 1981.

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