Fencing timber

Consider what timber is available, as it may be possible to combine the need for fencing timber with woodland management work. Oak and sweet chestnut coppices are the traditional and best source of naturally durable fencing material, but other types of woodland and other species may also be usable. Larch and Douglas fir are moderately durable, and other species can be treated with preservative to prolong life as required. Non-durable local timber may make ideal fence posts for temporary or short-term fencing of young trees or newly-cut coppice.

The use of materials which support the management of traditional local woodlands should be a priority, as such timber is an environmentally friendly material. The use of naturally durable timbers avoids the use of toxic chemical preservatives, which contain carcinogens. During the lifetime of treated timber some toxic chemicals can leach into the soil where they remain as non-biodegradable compounds. More are released when the timber eventually rots. Much preserved timber is imported, and the environmental cost of transport, as well as the cost to the communities and environments where the timber is treated should also be considered.

Second-hand materials may be available. The most commonly used are railway sleepers and telegraph poles, which if still in good condition should give many years’ useful life as gate posts or straining posts. A simple method of testing such timbers is to knock them at intervals along their length with a hammer. A ‘ringing’ noise indicates sound timber, whereas a dull thud indicates rotting timber.

Timber decay

Wetting and aging do not in themselves cause timber decay. Decay is caused by fungi that feed on the timber. As described below, these fungi also require moisture, oxygen and warmth in order to live.

• Food. Timber which is naturally durable against fungal attack contains substances, produced by the tree, which are harmful or toxic to fungi. Treatment of non-durable timber with preservative produces the same effect.
• Moisture. Freshly felled timber may contain as much water as wood, when it is described as having a moisture content of 100%. Once dried to below 20%, timber will not decay as there is not sufficient moisture for the fungi to live. Timber dried to 20% will not re-absorb water from damp air only, but direct rainfall, splashing or the presence of any ‘trapped’ water, for example at timber joints, will cause the moisture content to rise above 20%. Thus non-durable, non-preserved timber used for fencing is subject to fungal attack, although the degree will vary with the location and the site. A fence in damp woodland will rot more quickly than a fence in an open, windy site. Timber in contact with the ground rots more quickly than timber not in ground contact.
• Oxygen. Fungi only require minute amounts of oxygen in order to live, and it is not possible to exclude oxygen from the wood by any form of surface coating such as paint or varnish. Timber totally immersed in water, or in an impermeable, heavy soil may avoid fungal attack due to the lack of oxygen.
• Warmth. Fungal growth and timber decay is most active at around 20 degrees centigrade. Below 5 degrees it becomes dormant, so although the rate of timber decay is reduced in cold winters, the British climate is not cold enough to control it.

Wood is also attacked by insects, but in exterior use, timber treated against fungal attack will also be proof against insects.

Heartwood and sapwood

Bark is the external corky layer of the trunk. It is impermeable, and must be removed before the timber is dried or treated.

Sapwood is the outer layer of wood. Its pale colour makes it easily recognisable in species which have a dark heartwood, but difficult to distinguish in pale timbers such as spruce. The presence of bark can be a useful indication of sapwood. Sapwood of all species has a low resistance to decay, but on the other hand it is permeable and relatively easy to treat with preservative.

Heartwood is the central part of the trunk, and is variable between different species. Some have naturally durable heartwood, while others are perishable, and likewise their permeability to preservative treatment varies from high to low (see table 4a).

Round timber, with a protective layer of sapwood that can be deeply penetrated with preservative, is the usual material for fence posts. Square posts of treated softwood are also suitable, but those containing a proportion of non-durable heartwood are more difficult to treat effectively. Round timber also has advantages in the construction of strained wire fences.

Cleft timber is split along the grain, and the cleft surface sheds water better, and therefore rots less quickly, than the same timber sawn.

Natural durability

Table 4a shows the natural durability of the heartwood of British native and introduced species. Durability will vary according to the site and weather conditions, and is also proportional to the size of the timber. For example, a post of cross section 100 x 100mm would have twice the life of one 100 x 50mm.

Ease of preservation

Table 4a shows the permeability to preservative of different heartwoods. All sapwoods are in the categories of either moderately resistant or permeable, and can therefore be effectively treated.

Timber must be dried to 25-30% moisture content before treatment. This is done either in the open air or in a kiln, which is a very much quicker and more accurate operation.

Preservative treatments

The following section describes the various types of preservative and methods of application.

Waterborne copper chrome arsenate (CCA)

CCA preservatives can only be applied by vacuum pressure impregnation. Within about a week of treatment the preservative becomes chemically bonded to the timber and is resistant to leaching. Timber should never be supplied still wet from treatment. The treated and dried timber has a clean finish of a pale greenish-grey colour, and the surface can be treated with a stain or other coating.

‘Tanalised’ timber, treated with a CCA product called Tanalith, is widely available from suppliers of fencing. ‘Jakcured’ timber (Jacksons Fencing), and other CCA treated products are available.

Table 4a British Timbers (Heartwood)

Application methods

Commercial methods using pressure and heat are very much more effective than any ‘DIY’ method of treatment. If you have sufficient quantity, home-produced timber can be taken to a timber preservation company for treatment.

Vacuum pressure

Under this system timber is put into a sealed cylinder, and then a vacuum is used to draw air out of the timber. The cylinder is then filled with preservative, and pressure applied to force it into the wood. This method is suitable for CCA and creosote preservatives, and gives the highest standards of protection.

Hot and cold open tank

This process is only used for applying creosote. The timber is immersed in an open tank of creosote, which is then heated to 90 degrees centigrade, and kept at this temperature for one to three hours. The heat causes expansion of the air in the timber. The tank is then left to cool, and as the timber cools it draws in the creosote.

End-sealing

Avoid cutting treated timber unless necessary for the construction of the fence. Even under pressure, the preservative only penetrates the outer layer of wood, and any cut across the grain will expose untreated timber. Don’t finish off the fence by cutting the tops of the stakes just to give a neat and even appearance. Where cuts are unavoidable, such as in fitting struts to strainers, treat the cut area by brush application.

Special products are available for the treatment of cut areas. Check with the supplier of the posts which product they recommend. When applying by brush, flood the cut area with preservative, and as soon as the first coat has started to dry on the surface, apply a second coat, to keep the preservative moving into the wood. When applying any timber treatments a COSHH assessment will be necessary to identify the controls appropriate for the product. These might include the use of personal protective equipment, and limiting access to the area during the treatment process.