Examining traditional wood fence post deterioration

IN what could be a big blow to commercial forestry and timber, the Association of Fencing Industries (AFI), the trade body representing fencing installers, is telling contractors to consider alternatives to wood when installing new fence posts. This follows a survey of its members to investigate the scale of traditional wood fence post deterioration from fungal decay and insect attack.

An article surrounding this story recently appeared in Farmers Weekly; perhaps the worst possible place for it to impact on the forestry and timber industries because it will be read or relayed to just about every farmer in England.

Wooden fence posts have been on the defensive from fungal rots and wood-destroying insects ever since the European Commission banned the use of CCA (chromated copper arsenate) timber treatment in 2006, leaving most wood fence post producers to supply UC4 timber products comprising kiln-dried wood subsequently treated with a wood preservative.

The AFI survey, which received more than 200 responses, showed installers experiencing escalating incidences of fence post failure. 75 per cent of respondents experienced post failures between 2006 and 2012, rising to 90 per cent since 2012. 80 per cent of respondents had considered concrete, steel or even plastic alternatives to wood.

A scene from Suffolk Coastal District. UC4 preservative-treated pine fence posts and outdoor pig rearing shielded by a windbreak of pine trees. Will the pigs have to get used to plastic fence posts?

READ MORE: Abbey Timber: Taking traditions forward

UC4 post suppliers offer a 12–15-year guarantee for their products but the AFI says such guarantees do not cover the full replacement cost and often come with lots of provisos, limiting the value of the guarantee. The AFI told Farmers Weekly that, unless ‘proper guarantees’ covering the full cost are offered, it would have no option but to recommend installers only use chemically treated wood in instances where timber is in contact with the ground and make it plain to the client that fence post failure due to microbial-induced rotting and/or insect attack and damage is not the responsibility of the installer.

The Wood Protection Association (WPA) strongly disagrees and said the wood preservative systems used by its members are effective, provided they are applied in accordance with manufacturers’ instructions and installed correctly. The WPA emphasised how these biocidal products are subject to rigorous testing by the Health and Safety Executive prior to approval, while acknowledging recent examples of premature post failure of preservative-treated timber in contact with the ground. It said this could have a negative impact on the perception of the preservative treatments adopted since CCA was banned 15 years ago. However, they appeared to put most blame on users going for cheaper products rather than paying more for higher quality.

LOOKING AT QUALITY THE WRONG WAY ROUND

This last comment around efficacy, quality and price is especially pertinent because quality should be looked at first and foremost in relation to the wood and not the processing and preservative. A range of softwood species are used to produce UC4 timber, although what the industry calls ‘redwoods’ appear to be most popular. These include western red cedar and Scots pine, the latter sourced from northern Europe including Scandinavia, the Baltic states and Russia. Known as European redwood or Scandinavian redwood, this is considered ideal due to added strength from slower growth rates than Scots pine grown further south. The inherent open cell structure of these ‘redwoods’ makes for easier impregnation with wood preservatives.

Sweet chestnut is already used in a big way for fencing – finished fence posts ready for dispatch from a sawmill in Essex.

The wood is kiln-dried first to drive off significant amounts of water, a good thing in its own right to reduce the risk of microbial decay, but additionally to allow deeper penetration of the pressure-applied wood preservative with which it is subsequently treated. Kiln drying is used to reduce the moisture content of the wood to around 28 per cent.

Resins and tannins are natural chemicals known to confer resistance to insect pest and fungal attack. Western red cedar is fairly rich in tannins but doesn’t contain any resin whereas Scots pine is rich in resin but low in tannins.

HARDWOOD GEMS FOR FENCING

However, there are hardwoods which show good resistance to rotting while in contact with the ground due to high tannin content and inherently low proportions of rot-susceptible sapwood, which makes them ideal for fencing. There are clearly higher priority uses for these generally more expensive hardwoods, but some are already used to make fence posts with surplus going to biomass or firewood.

A prime example is sweet chestnut, dripping with natural tannin and possessing a comparatively low proportion of sapwood. There are plentiful supplies of chestnut coppice across south-east England, much of which is overstood with dimensions ideal for fence posts, whether cleft or sawn. Much of this chestnut resource is currently used as biomass.

When winter-cut to ensure the sap is down, sweet chestnut yields a very durable wood. Large 8-inch posts can still be found intact after 30 years in the ground. As a general rule of thumb, a good quarter-cleft stake (5–6 inches standard) can be expected to last 8–10 years. Despite the high level of natural resistance to microbial-induced rotting, chestnut posts can also be treated with preservative to extend their natural life in the ground. Natural regeneration of coppiced chestnut means continual supplies from existing plantings, with users unlikely to get a better look for the farm landscape.

Invasive false acacia growing a long way from its native North American home, at Bridgwater in the Adelaide Hills, South Australia.

Sweet chestnut is not a native species, but is long naturalised in the British Isles. However, there is another exotic and more recently naturalised tree with an even better reputation for resilience in the ground, although this particular quality of false acacia (Robinia pseudoacacia) timber is not widely known.

Robinia pseudoacacia, a native of North America, was brought to Britain in the 17th century. A range of mechanical and chemical characteristics combine to produce wood which is exceptionally durable when in contact with the ground. False acacia is heavy, hard, strong and close-grained, and contains flavonoids in the heartwood. A combination of these physical characteristics and chemical profile allows the wood to last for many decades. As a young man, Abraham Lincoln is credited with spending much of his time splitting false acacia logs for rails and fence posts on his father’s farm in Kentucky.

False acacia is a tree that fence post producers will never be short of. The species has been widely planted in Europe and is now highly invasive in many areas. Conservation groups curse false acacia for its alien credentials while landscapers plant the tree for the highly attractive foliage and flowers. But largely hidden and not exploited is one of the most durable of temperate woods for outdoor fencing.

Ironically, one of our traditionally best timbers for durability in outdoor situations was consigned to history by the combined activity of an insect pest and a fungal disease on the living trees. This was English elm, destroyed in the 1970s by Dutch elm disease caused by a fungus ferried between elm trees by bark beetles.

AUTHOR’S NOTES:

1. Chromated copper arsenate (CCA) proved to be highly effective as a wood treatment against fungal decay and insect attack. Use of CCA as a timber treatment was banned in 2006 (by the EU) on safety grounds, mainly due to the arsenate in the chemical product. Copper as a standalone chemical in compounds like cuprous oxide, copper oxychloride and copper sulphate is a highly effective fungicide and bactericide and has been used effectively and safely in agriculture and forestry for well over 100 years. Some contemporary approved wood preservatives contain copper, including those based on copper-triazole technology.

2. The end use of timber treated with wood preservative is classified into one of five main categories defined in BS EN 335-1, and based on the potential threat to the timber of fungal decay and/or insect attack in its eventual application. The use classes for the treated timber are:

UC1 – internal, dry (e.g. upper floor joists).

UC2 – internal, risk of wetting (e.g. roof tile battens).

UC3 coated (3.1) – outdoors, coated above ground (e.g. soffits/cladding).

UC3 uncoated (3.2) – outdoors, uncoated, above ground (e.g. cladding and fence rails).

UC4 – direct contact with soil or fresh water (e.g. fence posts).

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