Get off the fence
- Date: 21/09/2007
Building Magazine: Whether it’s timber, concrete, steel or plastic, a garden fence has to be sturdy and secure. Peter Mayer of Building LifePlans looks at the life-cycle costs.
Which side of the fence are you on? On the one side, fences can be specified to BS 1722, which is a recently updated suite of standards that cover common timber, steel and concrete components. On the other side, numerous outlets offer fencing products of non-specific performance, usually at a lower cost but an uncertain durability.
The specification of fencing is about finding a balance between appearance, security, privacy and suitability for a given location. The whole-life cost is related to service life, maintenance and the risks associated with vandalism and weather.
BS 1722 gives advice on the applicability, design and installation of fence types. The expectation is that the fencing will be installed by trained installers.
The most common wooden fence styles are covered by BS 1722. They include cleft chestnut pale, post and rail, close-boarded, palisade and prefabricated panels.
Specification to BS 1722 ensures the minimum size of timber components, the quantity of timber, a maximum water content of 28% and wind resistance.
The standard life expectancy of timber fences to BS 1722 is 15 years, with an option to specify for 30. Some manufacturers offer guarantees for periods between these limits. Durability is achieved by using durable timbers, such as oak, to BS EN 350, or moderately durable softwoods that are treated with preservative to BS 8417. Regular treatment or decoration is a matter of choice.
Components can be in one of two service environments. Posts and gravel boards in contact with the ground are in use-class four to BS EN 335. These need a higher degree of preservative treatment than components not in contact with the ground, such as rails or panels defined as use-class three.
Precast or prestressed concrete fencing elements should be specified to BS EN 12839. This details requirements for mechanical properties and appearance. Where solid panels are proposed, ensure the correct class of post is specified for the expected wind loads and fence height; there are three classes.
BS 1722 adds a practical aspect to the performance-based BS EN 12839 by stipulating minimum dimensions of concrete posts and gravel boards as well as the materials and method of manufacture of precast concrete.
The service life of concrete components is not stated but, in non-aggressive conditions, 25-35 years would be reasonable.
Low-carbon steel, continuous bar and hurdle, vertical bar, palisade, strained wire and mesh, open-mesh and anti-intruder chain-link fences are covered by BS 1722.
Service life is confirmed by galvanizing solid steel components to BS EN ISO 1461 after fabrication. The thickness of the galvanizing is related to that of the steel. Service life depends on the corrosivity of the atmosphere where the fence is located and thickness of galvanizing. The period to first maintenance would be 25-55 years for steel sections that are 1.5-3mm thick. Applying paint increases the period to maintain the galvanizing, but paint coatings typically have a redecoration cycle of three to eight years.
Steel wire is specified to BS EN 10223 and BS EN 10244 or BS 4102, which detail quality of steel and corrosion protection depending on wire diameter. Corrosion resistance is provided by coating with zinc or zinc alloy; durability can be enhanced by the addition of an organic coating such as powder coating. Service lives for wire fences are in the range of 10-30 years. Longer lives may be achieved with heat-applied coatings.
UV-resistant, recycled plastics are being used for fencing, but there are no standards by which to guarantee performance. Claims of 30-40 year service lives have been made for solid plastic section fencing, which should be distinguished from hollow section plastic profiles. Thin hollow profiles are unlikely to last as long, particularly where there is risk of impact damage.
Manufacturers should provide guidance on the design, spacing and fixing of component parts and take account of the high coefficient of thermal expansion associated with plastics.
The benefits of plastic fencing are reduced maintenance costs from not requiring decoration where used in public spaces and reduced costs associated with vandalism.