Timber has always featured widely in landscape design, due both to its physical properties and to its attractive appearance, which harmonise naturally with the landscape.
Examples of the uses of timber in landscape architecture include:
- fencing, gates, retaining walls
- pergolas, screens, arbours
- stairs, decks, boardwalks, ramps
- bollards, barriers, handrails, balustrades
- fixed benches and tables
- bin enclosures and planters
- shelters and bus stops
- signs and signposts.
These all have to resist the elements as well as wear and tear by people and/or animals. Designers and specifiers of landscape architecture, therefore, benefit from a working knowledge of the factors which must be considered when using timber outdoors.
Specifying for desired service life
- Specifiers can achieve a long service life by careful selection of species, treatment and finishes.
- Preservative-treated softwoods can achieve good results, while hardwoods offer extended durability, better resistance to wear and tear, and higher strength.
Specifying for wear and tear
- For wood in contact with the
ground, use ‘durable’ or ‘very durable’ species. Species that are less than ‘durable’ require treatment with preservatives. Modified wood may be a preferred option to preservative-treated wood.
- Some timber species can accelerate the corrosion of metal fixings, especially exposed mild steel. Hot galvanised metal components are usually sufficient, but greater protection against corrosion and longer service life can be achieved by specifying a suitable grade of stainless steel.
- Use stainless steel screws and nails.
Importance of moisture in wood in service
- The moisture content in wood fully exposed to the weather can vary from below 12% to above 20% in the course of the year, so timber components for service outdoors in the landscape should be installed at approximately 14%–18% average.
- Allow additional fixings to restrain warping and gaps for wood to expand due to seasonal variations in moisture content.
Finishing timber out of doors
- Finish can be natural (weathered), translucent (grain exposed) or opaque (grain concealed).
Detailing and maintenance
- There are certain basic principles of good practice to follow in order to achieve a long service life at reasonable cost; correctly specified timber used above ground will not generally decay unless poor detailing permits moisture to become trapped.
- Designers can extend the life of timber components considerably by attention to detail.
TRADA (Timber Research and Development Association) is an international membership organisation dedicated to inspiring and informing best practice design, specification and use of wood in the built environment and related fields. This is an extract from its WoodInformation Sheet Timber for landscape architecture. For more comprehensive information, please visit TRADA.co.uk.