The timber deck or boardwalk is an increasingly popular feature of both domestic and commercial landscape projects. Where such structures are intended to be permanent, correct timber selection is the crucial first stage to achieving a long and safe service life.
For quality outdoor structures, a service life of 15 years is considered to be the minimum design standard by the TDA, the independent technical and advisory association for timber decks, boardwalks and associated landscape joinery. Longer service lives of 30 years and 60 years are also readily achievable when required such as on new homes. The TDA produces detailed guidance about materials selection, design and installation good practice and its recommendations are accepted as the industry benchmark by organisations like NHBC.
When it comes to selecting timber for outdoor landscaping the TDA guidance is very clear: Only specify timber that:
A) has appropriate natural durability or
B) has been made suitably durable by an appropriate industrial wood protection process.
C) is from certified, legal and sustainable sources
Another key design caveat, often overlooked, is the need to specify that components destined for outdoor use must have a moisture content below 20% at the time of installation – more about this later.
Naturally durable timber
It is mainly hardwoods that British Standards rate as being naturally durable however, Western Red Cedar, a softwood, is also rated in this category. Designers need to be aware that natural durability ratings only apply to the heartwood of any timber species. As such, specifications for hardwoods, whether they are temperate or tropical, should always contain a clause requiring the use of components from which all sapwood is excluded.
Hardwoods with a long history of out door structural use are set out in the Timber Design Code BS5268:2. Those considered most suitable for decks and boardwalks are listed in Figure 1 below:
HARDWOODS: BS5268:2 listed For outdoor applications
Balau (yellow)
European Oak
Iroko
Jarrah
Karri
Opepe
Teak
Other suitable hardwoods
Cumaru, Garapa, Ipe, Keruing, Kempas, Massaranduba, Tatajuba
The major suppliers of hardwoods such as those covered by the DeckMark™ quality assurance scheme operated by the TDA can provide designers with details about the performance characteristics of any of the hardwoods they supply.
The use wood protection technology
Enhancing durability by an industrial wood protection process is typically associated with softwoods. Because the sapwood of most commercial softwoods can be treated, it makes economic sense to convert the entire log into useable timber products – a list of suitable softwood species is set out in Figure 2. below.
The principal, tried and tested, method of protecting softwoods destined for outdoor applications is pressure impregnation. Wood preservatives containing arsenic and chrome are no longer used -today the product of choice is a water based solution containing copper and organic biocides. Wood treatment is a more scientific process than ever before. It is tailored to the species, the end use and the service life required 15, 30 or 60 years. Full technical standards can be found in BS8417 and in the specification manual of the Wood Protection Association. BSEN335-1 defines five “Use Classes” for constructional timber. When buying or specifying timber for outdoor structures make sure the component has been treated to the correct use class standard. Use Class 3 is suitable for external applications where the component is out of ground contact; whereas a component that is in permanent contact with the ground or freshwater must be treated to “Use Class 4 standard. It is the responsibility of the treater to achieve the designer’s specification and as such the TDA recommends the use of treatment companies registered by the Wood Protection Association.
A list of suitable softwoods for deck and boardwalk construction is shown in Figure 2. Below.
SOFTWOODS:
For outdoor applications
British pine/European Redwood
Corsican Pine
Radiata Pine
Southern Pine
Douglas Fir
Larch
Spruce/European Whitewood¹
Western Red Cedar²
¹ Spruce is not recommended for ground contact uses when a service life in excess of 15 years is required.
² Western Red Cedar containing sapwood must be treated with preservative
In addition to preservative impregnation new forms of enhanced performance timber is now emerging known as modified wood. Modified wood describes softwoods whose structures have been modified by a physical agent such as heat or by biological means to improve durability – Accoya™- acetylated wood.
Follow manufacturers installation instructions
Research carried out by the TDA last year indicates that the performance of treated wood is being compromised by a failure to follow manufacturers installation instructions. Failing to reseal timber that has been cross cut or notched on site means that the potential for moisture penetration and wood rot are increased significantly. . If a structural component is involved then the ongoing safe use of the deck becomes a major issue. According to the TDA there is a belief, particularly in the DIY and jobbing builder sector that if wood is green treated it can be used and abused it in any situation. The TDA is quick to point out that this is a myth and like any other manufactured product, if treated wood is not used and installed properly it will not deliver the performance that is required by the designer and the customer. Users who ignore manufacturers instructions are inviting customer problems with all the time, trouble and expense that is then involved in putting them right. Make sure end sealing is built into contractors site instructions.
Another important, but often overlooked, factor in specifying timber for outdoor applications is moisture content.
Installation moisture content
Wood is a natural material – it is hygroscopic – in other words it gives off and takes on moisture. When the moisture content of wood is in balance with its surroundings it is no longer gaining or losing moisture. This is known as the Equilibrium Moisture Content (EMC) level. In the UK, the EMC ranges from 19% in winter to 12% in summer. To minimise defects such as warping cupping, cracking caused when timber shrinks to reach its EMC all timber components used for outdoor structures must have a moisture content less than 20% at the time installation. Shrinkage defects are exacerbated in summer as wood with a high moisture content will shrink rapidly and this will be made worse if the deck surface is exposed to summer sunshine. Designers should always protect their customers interests and their own reputation by making sure installation moisture content is detailed in sub-contracted work.
The TDA is working with other timber industry organisations like the Wood Protection Association and Wood For Good to help raise supply chain knowledge about specifying and installing pressure treated wood for decks and landscape joinery. This includes access to a one- hour online training programme access to which can be had through the TDA web site www.tda.org.uk
Contact Steve Young 01977 558147. director@tda.org.uk
Picture caption:
Re sealing cross cuts made on site ensures the long term, safe life of pressure treated wood. It also ensures any guarantee given by the manufacturer remains valid.