Timber Building
20 August, 2008
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All decked out
Winter 2006
Published:  10 December, 2006

Oak walkway at software company SAS Ltd headquarters near Marlow Brocklehurst architects ltd

Timber decking continues to be popular in public and private applications. TRADA consultant architect Patrick Hislop RIBA examines trends and highlights points of good practice for design, installation and maintenance

As with any timber product, certain standards of design, manufacture and materials for timber decking must be maintained. With the growing trend towards large public areas of decking and walkways, this is even more important.

In the past five years or so, the decking market has moved on from the simple domestic decks built by keen DIY buffs or local carpenters. Wood decks have become an eye-catching feature of a wide range of projects, including major public buildings, hotels, leisure buildings and retail outlets. The majority of these decks may also have to satisfy Building Regulations, where they provide primary access or escape routes and they will have to meet mandatory safety standards.

Architects and landscape architects, who are increasingly using wood for its ecological benefits over other materials, are now frequently turning to wood decking rather than using hard paving. Examples range from the Welsh Assembly building by the Richard Rogers Partnership, to the wood decking around the Cutty Sark at Greenwich. Large shopping centres such as Lakeside in Essex have also used extensive areas of wood decking, both internally and externally.

Other examples of the non-domestic use of wood decking are Colchester Barracks and the SAS building by the Thames near Marlow. A recently planned multi-storey block of flats, for instance, will have virtually the whole roof area decked to provide recreational and exercise areas for the residents. There is also a notable increase in the use of timber decking for external access or individual balconies on high-rise residential developments.

While many softwoods are adequate for quite heavily trafficked areas, many of the decks on these projects will have to stand up to exceptionally high levels of pedestrian use (and machine cleaning) and this is increasing the interest in hardwoods.

Working closely with Stoke-on- Trent City Council, WOODSCAPE designed this deck area and seating in FSC cumaru
In 1999, when TRADA’s first timber decking manual was written, other than temperate hardwoods such as European oak, there was only a limited choice of suitable hardwoods – particularly as there were concerns about the sustainability of traditional species such as iroko and ekki.

Seven years on, a wide range of hardwoods suitable for decking is available, with Forest Stewardship Council certification. These timbers are often lesser known species from Central and South America, but they have all the qualities of durability, density and stability necessary to meet the requirements of these public decks. Even traditional decking woods like teak are available from certified plantations, as are some African species like iroko and Australian hardwoods such as jarrah.

A possible disadvantage in using many of these very hard and dense timbers is that under high levels of pedestrian traffic they can tend to polish rather than wear, which can reduce their slip resistance unless sufficient precautions are taken. Temperate hardwoods such as European oak or sweet chestnut, available in certified form, from the UK and Europe, offer a suitable alternative and although they are less hardwearing, the roughening of the surface that occurs due to exposure and traffic can ensure that a good level of slip resistance is maintained.

Most softwoods have been available from sustainably managed forests for some time. However, some species such as Siberian larch have only recently become available with FSC certification.

Deck at Kew Gardens sited to create a viewing platform
Nowadays, for both public and domestic decks, a much larger range of prepared decking boards in hardwood and softwood is commercially available from merchants and DIY outlets, including both flat and grooved boards of various profiles and widths. It is important, however, to check that on the profiled boards, grooves are of an adequate size to drain the surface of the board effectively and not to become easily blocked by dirt.

Concern is often expressed that wood decks can become slippery. To address this, non-slip inserts are widely available. For most decks, however, good drainage and washing can prevent them becoming slippery. If decks are overhung by trees, or in close proximity to planting, it may be advisable to apply an algicidal wash occasionally to inhibit growth of algae. If areas of a deck are perpetually in shade, or subject to a high level of wetting, the application of a penetrating moisture-resistant finish will reduce the moisture absorption of the boards and, again, reduce the risk of slipperiness. Both will have to be repeated at intervals to maintain their effectiveness.

A number of systems for fixing boards with concealed clips are now available, many of which have been used in North America for some time. The only reservations on the use of some of these is that they reduce the gap between boards to the extent that drainage may be limited, and there is risk of the small gaps becoming filled with dirt. It may also be difficult to remove single boards if necessary in the long term for maintenance. A particularly ingenious system, however, has been developed for heavily trafficked areas such as bridges, using a bolt and wedge principle. In the long term, this system can allow for both adjustment and the removal of individual boards. For softwood decks, nailing with stainless steel annular ring shanks is normal practice, but stainless steel or epoxy-coated screws in pre-drilled holes are generally used for hardwood boards.

Deck boards freshly treated with water repellent
Two fixings should be used in any boards exceeding about 75mm width, and they should preferably be at about 400mm centres in the length of the board. This dictates the centres of the supports, but this is not so much related to the spanning properties of the board, but the need to hold deck boards flat and square to the supporting structure. Quite often TRADA sees examples of the use of thicker boards, particularly in hardwood, to achieve greater span and more widely spaced supports. Not only is this expensive because of the increased cost of the hardwood, but it leads to the risk of the boards twisting or bowing between the supports, and increases the risk of tripping.

Another aspect of deck design that is often forgotten is that, in some locations, a considerable amount of dirt and rubbish may accumulate under the boards. To clean this out it may be necessary at some point to remove the deck or individual boards. In decks for public access it may be advisable to design the deck as a number of separate panels that can be individually lifted to provide access to the space below for cleaning.

As with any project, the key to successful deck design is close attention to detail at an early stage by the project team.

• TRADA Technology has recently published a second edition of Timber Decking, which is endorsed by the Timber Decking Association. The publication costs £20 and is available from TRADA Technology by contacting [email protected] or visiting www.trada.co.uk/bookshop.

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