Landscape and exteriors Decking
Considerations The countersunk heads of the screws will hold the board down adequately, despite the clearance around the screw shank, but the screw gauge must be sufficient to ensure that the countersunk head overlaps the pre-drilled hole. If the board surface is grooved the fixings should be inserted within the grooves to prevent the fixings from protruding in the event of in-service shrinkage and becoming a trip hazard. Screw heads should be set flush with the base of the groove so that they do not impede the flow of water. If a larger hole is necessary to take up the additional shrinkage of wide or ‘undried’ boards, it may be necessary to fit washers beneath the screw head that are recessed into the board. These washers should be stainless steel with either a hole larger than the screw shank, or a recess in the board that will allow for sufficient lateral movement of the board without the washers binding on the edges of the recess. In this case, round or panheaded screws may be more appropriate than countersunk. Screws have the advantage that, if there is shrinkage in the board thickness, they can be tightened and boards can be removed if necessary. Countersunk screw heads should be slightly recessed below the wearing surface of plain boards, but not to the extent that the holes can accumulate standing water. Fixing screws should preferably be kept at least 25mm from the ends of boards to avoid the risk of end splitting. If boards are butted end-to-end this may mean that they cannot be fixed to a single joist.
Hidden fixings A possible alternative method of fixing hardwood decks is to screw battens to the underside of the boards (allowing an oversize clearance hole in the batten) to form a panel. The panel can then be reversed, laid on the substructure, and the battens fixed to the substructure by screwing through the gaps between boards. Besides eliminating all surface fixings, it is a useful way to either fix a timber deck to a steel substructure, or to provide panels that can be removed for cleaning purposes. This method will give a ‘panelled’ appearance to the deck as the ends of all the boards will coincide. As an alternative to screws, several hidden decking clips are available for use in grooved boards. Deck clips vary considerably in design; some clips are made from plastic or thin steel and are only designed for softwoods, with clips designed for hardwoods being made from stronger materials of sufficient thickness to resist bending or pull-out forces caused by board movement or distortion. High-density, large-movement tropical timbers may require substantial fixings to restrain distortion and most decking clips are not designed for such timbers. www.trada.co.uk
IPE decking installed with HardWood Clip S (3mm). Photo: HardWood Clip®/NÖVLEK
Generally, decking clips are designed to be used with dried timber, and are not designed to accommodate the larger movement expected of ‘green’ or undried boards. Most clips incorporate in their design a method of preventing the boards from being installed too tightly together, typically in the form of pins or ridges that will bend or compress into the timber as the boards swell, but some clips will require careful spacing of the boards during installation to ensure adequate gaps are provided for expansion of the boards. As the design of clips can vary widely, the method of fixing, its suitability for the timber species chosen, and other detailed information should be obtained from the fixing clip manufacturer. n
About the author
Aron Searle Technical Officer BM TRADA
Further information More information on timber decking can be found in TRADA’s recently published third edition of Timber decking: The professionals’ manual. Go to bookshop.trada.co.uk to find out more and purchase a copy. To find out more about choosing the most suitable wood species for your project, visit TRADA’s Wood Species Database at www.trada.co.uk/wood-species. Timber 2019 Industry Yearbook
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