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Timber slat decks – design focus

This article covers the first three parts of section six of Bulletin 611 Timber slat decks. NZ Landscaper Magazine covered the first five sections in Issues 17 and 18

Timber slat decks are supported by joists, bearers and piles and have gaps between the decking timbers, so rainwater can drain away. A deck less than 1.5m above the ground does not need a building consent but must comply with the New Zealand Building Code and may require a resource consent. A barrier is required if it is possible to fall more than 1m from the deck.

Part six of the bulletin covers deck design.

6.0 Deck design

6.1 Stringers/ribbon boards

6.1.1 Decks attached to a building can be supported by either:

  • Stringers fixed to a concrete or concrete block foundation wall.

  • Ribbon boards (also commonly referred to as stringers – see Figure 4) fixed to timber piles, timber wall or subfloor framing.

  • A row of piles adjacent to the building – note that decks that are separate from the building structure must have a fully braced subfloor using braced piles or anchor piles.

6.1.2 Ribbon boards should be packed out from the wall cladding by a minimum of 12mm to provide drainage between the wall and deck. Packing must be fixed into either:

  • A stud.

  • Solid framing for direct-fixed cladding.

  • A batten for a cavity system.

6.1.3 Packers must be sealed to the wall cladding at the bolt fixings. For best practice, it is recommended that EPDM washers be used:

  • Between the packer and the cladding.

  • Between the packer and the ribbon board.

  • Between the washer and the ribbon board.

6.1.4 Stringer sizes and fixing spacings must be in accordance with NZS 3604:2011 Table 6.5 and are dependent on the span of the deck joists. Stringers and ribbon boards must be fixed with M12 bolts in accordance with Table 6.5.

6.1.5 BRANZ has designed and tested alternative stringer fixings, using pairs of 200mm long coach screws at spacings given by NZS 3604:2011 Table 6.5, fixed into the jack stud framing or main stud framing of the building.

6.1.6 Under E2/AS1, a ribbon board cannot be attached to the face of EIFS, stucco or profiled metal claddings.

6.2 Piles

6.2.1 The vertical load from a deck is generally supported on piles, which may be one or a combination of:

  • Ordinary piles.

  • Driven timber piles (cantilever piles).

  • Braced piles (two piles with 450mm deep footings and a diagonal brace between them).

  • Anchor piles.

6.2.2 Timber piles must be treated to hazard class H5 and may be either:

  • 140mm minimum diameter round timber.

  • 125 x 125mm square sawn timber (note that 100 x 100mm posts are not included in NZS 3604:2011 for deck support and, if used, must be specifically designed and H5 treated if in ground contact).

6.2.3 Piles (other than driven timber piles) must have a minimum 17.5MPa concrete footing in which they are embedded, with a minimum of 100mm of concrete beneath the pile. Minimum concrete footing depths are:

  • 200mm for an ordinary pile.

  • 450mm for a braced pile.

  • 900mm for an anchor pile.

6.2.4 The minimum plan dimensions of the pile footing are given in NZS 3604:2011 Table 6.1. For braced and anchor piles, the minimum footing dimensions are:

  • 350 x 350mm for square piles.

  • 400mm diameter for round piles.

6.2.5 Under NZS 3604:2011 clause 6.4, the maximum permitted height of piles above cleared ground level to the top fixing is:

  • Ordinary and braced piles – 3m.

  • Cantilever piles – 1.2m.

  • Anchor piles – 600mm.

6.2.6 NZS 3604:2011 limits the minimum pile height above ground to 150mm. No piles may be cut or drilled (such as for fixings) within 150mm of the finished ground level.

6.3 Bearers

6.3.1 Bearer size depends on pile spacing and joist span. Use NZS 3604:2011 Table 6.4 (b) to size bearers for SG 8 timber and Table A6.4 (b) for SG 10. For fixing and other requirements, refer to clause 6.12 of NZS 3604:2011.

This article was first published in BRANZ Bulletin 611 Timber Slat Decks and is reproduced with permission. It can be read at


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