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Timber slat decks – design focus

This article covers the second half of the last section, section six, of Bulletin 611 Timber slat decks. NZ Landscaper Magazine covered the first five sections in Issues 17 and 18 and 19

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.

The final article in the timber slat deck series covers joists, bracing and deck design.

6.4 Joists

6.4.1 Use NZS 3604:2011 Table 7.1 (b) to size joists in SG 8 timber (Table A7.1 (b) for SG 10). Boundary joists that will be supporting a cantilevered barrier must be no less than 190mm x 50mm and have the same width and depth as the deck joists.

6.4.2 Joist set-out must allow for adequate vertical separation between the finished deck level and the floor level. For non-cantilevered slatted decks, the decking surface may be at the same level as the floor.

6.4.3 The maximum practical joist spacing is influenced by decking thickness:

  • 32mm thick (finished) timber decking – 600mm maximum joist centres.

  • 19mm thick (finished) timber decking – 450mm maximum joist centres.

6.4.4 Deck joists may be cantilevered beyond the outer bearer. Joist size is given by the right-hand column of NZS 3604:2011 Table 7.2. Note that, if a cantilevered barrier is attached to the ends of these joists, the minimum joist depth must be 190mm. While the back span of cantilevered joists is not defined by NZS 3604:2011, BRANZ recommends a minimum back span of 1.5 times the cantilever length (see Figure 5).

This is to provide sufficient resistance to overturning – for example, if people congregate at the overhanging deck edge.

6.5 Bracing

6.5.1 Decks projecting no more than 2m from the building with stringers (or ribbon boards) and joists bolted to the building do not require subfloor bracing.

6.5.2 Decks that project more than 2m from a building and freestanding decks (not supported by the building) must have a braced subfloor. This must provide 50% of the earthquake bracing demand required by NZS 3604:2011 Table 5.8 for ‘light/light/light’ cladding, 0° roof slope and for ‘subfloor structures’, modified by the factors for soil class and earthquake zone.

6.5.3 Where the deck must be braced to resist earthquake loads, this may be provided by:

  • A braced pile system (see Figure 6). Anchor piles, if the highest connection is less than 600mm above cleared ground level.

  • Driven timber (cantilever) piles, if the deck is less than 1.2m above cleared ground level.

6.6 Decking

6.6.1 Timber species suitable for decking are given in NZS 3602:2003. The most commonly used timbers in New Zealand are shown in the table on page 12. They are:

  • Radiata pine (must be treated H3.2).

  • Kwila.

  • Vitex (also known as vasa).

  • Cypress species such as heart macrocarpa.

  • Jarrah.

Note that only radiata pine is provided for in NZS 3604:2011.

6.6.2 Traditional decking boards are 19 or 32mm thick and 90mm wide. Wider boards:

  • Require larger gaps between the boards to allow for increased movement (6mm is recommended).

  • Have increased risk of cupping.

  • If smooth, are likely to be more slippery when wet.

6.6.3 Installing timber decking:

  • Leave a minimum 12mm gap between decking and the building.

  • Leave a gap between boards (for example, for 90mm boards, use a 100 x 4mm nail as a spacer) to allow:

  • Movement in the boards from moisture changes (allowance must also be made for the initial shrinkage of unseasoned timber).

  • Water to drain from the deck.

  • Fix with annular-grooved hot-dip galvanised nails – use stainless steel annular-grooved nails with alkaline copper quaternary and CuAz-treated timber. Stainless steel fixings are also recommended in exposure zone D.

  • To minimise the risk of splitting, pre-drill fixing holes before hand nailing and where screws are not self-drilling.

  • Nail boards at each joist with two nails:

  • 60mm long for 19mm thick decking.

  • 75mm long for 32mm thick decking.

6.6.4 Gun nailing is not recommended by BRANZ because:

Using gun nails in dry decking may cause the decking to split and splinter around the nails.

Gun nails may not be sufficiently durable when used externally and with treated timbers.

6.6.5 Proprietary clip fixing systems provide concealed fixing of the decking but are outside the scope of NZS 3604:2011 and must be treated as alternative methods.

6.6.6 Tropical hardwood decking:

  • Should be sourced from sustainable forests.

  • May splinter.

  • May vary considerably in durability between boards.

  • Can be difficult to sort, since the colour difference between lower-durability sapwood and durable heartwood can be indistinct.

6.6.7 Deck timbers may be left unfinished or may be given some protection from weathering and UV light. This involves the application of decking oil, timber stain, non-slip paint or non-pigmented sealer (which does not protect against UV light). To enhance the performance of the decking:

  • Avoid dark stains.

  • Ensure the space under the deck is well ventilated.

6.6.8 Composite decking boards are manufactured from recycled plastic (HDPE) and waste timber fibre, which is heated, compressed and then extruded into a decking timber shape. Composite decking is outside the scope of NZS 3604:2011 – check for verified performance information such as a BRANZ Appraisal. It is prefinished and is marketed as:

  • UV resistant and resistant to insects, mildew and moisture.

  • Low maintenance (no rotting, splintering, warping or loss of colour).

6.6.9 Install composite decking:

  • With joist spacing to the manufacturer’s recommendations.

  • With a 12mm minimum gap between decking and cladding for drainage and maintenance.

  • To the manufacturer’s instructions.

6.7 Barrier Design

6.7.1 Typically, a barrier rail is supported by posts fixed to the deck structure (a ‘cantilever balustrade’, NZS 3604:2011). When following B1/VM1, the barrier must be designed to resist the horizontal and vertical loads and wind loads given by AS/NZS 1170, without undue deflection, so its connection to the deck structure is critical.

6.7.2 No Acceptable Solution is currently available for cantilevered post fixings. Published MBIE guidance states that timber barriers complying with Guidance on Barrier Design paragraph 4.2.7 meet the performance requirements of NZBC clauses B1, B2 and F4. NZS 3604:2011 clause and Figure 7.10 (a), (b) and (c) describe the support details for cantilevered barriers. Some of the fixing details have been modified by B1/AS1 Amendment 15. Other barrier types require specific engineering design.

6.7.3 BRANZ has designed and tested a selection of cantilevered deck barrier support details (posts) (see one example in Figure 7) and one detail where the H5 deck foundation post is extended to form support for the handrail. These details satisfy the loading and deflection criteria of AS/NZS 1170.

6.7.4 A range of other barrier materials are included in Guidance on Barrier Design but are outside the scope of this bulletin. Such materials include glass, aluminium, steel and wire infill.

6.8 Stairs to decks

6.8.1 Stairs that are on the access route to the first entry point of a house or used by the general public must comply with D1/AS1 requirements for common stairways.

Other exterior stairs are classified as main private stairways.

6.8.2 The parameters given in D1/AS1 Table 6 for both common and main private stairways are:

  • 37° maximum pitch.

  • 190mm maximum riser.

  • 280mm minimum tread.

  • A D1/AS1 compliant handrail (on both sides where stairs are wider than 2m) – handrails may be omitted on stairways of two or three risers within or giving access to a household unit.

6.8.3 All steps in a flight must be uniform. Treads must have a level surface and a slip resistance in accordance with D1/AS1 Table 2. Where open risers are used, the open space between treads must be small enough to prevent a 100mm diameter sphere from passing through. Safety is enhanced where a non-slip nosing is installed and treads are a contrasting colour.

6.8.4 Stair barriers have the same requirements as decks. In addition, barriers to stairs should:

  • Be a minimum 900mm high, measured vertically from the pitch line (the front edge of the treads, steps or stair nosing).

  • Not allow a 150mm diameter sphere to pass through the triangular opening formed by the stair riser, tread and bottom of the barrier.

  • Have handrails with a profile complying with D1/ AS1 Figure 26 (a), where required.

6.8.5 A stair handrail may transition up to a height of 1000mm on intermediate landings for housing.

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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