Deck design to NZS 3604

A timber deck is a full load path in miniature — piles, bearers, joists, decking and bracing all working together. NZS 3604:2011 §7.4 sets out how to size and fix each part, and getting the framing, drainage and connections right is what keeps a deck standing, dry and off the consent radar where it can be.

Consent: when you need one

Deck height off the ground is the trigger for consent and fall protection.

• Decks less than 1.5m above ground are mostly Schedule 1 exempt — no consent required.
• Above 1.5m, you need a full Building Consent plus F4 fall protection.

Sizing the framing

Bearers and joists are read off the NZS 3604 span tables for your span, spacing and timber grade. Deck timber is H3.2 SG8, and the wet-in-service tables apply.

• Bearers: read the required deck-bearer size off NZS 3604:2011 Table 6.4 Part (b) (wet-in-service) for your bearer span, loaded dimension and timber grade — consult your licensed copy of the standard.
• Doubling up: a pair of narrower members nailed together can carry the same load as a single wide member, often more cheaply (NZS 3604:2011 cl 2.4.4.7) — confirm the equivalence and maximum span in your copy of the standard.
• Joists: read the required deck-joist size off NZS 3604:2011 Table 7.1(b) (wet-in-service) for your span, joist spacing and timber grade. Bigger joists, or closer centres, span further.

Decking, fixings and gaps

Board thickness has to suit your joist centres, and every gap is doing a job — drainage, movement and drying.

• 32mm decking is required for 600mm joist centres; 19mm decking is only OK at 450mm joist centres.
• Leave a 3–6mm gap between boards — a 100×3.75 nail head works as a spacer.
• Fix with 75×3.15mm HDG ring-shank nails OR 65×3.5mm SS deck screws (stainless steel is mandatory in Zone D).
• Lay boards parallel to the house — perpendicular to the joists — for maximum stiffness.
• Seal all cut end-grain with copper naphthenate as part of H3.2 maintenance.
• Keep a minimum 12mm gap between decking and cladding (E2/AS1 §7.1.1) for drainage.

Slope, drainage and the house connection

Water has to run off and away, and the deck must tie back to the structure — not the cladding.

• Fall the deck at 1:100 (1cm per metre) AWAY from the house, pitching toward the open side or a drainage gutter.
• Set bearers slightly lower toward the outside — the bearer slope can deliver the fall.
• Fix the stringer to the foundation wall ideally, packed off the house rather than into the cladding.
• Run a membrane behind the stringer, with flashing tape against the weatherboard to seal penetrations.

Bracing and cantilever balustrades

Whether a deck needs its own bracing depends on how it projects from the house, and cantilever balustrades carry their own fixing rules.

• Decks attached to the house and projecting 2m or less need NO bracing — the house provides lateral restraint.
• Decks with a larger projection, or free-standing decks, need bracing. Work out the demand from NZS 3604:2011 §5/§7.4 (decks use a reduced fraction of the subfloor demand) and provide it with anchor piles or cantilever piles.
• The bracing capacity each pile type contributes, and the connection capacity it needs, come from NZS 3604:2011 §6 — read the figures off your licensed copy, or let GIB EzyBrace or your designer confirm.
• For cantilever balustrade decks (a BRANZ recommendation), NZS 3604:2011 §7.4.1.3 and Figure 7.10 cover the cantilever balustrade fixing, and B1/AS1 refines the cantilever/balustrade connection and fixing requirements that sit over §7.4.1.3 — always check both.