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Residential decks: framing, materials, and compliance

Australian residential deck guide: AS 1684 framing, balustrade heights, timber vs composite, council permit thresholds, durability classes, NCC 2022.

By Dan Waring. Last updated 8 May 2026. Verified 8 May 2026.

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TL;DR

A residential deck is a Class 10a structure under the NCC and is framed per AS 1684.2 (non-cyclonic) or AS 1684.3 (cyclonic). A balustrade is mandatory where there is a fall of 1 m or more from deck surface to ground: minimum height is 1,000 mm per NCC 2022 Housing Provisions Part 11.3 (verified 2026-05-08). Exempt development thresholds vary by state: NSW is under 25 m2 and floor height under 1 m (SEPP 2008); Victoria requires a building permit for most decks attached to a dwelling; Queensland requires approval for floor heights over 1 m or plan area over 10 m2. Supply-and-install cost runs roughly $120 to $250/m2 for treated pine or merbau hardwood, and $250 to $450/m2 for composite, ex-GST (2026). The most common defect is underframing: bearers and joists sized for floors, not decks, combined with inadequate footing depths for posts, cause deflection and long-term rot at connections.

When you do this

Deck work typically falls into three scenarios on a residential build:

  1. New build: deck framed as part of the main contract, scheduled after the main structure is weather-tight and before landscape.
  2. Owner-commissioned addition: existing homeowner adds a deck post-occupation. Separate building approval usually required (see Approvals section below).
  3. Replacement: rotted or failed deck stripped and reframed. Same compliance obligations as new; check footing depths before assuming existing piers are reusable.

Who’s involved

  • Builder or licensed contractor: framing and structural elements. Most states require a builder’s licence for decks attached to a dwelling where work value exceeds the licence threshold.
  • Chippy: often the trade doing the actual framing and boarding.
  • Structural engineer: required for decks on engineer-designed footings, high decks (over 3 m above ground), or where deck loads transfer to the main building structure.
  • Certifier or private building surveyor: inspects framing before boarding up; issues completion/occupancy on approval.
  • Client: homeowner signs off on design intent; should sight final inspection documentation.

Approvals

Approval thresholds vary by state. Always check with the local council or a licensed building surveyor before assuming a deck is exempt.

NSW

Under the State Environmental Planning Policy (Exempt and Complying Development Codes) 2008, a deck may be exempt development if:

  • Floor area 25 m2 or less (all decks on the lot combined)
  • Floor height 1 m or less above existing ground level
  • Not more than 3 m at its highest point
  • Set back at least 900 mm from the lot boundary (5 m in rural zones)
  • Does not extend beyond the building line of a road frontage

Any overlay (heritage, bushfire BAL, flood, biodiversity) likely removes the exemption. Check the NSW Planning Portal before starting (verified 2026-05-08).

Victoria

A building permit is required for most decks attached to a dwelling in Victoria. The VBA’s guidance confirms that a registered builder must be appointed where total labour and materials cost exceeds $10,000 (verified 2026-05-08). Some councils allow minor, unattached structures under 10 m2 without a permit; verify with your local council before assuming exemption.

Queensland

Under the Building Act 1975 (Qld), building approval is required for decks where any of the following apply (verified 2026-05-08):

  • Plan area over 10 m2
  • Floor height over 1 m above natural ground surface
  • Overall height over 2.4 m, or mean height over 2.1 m
  • A side longer than 5 m
  • Attached to another building or structure

Cyclonic areas: all decks require approval regardless of size.

Steps

1. Establish footing type and depth

Deck posts bear onto concrete footings (piers or strip). Footing size and depth is driven by:

  • Soil classification per AS 2870 (site classification from the slab/footing engineer or from the original DA documentation)
  • Tributary load area per post
  • Termite risk: timber posts must be isolated from soil; steel stirrups embedded in concrete are standard for treated-pine post bases

For decks under 1 m above ground, table-based solutions in AS 1684.2 span tables apply. For higher decks or where soil is reactive, get an engineer to size footings.

2. Set posts

Posts for low decks (under 1 m) typically use 100 x 100 mm H4-treated pine or equivalent seasoned hardwood (DC1/DC2 per AS 5604:2022) where ground contact or moisture exposure applies. H4 is the minimum for posts set in or adjacent to soil; H3 is not adequate for in-ground applications (verified 2026-05-08).

Post heights should account for the finished deck height less decking board thickness less joist depth less bearer depth.

3. Fix bearers

Bearers run parallel to the house and span between posts. Size from AS 1684.2 span tables:

  • Table 49 applies for deck floors greater than 1,000 mm above finished ground level (design live load 2.0 kPa per AS/NZS 1170.1:2002)
  • Table 5 (floor bearer tables) applies for decks at or below 1,000 mm above ground

Typical bearer sizes for 1.8 m centres at spans of 1.8 to 2.4 m: 190 x 45 or 240 x 45 MGP10 treated pine, or equivalent seasoned hardwood. Use span tables, not rule of thumb.

Bearers must be H3 minimum for external above-ground exposure per the treatment specification. Where the bearer is in close proximity to soil or in a damp location, H4 is preferred (verified 2026-05-08).

4. Fix joists

Joists span between bearers and carry the decking boards. Size from AS 1684.2 span tables:

  • Table 50 for deck floors over 1,000 mm above ground (2.0 kPa live load)
  • Table 6 for decks at or below 1,000 mm

Spacing is typically 450 mm for 19 mm boards over 90 mm joists, or 600 mm where the board species and thickness allows. Check the decking board manufacturer’s spec for maximum joist spacing before pricing.

Joists should be H3 minimum for external exposure.

5. Attach to house (where deck is attached)

Where the deck bears against the house wall, a waling plate is fixed to the house framing. NCC 2022 Housing Provisions Part 12.3 specifies (verified 2026-05-08):

  • Timber waling plate: 90 x 35 mm minimum, F5 or MGP10, fixed to timber frame using No. 14 partial-threaded self-drilling screws at maximum 450 mm centres
  • Where fixing to masonry: 140 x 35 mm waling plate anchored with M12 chemical or mechanical anchors at maximum 300 mm centres staggered
  • Nearest joist span to wall: 3 m maximum (single or continuous span)
  • Joists bear off the waling plate and must be not less than 90 x 45 mm F5 or MGP10

Flashing is required where the deck-to-wall junction could allow water entry. Use flashing per AS/NZS 2904 or manufacturer-approved membrane product.

6. Install decking boards

Common deck board options:

MaterialTypical sizeTreatment/ClassNotes
Merbau hardwood90 x 19 or 140 x 19 mmNatural DC1 (no treatment needed)Bleeds tannin when wet; pre-wash before install
Treated pine90 x 19 or 140 x 19 mmH3 above-ground minimumCheck board is H3, not H2
Modwood composite138 x 23 or 88 x 23 mmN/A (wood-plastic composite)Follow manufacturer’s joist spacing limits
Trex compositeVariousN/AFollow manufacturer’s install guide for AU conditions
Ekodeck compositeVariousNCC-compliant per manufacturerCheck current product compliance statement

Gap between boards: 3 to 4 mm is standard for long-term performance, allowing drainage and board movement. Tighter gaps trap debris and moisture. Wider gaps (over 6 mm) create a trip hazard and are typically unacceptable at PCI.

Board direction should be chosen to shed water away from the house and to minimise waste in cuts.

Fastening options: face-screw with 316 stainless (standard), hidden clip systems for composite (follow manufacturer’s instructions), or hand nail (hot-dipped galv minimum) for hardwood. Never use T-nails on deck boards.

7. Install balustrade where required

A balustrade is mandatory where there is a potential fall of 1 m or more from the finished deck surface to the surface below. NCC 2022 Housing Provisions Part 11.3 requirements (verified 2026-05-08):

  • Minimum balustrade height: 1,000 mm above finished floor
  • Openings in the balustrade: must not permit a 125 mm sphere to pass through
  • Where fall exceeds 4 m: horizontal elements between 150 mm and 760 mm above floor must not facilitate climbing (non-climbable zone)

Common balustrade systems: framed timber, aluminium, tensioned wire (316 stainless), toughened glass panel. All must be structurally adequate to resist the loads specified in AS 1170.1 (refer to your engineer for post and rail sizing on any elevated or high-load deck).

8. Finish and drainage

  • Seal or oil hardwood boards immediately after installation (merbau in particular absorbs the first oil application fast; two coats standard on day 1)
  • All composite decking requires specific gap allowance for thermal expansion and contraction; follow manufacturer’s guidance for your climate zone
  • Ensure drainage away from the house and any adjacent garden beds: minimum 1:100 fall on deck surface is standard practice
  • Install insect barrier mesh or solid skirt boards at the deck perimeter to prevent possum, bird, or rodent access under low decks

Tolerances and acceptance

Workmanship tolerances at PCI for residential decks:

ElementAcceptableCommon defect
Deck board surface (level)Generally within 3 mm in 3 mHumps or dips at joist positions from board cupping
Board gap3 to 6 mmTight boards (less than 3 mm) trap moisture; excessive gaps (over 6 mm) are a trip hazard
Balustrade height1,000 mm minimum (NCC 2022 HP Part 11.3)Under-height handrail posts
Balustrade opening125 mm sphere must not passHorizontal rail spacing too wide
Post plumbGenerally within 3 mm per 1 m heightRaked posts from inadequate footing or post base
Board end cutSquare, within 2 mmAngled butt joints showing

Specific HIA Guide tolerances for deck surface flatness and board line will be verified when HIA membership is active. [HIA-053]

Documents needed

  • Approved building permit (where required)
  • Site classification report (if footing design is required)
  • Structural engineer’s certificate (for elevated decks or engineer-designed footings)
  • Manufacturer’s technical data sheets for composite decking and balustrade systems
  • Certifier’s inspection checklist (framing stage inspection is mandatory in most states before boarding up)
  • Final inspection certificate

Common holds

  • Framing inspection not booked: boarding up before the certifier signs off on framing is the most common hold. In most states the certifier must inspect the framing before the surface is covered.
  • Balustrade height non-compliant: posts fixed before measuring finished board height, leaving the handrail under 1,000 mm when boards are added.
  • Post treatment wrong: H3 posts used where H4 is required for near-ground or ground-adjacent applications.
  • Missing flashing at wall junction: water entry behind the ledger board is a common defect and difficult to fix post-boarding.
  • Composite joist spacing exceeded: composite boards have stricter joist spacing requirements than timber (often 300 to 400 mm versus 450 to 600 mm for timber). Check before framing.

References

See also

Last updated: 2026-05-08. Verified: 2026-05-08. Quarterly review for currency.

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