The following information covers the first three sections from a new BRANZ bulletin, which replaces Bulletin 611 Timber slat decks. NZ Landscaper will cover the remaining four sections in subsequent issues
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.
1.0 Introduction
1.0.1 This bulletin details the requirements for timber-framed decks with a slatted timber walk-on surface within the scope of NZS 3604:2011 Timber-framed buildings. To fit within that scope, a deck must:
Be designed using a floor loading of 2kPa.
Not require specific engineering design.
Not be enclosed by roofs or walls but may have a separately supported veranda over it.
Have a maximum height of 3m from cleared ground to deck surface.
1.0.2 Specific engineering design is required for decks:
Supported on piles where the deck surface is more than 3m above cleared ground.
Where joist and bearer spans are outside the limits of NZS 3604:2011 Table 7.1.
Supported on joists cantilevered past a bearer more than is allowed in Table 7.2 of NZS 3604:2011.
Supporting a spa pool or similar water-retaining structure.
That have a floor load exceeding 2kPa (such as vehicle decks).
Not founded on good ground.
1.0.3 Timber slat decks supported on floor or other joists cantilevered past the external wall of the building are outside the scope of this bulletin.
2.0 General design
2.1 New Zealand Building Code Requirements
2.1.1 New Zealand Building Code (NZBC) clauses and supporting documents that apply to open, slatted timber deck construction:
B1 Structure – Acceptable Solution B1/AS1 cites NZS 3604:2011 for timber deck construction not requiring specific design. It gives the sizing of deck joists and bearers. Amendment 15 (January 2017) amended the requirements for connections between deck framing members where posts (balusters) are attached. For specific engineering design, B1/VM1 references AS/NZS 1170.1:2002 Structural design actions – Part 1: Permanent, imposed and other actions and NZS 3603:1993 Timber structures standard. Clause 3.6 of AS/NZS 1170.1:2002 as modified by B1/VM1 2.2.7 applies to barrier loads. The MBIE publication Guidance on Barrier Design provides guidance on a range of barrier types. It gives an approved alternative method for timber deck barriers.
B2 Durability – NZS 3604:2011 section 4 provides a means of compliance for the selection of metal fixings and fastenings. NZS 3602:2003 Timber and wood-based products for use in building provides a means of compliance for timber selection.
D1 Access routes – Acceptable Solution D1/AS1 provides a solution for stair design and defines the level of slip resistance where required.
E2 External moisture – covers waterproofing to the building envelope, particularly where any deck structure is attached to the building.
F2 Hazardous building materials – requires that people be protected from illness or injury caused by exposure to hazardous building materials (such as glass barriers).
F4 Safety from falling – Acceptable Solution F4/AS1 sets out the minimum requirements for heights, maximum opening sizes and other critical dimensions for barrier construction, including decks incorporating fixed seating.
2.2 NZS 3604:2011
2.2.1 Under NZS 3604:2011, the sizing of bearers, joists and decking is to accommodate a maximum floor load of 2kPa.
2.2.2 Decks extending more than 2m from the face of the building must have subfloor framing braced at half the earthquake bracing demand given in NZS 3604:2011 Table 5.8. There is no requirement for bracing to resist wind loads.
2.2.3 Decks attached to a building and projecting 2m or less from it do not require subfloor bracing.
2.2.4 NZS 3604:2011 clause 7.4.1.3 and Figures 7.10 (a), (b) and (c) describe joist configuration and fixings required for cantilever barrier supports. MBIE modified these figures in Amendment 15 of
B1/AS1 (January 2017). Designers must ensure that the modified version is used.
2.3 Timber grading
2.3.1 The spans and spacings for joists and bearers must be read from the ‘wet in service’ tables in NZS 3604:2011. SG8 is the default timber grade used.
3.0 Consents
3.0.1 A building consent is required for a deck where a person could fall 1.5m or more. A safety barrier is necessary if there is a fall of 1m or more.
3.0.2 Decks not requiring a consent must still comply with NZBC requirements.
3.0.3 The consent application must identify the NZBC clauses relevant to the proposed deck and should also include:
Drawings and specifications.
Calculations for any specific design.
Construction aspects such as structure, durability and weathertightness at the intersection of deck and building.
Access (stairs).
Slip resistance.
Safety from falling (barrier design).
3.0.4 Where appropriate, manufacturers’ data, engineering and design calculations, product appraisals and technical statements and warranties specific to the project should also be included.
3.0.5 The drawings should include:
A site plan showing site and boundary locations, location and size of the deck and other buildings, and location of services.
Deck elevations, including height above the ground, barrier height and steps.
A section through the deck.
Construction details including:
- Subfloor bracing (if required).
- Pile foundations.
- Sizes, spans and fixing of bearers, joists and decking.
- Barrier details and calculations.
- Stair details if required.
3.0.6 A resource consent may be required from the territorial authority. Check before starting the project.
This article was first published in BRANZ Bulletin 611 Timber Slat Decks and is reproduced with permission. It can be read at www.branz.co.nz/pubs/bulletins/bu611