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Timber slat decks – safety focus

Shadowdeck decking system pictured

This bulletin replaces Bulletin 489 Timber slat decks/ balconies and covers part four of seven sections, which focus deck safety - from falling and slipping. NZ Landscaper covered the first three sections in Issue 17

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 four of the seven-section bulletin covers deck safety.

4.0 Safety

4.1 Safety from falling

4.1.1 Protection must be provided where people could fall 1m or more. This may be a timber barrier constructed in accordance with Guidance on Barrier Design paragraph 4.2.7 (to be submitted for consent as an alternative method) or by specific design to B1/ VM1.

4.1.2 Where fixed seating no more than 500mm high is incorporated at the deck edge and the deck is less than 1m above ground, a barrier is not required to the seat. This applies even if the potential fall height from the seat is more than 1m (Figure 1a).

4.1.3 If the deck is more than 1m above ground level, a seat back at least 760mm high is required.

The seat back must be of solid construction or vertical members with gaps of no more than 100mm. An alternative option could be horizontal slats with a maximum gap of 15mm. There must be no openings beneath the seat that a 100mm sphere can pass through (Figure 1b). Where ground slopes away from the deck, MBIE recommends that the fall height is measured 1200mm out from the edge of the deck (Codewords 32).

4.1.4 Barrier loads are set out in AS/NZS 1170.1:2002 clause 3.6. To be used in the design of a barrier system, these loads must be multiplied by the appropriate combination factors for both the ultimate and serviceability states. These are given in section 4 of AS/NZS 1170.0:2002 Structural design actions – Part 0: General principles in conjunction with B1/VM1, which modifies some of the loadings of AS/NZS 1170.

4.1.5 Barriers should:

  • Be continuous and extend the full length of the hazard.

  • Be at least 1m high.

  • Have adequate rigidity, both in the barrier itself and its connection to the supporting structure.

  • Have strength to withstand wind loads, human impact and, where required, the static force from people pressing against them (refer to AS/NZS 1170 for specific design criteria).

  • Prevent children falling through or climbing over them by having:

    • No openings that will permit the passage of a 100mm diameter sphere.

    • No components between 150mm and 760mm above the deck (or stair nosing) that can provide a toehold.

    • Continuous mesh, perforated sheet or trellis can only be used if openings are no larger than 35mm.

    • No gaps greater than 15mm between horizontal slats.

4.1.6 Proprietary barrier systems must either comply with B1/VM1 or be submitted for consent as an alternative method. Evidence must be provided to demonstrate compliance with the requirements of the Building Code for both the barrier and its connections to the deck framing.

Usually, the supplier of the proprietary balustrade system will provide a producer statement (PS1), which must be site and job specific. The deck designer must be satisfied that:

  • The proprietary balustrade system meets durability requirements.

  • The supporting structure is capable of accommodating the loads applied to the balustrade system without collapse or excessive deflection.

4.2 Slip resistance of decking

4.2.1 Where a deck is part of the main access route into a building, D1/AS1 requires the surface to have adequate slip resistance under all conditions of normal use. From January 2017, D1/AS1 uses the term slip resistance value (SRV) instead of the previous term coefficient of friction.

Compliance may be demonstrated:

  • By test, using the wet pendulum test method of AS 4586-2013 Slip resistance classification of new pedestrian surface materials Appendix A with the Slider 96 rubber, or

  • By using materials listed in D1/AS1 Table 2 as having ‘acceptable wet slip’ (D1/AS1 Table 2 simply says yes or no when identifying slip resistance and does not give an SRV.)

4.2.2 A coefficient of friction of 0.4 equates to an SRV of 39 measured using the test method above. All external stairs and ramps are required to have both wet and dry slip resistance.

4.2.3 Timber decking is typically profiled on one face and smooth on the other. To provide sufficient slip resistance where the deck is part of the main access to the building, decking should be laid perpendicular to the direction of travel with the grooved face uppermost. Smooth timber and profiled timber that runs in the direction of travel does not comply with 4.2.1 above. However, adding anti-slip tapes and sand or grit to a coating will meet the requirement.

4.2.4 There is no specific requirement to provide slip-resistant surfaces elsewhere, but BRANZ recommends that profiled timber decking should run at right angles to the direction of travel where practicable. If a deck is not part of an access route, the smooth side, which is easier to keep clean, may be laid face up. Consideration should also be given to the slip resistance of the surface when worn or wet.

This article was first published in BRANZ Bulletin 611 Timber Slat Decks and is reproduced with permission.

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