Durability of metal components for decks or balconies

By - , Build 101

The durability of metal components in open timber-slatted decks and balconies is just as important as the timber.

Figure 1: Durability requirements for deck elements.

The fixings for decks and balconies designed within the scope of NZS 3604: 1999 Timber framed buildings must meet the requirements of the New Zealand Building Code Clause B2 Durability. For decks, the durability is shown in Figure 1 and is not less than:

  • 50 years for structural elements and associated fixings, including all framing members (posts, bracing, stringers, bearers, joists, balusters or support posts for handrails) and handrails
  • 15 years for all other elements and associated fixings, including decking and balustrade infill.

Timber treatments

Timber members of a deck or balcony need to be treated to a minimum of H3.2 to meet durability requirements (NZS 3602). Members of a deck that are not structural should also be treated to at least H3.2 as they are exposed to the weather.

Where timber deck members are in ground contact, they should have a minimum treatment of H5.

Metal fixings risk corrosion

The main risk to the durability of a metal deck component is corrosion from moisture with one or more of:

  • copper in timber preservative treatments
  • chloride (salt) in the atmosphere
  • geothermal environments
  • incompatible material selection.

NZS 3604: 1999 Section 4 sets out a corrosion classification for different environments. These were derived from research around New Zealand and generally depend on the degree of exposure to marine salts or geothermal gases.

Figure 1: Durability requirements for deck elements.

Stainless fixings often needed

Components must be stainless steel for structural elements, where a durability of not less than 50 years is required. This includes nail plates, brackets, nails and bolts.

Because of the smoothness of stainless steel nails, annular grooves are required on the shank to ensure they have similar withdrawal resistance as rougher hot-dipped galvanised nails.

Recent research has highlighted the aggressive nature of alkaline copper quaternary (ACQ) and copper azole (CuAz) treatments (see Build 98, February/March 2007, pages 68–70). When these treatments are used, stainless steel is recommended for fixings, even where a 15 year durability is required (such as for balustrade infill and decking).

Where copper-chrome-arsenic (CCA) treated products are used, hot-dipped galvanised, silicon bronze and stainless steel fixings will meet the 15 year durability requirement.

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Figure 1: Durability requirements for deck elements.

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