The simple answer is when it meets the 50-year durability standard set out in the New Zealand Building Code.
The New Zealand environment presents unique challenges to manufacturers of construction products. The traditional kiwi lifestyle engages the great outdoors, and New Zealand homes reflect this passion with demand for indoor/outdoor flow areas and desire for coastal living.
The salty air dramatically increases the acidity of our natural environment and, when combined with the volcanic nature of New Zealand’s central North Island, the resulting recipe places extreme demands on New Zealand building materials.
The New Zealand Building Code is among the most thorough construction regulations in the world. With a continued focus on quality building, evolving construction methods and timber treatments, it is critical that builders and inspectors understand the implications for fixings and fastenings.
Stakeholders such as manufacturers, builders, territorial authorities and home owners are all potentially affected by issues surrounding nail durability — after all nails are a critical element in the construction of New Zealand timber homes.
In recognition of this, the New Zealand Building Code sets out a durability standard for fasteners used in structural applications. It is:
NZBC B2.3.1 (a) 50 years durability on structural building elements including fasteners that cannot be easily accessed for maintenance and repair.
One method of protection for fasteners is galvanising. Galvanising is the process of bonding a protective layer of zinc to a bright steel object. The zinc forms a sacrificial barrier allowing the corrosive mix of air and salt laden moisture to react.
This reaction corrodes the zinc rather than the bright steel, extending the life expectancy of the fastener.
Treatments such as LOSP (Light Organic Spirit Preservative), CCA (Chrome Copper Arsenic) and Boron can have a dramatic affect on the life expectancy of nails and other fasteners.
By far the most common timber treatment in New Zealand is CCA. Unfortunately for nail manufacturers, CCA is one of the more aggressive treatments and can corrode some types of galvanised nail well within the specified 50-year durability requirement.
The industry must also consider the newer treatments becoming available that contain greater levels of corrosive material. These treatments include Copper Azoles and ACQ (Alkaline Copper Quaternary).
In New Zealand only the Paslode brand of collated galvanised nails has a BRANZ appraisal to exceed the building code. Senco has also been batch tested and meets the minimum standard.
However, some nails marketed as galvanised may not meet the 50-year durability code requirements as they stand today.
One reason for this is because there are various methods of galvanising that often progressively add cost to the manufacturing process. Complicating the issue is the application in which some of the lesser-coated galvanised products can be used.
Timber staining, unsightly rust streaks and structural failure all result from wrong nail selection. Wrong nail selection may not be evident for some period after the job has been completed, and by then it may be difficult to rectify easily.
There are also other nail types available. Bright nails offer no protective coating and cannot be used for many permanent external structural applications. However, bright nails will exceed the 50 years durability test for interior applications.
The most durable fastener is one constructed of stainless steel (304 grade or higher). Stainless steel nails are required for external structural use in sea spray zones (less than 500 metres from the coast), geothermal locations or environments where some other external acidic compound is present.
High grade stainless steel nails may very likely outlast the timber substrate.
In summary, many collated nails might be identified as galvanised on their packaging but are they code compliant? Identifying the right fastener is crucial for building professionals and inspection officers. How can you know?
One method is to request producer statements from the fastener manufacturer, while another method is to identify whether the fasteners have been BRANZ appraised.
Paslode remains the only BRANZ-appraised nail programme in New Zealand, and has incorporated a clever way of identifying that its galvanised nail has been used after nailing.
Paslode has stamped its “P” logo into the head of every D-Head galvanised nail so it can be identified as code compliant even after the nail has been embedded into the timber.
Zinc is a precious metal and hot dip galvanising is an intensive process that adds cost to the fastener. However, the price of having to repair or replace low grade galvanised nails because of structural integrity, inspection rejection or unsightly corrosion marks on timber and decking is reason enough for builders and industry professionals to take a second look at fastener construction.
Product certification and tighter regulatory controls place a burden on all industry stakeholders to shoulder responsibility to exceed industry standards — after all, they are the minimum requirements.
When is a galvanised nail a galvanised nail? When it meets the New Zealand building code standards of compliance.
Note: A review of the code is currently under way, with industry stakeholders expected to clarify elements relating to the durability of galvanised nails within the code and supporting standards.
Know your galvanising treatments
Below is a guide to nail galvanising treatments to help builders choose the right nail for the job. The most common galvanising treatments are electromagnetic galvanising, mechanical galvanising and hot dipped galvanising.
Electromagnetic Galvanising: This is a zinc-plating process where chromate reacts chemically with zinc when drawn through a special chemical bath. The result is a smooth finish with a galvanised layer typically up to 72g per sq m (10 µm) on small profile fasteners.
When used in H3 treated timber in the New Zealand environment a nail coated in this manner would have a life span of approximately six years when the fastener could then be expected to corrode as a result of a loss of the protective zinc layer. This coating type is ideal for interior applications where finishing nails are used.
Mechanical Galvanising: This is a cold process of tumbling bright steel components together with zinc, effectively beating the galvanising onto the bright steel. This is a very cost-effective method of coating very fine wire products such as screws or nails.
It is easy to determine whether a fastener has been mechanically galvanised by looking at the surface. It will appear relatively smooth and shiny with what appears to be flattened scales across its surface.
Typical mechanically galvanised fasteners have a thickness of up to 181g per sq m (25 µm) with an approximate life expectancy in H3 timbers of 10 to 25 years depending on the level of exposure to corrosive environments.
This method does not provide 50-year protection on some parts of the nail, including the head and point. These areas, if not entirely protected from the elements, can trap moisture, resulting in nail degradation.
Another limitation of this manufacturing process is a cracking of the galvanised coating during the tumbling process that creates a pathway for moisture to attack the bright steel beneath. Mechanically galvanised fasteners do not meet the 50-year durability standard.
Hot Dipped Galvanising: This is a heated process where nails are dipped into a bath of molten zinc. This ensures that all areas of the nails are coated. To meet the New Zealand code the fastener is left to soak for longer in the zinc bath, which has the effect of increasing the surface area of the nail and the quantity of protective zinc.
This extra time increases cost and is likely to be repeated for very high grade galvanised nails. With the greatest thickness, hot dipped galvanising can achieve a typical layer thickness of 360g per sq m (50 µm) or more.
This is currently the only galvanising process for small steel components that will provide an adequate level of protection to meet New Zealand building code requirements.