Understanding the mechanisms behind nailplate backout is essential for both homeowners and builders, as well as knowing how to address and prevent the issue.
In residential roof construction, particularly in timber-framed houses, roof truss connections are commonly secured with metal nailplates. One issue that can arise over time is nailplate backout, where the nailplates begin to loosen and protrude from the timber trusses.
Timber is a hygroscopic material, meaning it absorbs and releases moisture from the surrounding environment. As humidity levels fluctuate, the moisture content in the timber changes, causing the wood to expand and contract.
In high-humidity environments, timber absorbs moisture and expands, while in dry conditions, it loses moisture and contracts. Moisture exchange between wood and air depends on the relative humidity and temperature of the air and the amount of water in the wood.
This cyclical movement as the timber expands/contracts due to exposure to moisture and/or humidity can result in nailplate backout over time. These movements, though typically small, causes the nailplates to loosen and eventually back out of the wood over time.
Many of the reported examples of nailplate backout have occurred in unsarked tiled roofs and in roof spaces with evidence of water penetration. Test data generally shows that the installation of a moisture barrier (sarking) drastically reduces the humidity variation and slightly reduces the temperature variation in the roof space.
Roofs that are inadequately weatherproofed or do not have sufficient cross ventilation allow for the timber roof framing to be exposed to prolonged moisture and humidity changes over time.
Although the installation of whirlybirds or roof sarking is not a requirement in all situations, they are both useful for improving roof ventilation. Note, sarking became a requirement for tiled roofs under certain conditions in the Building Code of Australia (BCA) in 2008.
There are also many examples of nailplate backout caused by other issues, including the following:
Depending on the type of nailplate seperation observed in the roof space, the engineer should be able to determine the underlying cause of the issue. Each cause will have a distinct pattern of nailplate withdrawal from the underlying timber truss.
We encourage home owners and builders to learn more about this issue and to do their due diligence, particularly if exposed to a home with a tiled roof. There's no harm in checking if sarking is installed below your roof tiles and to see if any nail-plates have begun to withdraw.
We recommend that an engineering professional be contacted if evidence of this is observed in a roof space. A&A is able to assist in the initial investigation, potential remediation/strengthening of trusses and further recommendations/advice if nailplate back-out is observed.
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