Industry Update: Changes to AS 4440:2004 Installation of Nailplated Timber Roof Trusses

A revised Standard will help to eliminate the poor practices, particularly in bracing, that lead to roof truss problems, writes Kevin Ezard.

When roof trusses don't perform, it's usually due to incorrect installation, so readers should be aware that the Australian Standard for truss installation has been extensively updated.

The new document – AS 4440:2004 Installation of Nailplated Timber Roof Trusses – incorporates recent changes and all amendments to the Standard since it was first published in 1997.

Having a national Standard for roof truss installation is critical for ensuring the structural integrity of all roof frames.

AS 4440 is a secondary reference document to the BCA (Building Code of Australia), and the detailed provisions are not specifically included in the BCA.

While the Standard governs general rules for truss installation, truss system providers also produce proprietary information on their products and systems. This product-specific information complements the Standard.

More Than the Sum of the Parts

A typical roof truss is flexible when laid out flat, a fact that becomes obvious when you lift the truss up onto the frame. However, when the truss is standing vertically and braced correctly, it becomes an extremely rigid structural unit.

While trusses work through the natural stiffness of a triangle shape, the thin truss section members rely on adequate support and bracing to perform as designed.
Bracing elements that support the top and bottom chords include steel diagonal braces, roof battens and the underlying ceiling. It is critical that the wall-frame is adequate to transfer the load to the foundations, and that the top plate is level all the way around the external walls to ensure that final roof levels are correct.

The top chord diagonal bracing is usually formed steel bracing, which is positioned from the outer ends of the roof inwards at an angle of 30-45°, running from the top plate up to the ridge. This bracing is essential to prevent movement or buckling of the chords, and to provide overall stability to the roof under all load conditions.

Focus on Battens

Roof battens, which play a major role in bracing the roof, have contributed significantly to truss problems in the past.

Consequently, the updated AS 4440 pays special attention, as they are an important component of the bracing for top chords, and are incorporated in the engineering design of the truss roof.

For this reason, battens must be as long and continuous as possible in the areas towards the end of the gable roof, where the diagonal bracing is at its greatest distance apart.

Importantly, no more than one-third of battens should be spliced at a single truss and any two splices should not be adjacent at the same truss. This spreads the effect of lower strength at the batten joins, and ensures that most of the continuous lengths give adequate bracing support to the truss.

Top-hat trusses and north-light trusses also receive special mention in the new Standard to ensure that battens are applied correctly. With top-hat trusses, the battens must continue along the horizontal top chord join between the upper and lower sections of the trusses. In north-light truss installations, battens must be continuous along the top chord in the area of the raked 'light' section.

Bottom chord bracing is generally provided by the ceiling being fixed directly; however, if it incorporates battens they will provide bracing support and need to be rigidly connected as with top chord battens.

Wind uplift can also be a factor where the members may reverse from being 'in tension' to being 'in compression'.

Another common problem arising from poor installation is lack of accuracy in setting up the trusses. The installation must take into account specific tolerances for the allowable bow and skew, as well as plumb, due to the effect of any mis-alignment that would significantly affect the performance of the trusses when under load.

Large span trusses, in particular, can move over time in the direction of the mis-alignment, particularly when the bracing and battens have not provided adequate fixing. (Tolerances for truss alignment are also included in the Standard.)

All builders using timber roof trusses need to have copies of AS 4440:2004 available for supervisors and tradesmen on site. The document contains a great deal of detailed information and diagrams that will assist in ensuring trusses are correctly installed. This is a valuable opportunity to provide information to the installation team, which may protect you from the likelihood of problems in the future.

Kevin Ezard is a marketing and communications consultant, and founder of Musclebeam and Ezijoin process. Kevin also initiated Frame Australia in 1998. You can email comments of questions to him at ezard@bigpond.net.au

To order your copy of AS 4440:2004, phone 1800 623 214.

This article was reproduced with permission from Building Connection (Autumn Quarter March 2005) www.build.com.au

Building Connection is an independent source of quality technical information and current issues for the trade. The magazine reports on new products and technologies; State, national and local government regulation; best practice; market reviews; training/education; and effective business management. Published by Connection Magazines which has 20 years experience in trade publications, Building Connection has an audited circulation/subscriber base of 50,000+ nationally.

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