The amount of time, effort and money spent on correctly designing timber roof trusses by a truss manufacturer is huge. It can all be a worthless investment if proper on-site installation procedures are not followed.
Although trusses may be designed and manufactured correctly, failure can still occur if they are not installed correctly. The best case scenario of a failed truss may simply be the timbers becoming over stressed. The worst case scenario is the truss itself collapsing.
It has been stated in one of the Gang-Nail Guidelines published by MiTek; that a report in the United States found incorrect temporary bracing caused over 60 per cent of truss roof collapses. It is also reported by local engineers here in Australia who have inspected failed roof trusses, a large percentage of truss failures are related to poorly erected, braced or connected trusses.
Unfortunately for everyone involved, a roof truss will not usually present signs of failure until a long period of time has passed since its installation and the building is well beyond framing stage with the ceiling often installed. Obviously this makes for a difficult and costly repair.
There was a time when the responsibility was on the builder to follow a number of installation guides that were produced by nail plate manufacturers then the inspectors were relied on to double-check the installation. Today there is an Australian Standard available – AS 4440 – “Installation of nail plated timber trusses.”
The code covers supporting structures, truss installation, roof bracing, truss connection and overhangs. An abbreviated set of instructions produced by MiTek known as the ‘Fixing & Bracing Guidelines for Timber Roof Trusses’ is sent to site with every Bar-Truss job. It should be noted that this is not intended to replace the need to reference AS4440.
If a roof truss is installed correctly it will perform at its full design capacity. Correctly applying the details in AS 4440 will go a long way to ensuring the truss installer is not faced with costly failures in the future. The guidelines should not be ignored and AS4440 must be well read prior to the installation of any timber trussed roof system.
A copy of the MiTek guideline can be found at http://www.mitek.com.au
* All information is provided as a guide only. Please refer to industry and Australian standards.