Why do roof trusses fail?

by Tania Wannenburg
Why do roof trusses

Why do roof trusses fail? WALLS & ROOFS spoke to a couple of professionals in the industry to find out more.

Roof trusses should be a safe and straightforward way of erecting roofs, yet failures occur. While product and service providers carry a great deal of responsibility, ultimately, in terms of the National Building Regulations (NBR) it is the building owner who is held accountable for the roof and the necessary inspection thereof.

The majority of roof truss failures, about 85%, occur due to poor roof truss erection, where constructers don’t adhere to the provided roof system details, according to Uwe Schlüter, business development manager at MiTek. This figure also includes modified roofs that were never re-inspected. And while approximately 11% of failures occur due to incorrect truss manufacturing, where manufacturers either don’t adhere to the provided roof system details or use the incorrect structural material such as wrongly graded timber, actual design failures are very rare and represent only about 4% of all roof structure failures.

“The reason for this low risk is that most modern roof structures of timber or light gauge steel are designed with the help of sophisticated design software, which almost eliminates the possibility of failure,” explains Schlüter. “Only incorrect input of data could still cause failure.”

Inspections are crucial
In line with the NBR, where a rational design is used, a professional competent person must be appointed to carry out the design, Schlüter points out. “The roof inspection also forms an important part of the design process and therefore the complete design must be inspected and signed. Even roofs built in accordance with the deemed-to-satisfy rules of the NBR should be checked for compliance by the architect, owner or building inspector. Sadly this is often not done, which is why we get roofs that fail,” he says.

Dave Anderson, managing director at Hi-Tech Nail Plate, agrees. “Building owners need to adhere to the regulations where buildings need to be inspected on a regular basis, which generally doesn’t happen.”

Especially when building alterations are made, such as load changes when ceilings are modified or new air-conditioning units are installed. “A timber roof is probably more susceptible to such changes than a concrete or steel roof since one can take out a structural member with a simple saw,” Anderson explains.

“I also believe that to an extent, sometimes the architectural design or engineering concepts of the building is incorrect, for example when the practical constraints of timber roofing are pushed too far.

He adds that building owners and constructors must engage with specialists in the industry to understand what is practically achievable and safe over the long term and each roofing project should be assessed on its own merits.

Penny wise, pound foolish
“We see a lot of roofs in the domestic housing market which are failing because they haven’t been correctly designed and erected in the first place. More often than not the building owner hired an unskilled person from the street to install the roof rather than following a properly engineered solution,” Anderson states. “The latter might even turn out to be the more economical route from the start because a proper design and calculated connections might require less timber.”

He notes that a timber roof is probably the most economical in terms of cost compared to a concrete or steel roof. “If the risk is managed, and it is done practically within the constraints and bounds of timber roofing, a timber roof truss is a very safe and simple solution,” says Anderson.

Fred Wagenaar, Executive Officer of the Institute for Timber Construction in South Africa (ITC-SA), stresses the importance of using a reputable product and service provider.

He references an incident where a consumer had to incur the cost of removing and replacing a faulty timber roof structure after an ITC-SA inspection found that the roof was badly erected and was manufactured with components that weren’t tested in accordance with industry standards.

In another instance, during a routine inspection an ITC-SA roof inspector stumbled across timber that was used as structural timber in the manufacturing of an in-situ built roof structure in an upmarket residential home. The timber was marked as structural S5 as per the SANS specification, but it did not comply with any mark specifications. In addition, the finger jointing of the structural members was inferior and non-compliant. Although the necessary structural and compliance markings were present, they were fraudulent and done by a self-made stamp.

“The built environment should not allow this lack of integrity and un-consequential trading in our industry,” Wagenaar states. “We cannot tolerate this behaviour. We also need to consider the financial and reputational consequences on the industry that we serve.”

Role of the ITC-SA
The ITC-SA actively works towards exposing fraudulent practices and rescinding the effects and consequences of bad industry practices on consumers, its industry partners and ITC-SA accredited members that do comply. Internally, it has a defined Code of Professional Practice and deals with membership transgressions. The body also provides assistance to the statutory regulators in the investigation and exposure of fraudulent practices.

“Quality control together with proper risk management should be the order of the day,” states Wagenaar. “For those who do not comply there will be no room in the industry.”

Full thanks and acknowledgement are given to the Institute for Timber Construction, MiTek and Hi-Tech Nail Plate for the information given to write this article.

Reasons for failure:
•    Poor roof truss erection/construction.
•    Installations by unskilled constructers without proper designs.
•    Modified roofs.
•    Incorrect manufacturing.
•    The use of inappropriate structural material such as low-grade timber.

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