ACMO Blog

Roof Truss Plate Separation – Out of Sight Should Not Mean Out of Mind

When was the last time you thought about what holds up the roof over your head? Many people might not know what roof truss plates are, let alone how critical they are in modern roof structures. Traditionally, sloped roofs on houses included conventional framing consisting of sloped wood rafters supporting the roof surface and horizontal joists supporting the ceiling. With this construction, the larger the span, the larger the lumber used. Houses are still being constructed with these roofs today; however, starting around the 1970s, prefabricated wood roof trusses became more popular. Wood roof trusses consist of top and bottom “chords” and interior “webs”, usually constructed of 2x3, 2x4, or 2x6 lumber, forming a truss shape. They are designed and preassembled in a factory to match the desired size and shape of the roof. This process is well suited for complex roof geometry as well as higher production homebuilding and has been widely adopted by builders of large townhouse developments.

With a wood truss, the individual members are secured together with galvanized steel truss plates which have punched teeth that are intended to be permanently pressed into the wood at the time of fabrication. These truss plates are sized to meet the loading requirements of the joints and when properly installed can provide incredibly high strength connections. Roof trusses, by their nature, are an extremely efficient use of materials; however, as a result, there are no redundant members or connections. The trusses rely fundamentally on the interconnection of all their members, the absence of which compromises the strength of the entire truss. 

The truss plate teeth are relatively short (in the order of 8mm or 5/16” long) and are required to be fully embedded, with the plate tight to the face of the wood to achieve the intended design strength. These plates have tight installation tolerances, such that plates with an embedment gap of 1.6mm or 1/16” or more are considered ineffective from a design perspective.  Combined with the critical nature of each truss connection, the separation of even a single truss plate can significantly reduce the structural capacity an entire truss assembly. Although rare, in a number of cases involving large agricultural buildings, the separation of the truss plate connections has led to widespread structural collapses.

Truss plate separation is not just an issue for agricultural buildings and has been uncovered in individual houses as well as large townhouse developments. It can occur during truss transportation or installation, such as if the trusses are excessively bent out-of-plane, applying unintended loading on the connections. It can also occur over time as a result of inadequate initial drying of the wood, resulting in excessive dry shrinkage after fabrication. This shrinkage can essentially squeeze the truss plate teeth to the extent that they partially withdraw from the wood. Excessive dry shrinkage after fabrication can also result in checking (surface cracks) and twisting of wood, further stressing the truss plate connections.

Poor attic ventilation combined with discontinuities in the ceiling air and vapour barriers can lead to excessive condensation moisture exposure during the winter months and high attic temperatures during the summer. Over the life of a roof, seasonal moisture changes and related expansion and shrinkage cycling of the wood can also contribute to progressive truss plate separation. Unlike damage during transportation/installation, which will hopefully be caught during construction, this can occur slowly and go unnoticed within an attic over years.

Modern quality controls, improved installation practices, along with better attic moisture management can help to prevent this type of truss plate separation from starting; however, what happens when the damage is already done? An individual truss plate connection can usually be repaired with the use of properly designed plywood gusset plates using conventional nails. However, doing so is relatively labour intensive and the thought of repairing even a few plates on each truss throughout a townhouse development is daunting. In some cases, if the number of repair locations is high enough, the roof sheathing is mouldy, and the shingles are in poor condition, it may be cost-effective to completely replace the roof structure.

Ultimately, truss plate separation, though relatively uncommon, is not the type of issue you want to find out about once the trusses have already started to fail. So, it might be a good idea to have an inspection of that attic space to see just what might be lurking up there.

Stephen MacDougall P.Eng.
Principal Forensic and Structural Engineering
Brown & Beattie Ltd.

Brown & Beattie Ltd. was founded in 1998 and we specialize in the investigation, repair and improvement of building systems and structures. Brown & Beattie has worked closely with condominium corporations, property management firms, the public sector and the insurance industry for over 25 years, and we aim to provide experience-based value and honest Professional Engineering advice. Our Professional Engineering services includes (but not limited to): 

• Building Restoration Design
• Condition Assessments
• Specifications & Tendering
• Construction Review & Contract Administration
• Repair Planning
• Due Diligence Exercises
• Litigation Support
• Forensic Engineering

An example of our services can be found here: www.brownbeattie.com/services