Repairs, Maintenance and Renovations
Roof Truss Separation Anxiety
From the Volume 16, Summer 2023 issue of the CCI GHC Condo News Magazine
When was the last time you looked in your attic? If you said “never”, you’re not alone. Attics are dark, uncomfortable places, usually with access hatches hidden away in overstuffed bedroom closets. And what is up there to look at anyway? It’s not like the roof trusses are going to fall apart, right? But that might not always be the case. A chance inspection recently revealed that the metal plates used to secure the roof truss members together at one townhouse development had significantly separated from the wood, leaving the trusses structurally compromised and at risk of collapse.
The keen eye of an Engineer completing an inspection of an attic space to check for potential roof ventilation issues as part of an asphalt shingle roofing assessment uncovered that some of the metal roof truss plates had separated from the wood. This led to the reviewing of additional attics where the problem was found to be widespread throughout the townhouse development. But let’s back up. What is a truss plate and why do I care if it separates?
Traditionally, sloped roofs on houses included conventionally framed roofs consisting of sloped wood rafters supporting the roof surface and horizontal ceiling 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 in 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, before being delivered to the site where they are lifted in place once the exterior walls have been constructed. 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 wood members are secured together with galvanized steel truss plates which have punched teeth that are intended to be permanently pressed into the sides of the truss 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 inter-connection 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. The Truss Plate Institute of Canada requires that all teeth of each connector plate be completely embedded in each wood member. Truss plates that are
separated from the wood, resulting in an embedment gap equal to or greater than 1.6mm or 1/16”, are considered ineffective from a design perspective.1 This combined with the critical nature of each connection means that the separation of even a single truss plate can significantly reduce the structural capacity of the entire truss assembly. In a number of cases involving large agricultural buildings, the separation of enough truss plate connections has led to widespread structural collapses. So yes, the separation of even a few truss plates from the wood is a big deal!
1 Truss Plate Institute of Canada TPIC 2019. G.4.1.6 Tooth Embedment Gap
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.
But if the truss plates are permanently pressed into the sides of a truss when the truss is built, how do they separate from the wood? Truss plate separation can be related to issues during truss transportation or installation, such as if the trusses are excessively bent out-ofplane, applying unintended loading on the connections. In the case of this townhouse development, the separation appears to have been a result of inadequate initial drying of the wood, resulting in excessive dry shrinkage after fabrication. The wood truss members included widespread checking (surface cracks) and the members where the truss plates were most separated were significantly twisted, due to stress from uneven shrinkage of the wood. This dry shrinkage appears to have essentially squeezed the truss plate teeth to the extent that they partially withdrew from the wood, with further separation occurring where the members twisted out of alignment.
The service conditions within the attic space likely also contributed to progressive truss plate separation due to changes in moisture of the wood. The attics included poor exterior ventilation combined with discontinuities in the ceiling air and vapour barriers. These conditions, which are not uncommon in older townhouse developments, resulted in excessive condensation moisture exposure during the winter months and high attic temperatures during the summer. Over the life of the townhouse development, the seasonal moisture and related expansion and contraction cycling of the wood likely contributed to progressive truss plate separation.
Modern quality controls with truss fabrication, 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 more cost effective to completely replace the roof! Of course, replacing the roof structure, even on a single townhouse unit is a huge undertaking; however, given the structural implications of ignoring the issue, the decision may need to come down to what can be done before the winter snow arrives.
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 is a licenced Professional Engineer and Principal Forensic & Structural Engineer at Brown & Beattie Ltd. where he has worked for the past 14 years. He specializes in the assessment and repair of buildings damaged by a wide range of events from leakage to structural collapses, explosion and fire damage, along with vehicle impact and extreme weather. He and his colleagues help Condominium Corporations and Insurance Companies determine the cause and best course of action to address structural and building envelope related failures.
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