Are wood roof trusses with plywood gusset plates alright for house construction?
Monday, December 16, 2019
No, they are usually not acceptable, and the reason is that roof trusses for a home must be designed and certified to handle their anticpated live and dead loads by a professional engineer. Engineering drawings have to be submitted to the local building department as part of a building permit application.
We have seen trusses using plywood gusset plates, glued and nailed to the truss webs, that were constructed at the homesite by the carpenters, a few times in the 1980s in the Florida Keys. They appeared to be engineered, followed a standard web configuration, and the plates seemed adequate. But, more often, site-built trusses are like the ones shown in the photo above of a rural 1950s era home, with undersize gussets secured with only a few nails and definitely not following an engineer’s design for the web placement.
There are engineer’s plans now available on the internet for building your own trusses with plywood plates, so it is possible. But factory-built trusses are so economical by comparison, when you factor in the labor cost, that do-it-yourself trusses only make sense if you consider your labor to be free. Plus, your homemade trusses are likely to get agonizing scrutiny for compliance with the plans by the local building inspector.
A home inspector for a homebuyer will always call out site-built trusses with plywood gusset plates for evaluation by a licensed structural engineer unless stamped engineering drawings are provided by the homeowner.
Truss factories use spiked metal gusset plates, with their size, shape, and location specified as part of the engineering. The plates are sunk into the wood at connection points with heavy rollers or a hydraulic press to create an extremely stable connection.
The only time we see plywood gusset plates nowadays is for a repair of a manufactured truss with metal gusset plates that was damaged during installation, like in the photo below. The repair gussets must be specified by an engineer and are always oversize to allow enough stiffness and nailing surface for the connection.
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To learn more about roofs and attics, see these other blog posts:
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