Roof Truss Plate Dislodgment
Question:
I am investigating a roof failure in a 22-year-old structure. The connector plates have peeled open like a banana skin in several instances. The teeth do not appear sheared or torn at the wood surface. However, the plates will be in firm to one member and separated in the other (a gap of about 1/32 in.). Although there are large deformations, the roof is still standing. There was a heavy snow before the problem became apparent. Do you have any insights?
Answer:
There are two primary causes for plates withdrawing from lumber:
1. If the lumber is subjected to repeated wetting and drying, the plates may work their way out due to the resulting swelling and shrinking of the wood. This results in a more or less uniform gap between the lumber and the plates.
2. When overloaded, plates may exhibit “peeling.” The gap between plate and lumber will be at a maximum at the outside of the plate and taper off towards the inside. From what you said, the gaps on your trusses sound like the result of overloading. When truss plate manufacturers test their plates to come up with design values, one of the tests they perform is for lateral withdrawal resistance. Lateral withdrawal resistance refers to the truss's ability to resist “plate peeling.”
Determine who manufactured the truss plates that you’re dealing with (there are often markings on truss plates that identify the manufacturer) and contact that manufacturer for assistance with repairs.