- Q. What happens to trussplate connectors as the wood truss members expand and contract with temperature and moisture changes? The prongs on the plate don’t seem to penetrate very deeply, and it seems as if they could work loose over time.
A.There is no question that the wood shrinks away from the prongs on truss plates as the lumber dries, and this does reduce the strength of the truss somewhat. But trusses are designed with a large safety factor. For example, most trusses are designed for a roof load of 40 pounds per square foot, yet joist-and-rafter roof construction usually fails at less than 15 psf, and such failures are rare. Most of the loosening of the plates would occur during the initial drying period rather than from repeated moisture changes.
A greater problem is the lateral loads often placed on trusses as they bend and twist during handling and erection. Trusses are designed to resist vertical loads. The lateral loads that occur when they are taken off the truck, and when they are swung from an inverted position on the wall plates to the upright position, can loosen the plates, or even pop them off one of the members being joined.