Manufacturing Tolerances
Question:
I am a structural engineer on a project with a wood trussed roof. I have noticed some variations in the peak height in a run of identical trusses. Do you have any information on tolerances for the final dimensions of manufactured trusses?
Answer:
Yes, the Truss Plate Institute's ANSI/TPI 1 provides guidance on manufacturing tolerances.
It specifies that a Truss to Truss variance of identical trusses is ½” in length and ¼” in height. It is better for the framer if all the trusses are ½” too long or ¼” too high than one truss ½” too long or ¼” too high.
The truss manufacturer’s goal is to have all the trusses have the same height or length so that the plane they are forming is fairly flat. Once the jig is set in the truss plant this should be the result. If there is significant variation in the trusses from one to the next, one of the issues to check is the amount of shrink, swell, twist, warp or bow in each truss.
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