Allowable Variances In Trusses
Question:
My framers are always complaining that the “trusses are bad.” I am looking for tolerance information that not only addresses the allowable variance in length and height, but also allowable variation in the top chord with regard to straightness (i.e. how straight should a pull string line from the top and bottom of top chord be?). Also, if trusses are set on a perfectly level wall, what variation is allowed from truss to truss (i.e. if I put a 10 ft. straight edge perpendicular to the trusses, how much can they vary in height, not just at peak or bottom but all along the top chord)?
Answer:
As you know, one of the best reasons for using wood in construction is that it is easily cut and shaped and can be very forgiving dimensionally. One of the limitations is that it can shrink, swell, twist, warp and bow. The result is that once a component piece is cut, its final shape may change from what you originally intended. This is a fact of life when you are designing with wood and most construction and manufacturing details allow for some play in the final results.
The Truss Plate Institute's ANSI/TPI 1 is the guidance the industry provides regarding manufacturing tolerances. For example, table 3.5-1 specifies that a 1/2 in. span variance of identical trusses is acceptable as well as a 1/4 in. height variance. A 3/4 in. variation from the span of truss design dimensions is allowed as well as a 1/2 in. variation from height design. For example, Table 3.5-1 allows identical trusses to vary in span by as much as 1/2 in. and in height by as much as 1/4 in. Variance from the design dimensions is also allowed: 3/4 in. in span and 1/2 in. in height. Complying with the erection tolerances specified in SBCA’s BCSI-B1 Summary Sheet is critical to achieving an acceptable roof line.