Juvenile Wood as a Cause of Seasonal Arching in Trusses

Seasonal arching of roof trusses has become increasingly prevalent in recent years. Recent research projects have suggested that this phenomenon is caused by moisture differences between upper and lower chords of trusses and is aggravated, by the presence of juvenile wood. A series of 14-foot king-post trusses was constructed of green southern pine lumber with various combinations of adult wood/juvenile wood chord members. Assembled as a simulated roof system, the trusses were loaded to an equivalent in service dead load and dried to approximately 6 percent moisture content (MC) while measurements were made of mid-span deflection. Significant arching occurred only in the trusses containing juvenile wood lower chords, induced by the large longitudinal shrinkage of juvenile wood. The Purdue Plane Structures Analyzer II (PPSAII) was used to correlate the results of this study to arching caused by moisture differences in trusses. Suggestions for reducing seasonal arching include lowering the relative humidity of attic spaces, fabricating trusses with lumber at or below 15 percent MC, and reducing the amount of juvenile wood in the lower chord.

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