Warp reduction of SYP lumber by restrained drying Rubin Shmulsky ✳ Robert W. Erickson ✳ Philip H. Steele ✳ Donald Buckner Abstract Warp in dimension lumber is a significant cause for loss in grade and value. Among dimension lumber, the 2- by 4-inch size is typically the most susceptible to warp for a variety of reasons. Among the various sizes of southern yellow pine (SYP) dimension lumber, 2 by 4 is produced in the greatest volume. Two-by-four material sawn from young trees, thinnings, and stem tops contains a significant proportion of juvenile wood and is particularly notorious for warp. At the planer mill, warped 2 by 4’s represent a significant loss in value. In-service replacement of warped 2 by 4’s as wall studs or truss members is costly. In this research, a drying restraint system was evaluated as a means of warp reduction in 8- and 16-foot-long SYP lumber. Both restrained and control boards were dried to approximately 9 percent moisture content, representative of final in-service condi- tions. The lumber was then measured for crook, bow, and twist. Results showed a significant improvement in higher grade recovery for the restrained material. As a means of assessing long-term dimensional stability, the 16-foot-long restrained pack- age was remeasured for warp after both 1 and 9 months of warehouse storage. Because a large proportion of 2- by 4-inch lumber is cur- rently sawn from young small-diameter trees, problems asso- ciated with wood quality are significant. High percentages of juvenile wood in these young stems reduce grade retention. Juvenile wood is particularly known for its ability to induce crook and other forms of warp (Bowyer et al. 2003). The im- mediate result is noted at the planer mill where the affected lumber is downgraded from No. 2 and better grade to No. 3 and reject. Because this lumber is from young small-diameter trees, knots are generally not of sufficient size or frequency to cause downgrade below No. 2 grade. If dried free of warp, it is estimated that southern yellow pine (SYP) 2 by 4’s could be produced with approximately 90 percent as No. 2 and better, with degrade occurring from knots and wane. On average however, SYP 2 by 4’s are produced at only 73 percent No. 2 and better. The remaining 27 percent is No. 3 and lower. For this lumber, warp is a highly important predictor of value. Grade retention improvements are achievable through control of warp frequency and severity. The economic value loss for the lumber downgrade is sig- nificant, on the order of 50 to 150 U.S. dollars per thousand board feet (MBF). Based on this and previous studies related hereto, we estimate that 7 percent of the total SYP 2 by 4 production could be upgraded from No. 3 and lower via re- strained drying. At an average increase of $100/MBF and a total annual SYP 2 by 4 production of approximately 5.1 bil- lion BF (SFPA 2002), this represents an added value of $35 million annually. For a commonly sized small log mill (125 million BF per annum) that produces primarily 2 by 4 stock, this 7 percent increase in grade recovery translates to $900,000 per annum. In many instances, warp develops in dressed lumber that is stored at the planer mill for weeks or months. Storage regu- larly occurs when lumber prices decline and inventories accu- mulate. During storage, the lumber continues to moisture The authors are, respectively, Associate Professor of Forest Prod- ucts, Mississippi State Univ., Mississippi State, MS (rshmulsky@ cfr.msstate.edu); Professor Emeritus of Wood and Pap. Sci., Univ. of Minnesota, St. Paul, MN (erick117@umn.edu), Professor and Re- tired Research Technician of Forest Products, Mississippi State Univ. (psteele@cfr.msstate.edu; dbuckner@cfr. msstate.edu). Gra- cious acknowledgement and thanks is extended to Carl Neels, Inter- national Paper Co. and Gayle Bridges, Wellons Co. for their contri- butions to this research. FWRC manuscript No. FP322. This paper was received for publication in February 2004. Article No. 9847. ✳Forest Products Society Member. ©Forest Products Society 2005. Forest Prod. J. 55(9):37-41. FOREST PRODUCTS JOURNAL VOL. 55, NO.9 37