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How Wood Dries - Definitions The following definitions are presented as background for understanding the principles of wood drying and for understanding the practical aspects of drying as well. Definitions Related to Wood and Its Structure Longitudinal Wood has three directions, with different properties in each direction. The vertical direction in the standing tree, and therefore the length direction in lumber, is called the longitudinal direction. Two important characteristics of the longitudinal direction are that water moves up to 50 times faster in this direction (i.e., end grain dries much faster than side grain) and wood shrinks very little in this direction. (Note discussion of reaction wood and juvenile wood as well.) Radial The radial direction is along a radius of the circular cross section (from the bark to the center), like the spokes of a wheel. Water moves faster in this direction than in the tangential direction. Therefore, flatsawn or plain-sawn wood will dry faster than quartersawn or rift-sawn wood. Also, shrinkage is less (about one-half) radially than tangentially. Tangential The tangential direction is parallel to the annual growth rings (or perpendicular to the radial direction) in a circular cross section. Water moves most slowly in this direction. Shrinkage is greatest in this direction. Flatsawn or Plain-sawn Lumber that has its wide face parallel to the annual growth rings (i.e., its wide face is a tangential surface) is flatsawn. This lumber, as it dries or with changes MC after drying, will shrink more in width than quartersawn lumber, making it difficult to keep paint on flatsawn lumber when the lumber is exposed to frequent wetting and drying. Flatsawn lumber dries faster than quartersawn lumber. Flatsawn lumber is more likely to develop surface checking than quartersawn lumber. Quartersawn or Rift-sawn Lumber that has its wide face perpendicular to the growth rings (i.e., its wide face is a radial surface) is quartersawn. This lumber shrinks more in thickness than flatsawn lumber, but shrinks less in width. It also dries more slowly. There is virtually no risk of surface checking on the 12PDF Image | Principles and Practices of Drying Lumber
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