U^CX. 
 
 TECHNICAL 
 
 FOREST PRODUCTS LABORATORY 
 MADISON. WISCONSIN 
 
 N O 
 
 N U M B E R 2 3 4 
 
 UB#CD STATES J^RtF5T-S*RV ICE 
 
 The lengthwise shrinkage of wood 
 green to the oven-dry condition is normally somewhere be- 
 tween l/lO and 2/l0 of 1 percent. In drying to an average 
 air-dry condition of about 12 percent moisture content, the 
 normal shrinkage is only about half as much, the amount 
 varying from l/20 inch to l/lO inch in a board g feet long. 
 
 As a rule, therefore, the user does not need to make any 
 particular allowance for the longitudinal (lengthwise) shrink- 
 age of wood, certainly no such allowance as he must make for 
 transverse shrinkage. Nevertheless, trouble may easily arise 
 if the longitudinal shrinkage becomes abnormally large, as 
 sometimes happens. To prevent such trouble the following in- 
 formation on the main causes of excessive lengthwise shrink- 
 age, their identification, and means of avoiding or control- 
 ling them will be found useful: 
 
 1. Compression wood is a hard, heavy, brittle type of 
 wood generally formed on the lower side of branches and lean- 
 ing trunks of coniferous (softwood) trees. No fixed longi- 
 tudinal shrinkage value can be assigned to it, but its shrink- 
 age is in general excessive, in extreme cases running as high 
 5 to nearly 6 percent (green to oven-dry), which would mean a 
 shortening of 10 or 11 inches in a l6-foot length; this is 
 more than the average transverse shrinkage for many species. 
 More often its longitudinal shrinkage is less than 1 percent, 
 but well in excess of ordinary working tolerances. 
 
 The principal effect of compression wood, however, is 
 not direct end shrinkage so much as crooking or bowing of 
 lumber or dimension in drying, owing to its uneven distribu- 
 tion in the piece. Likewise an occasional streak of com- 
 pression wood that adjoins normal wood will pull itself 
 apart in drying, so as to form cracks across the grain. 
 Defects of this kind are as a rule well taken care of in 
 grading the material. Sometimes, however, the direct short- 
 ening causes trouble, as in the opening up of butt joints in 
 
house siding. The prevention of this trouble lies in seeing 
 that the material is thoroughly dry and fully shrunk before 
 it goes into construction. 
 
 Compression wood can usually be detected by the greater 
 thickness of the annual growth layers or rings in which it 
 occurs, the summerwood layer being especially thickened. 
 Sometimes the compression wood layers are grouped in close 
 succession, in other cases the thickening occurs sporadi- 
 cally. Compression wood seldom or never occupies the complete 
 circuit of a growth ring within the tree; in cross section it 
 is usually found as a scallop or half-moon pattern. Compres- 
 sion summerwood tends to be somewhat paler in color than 
 normal summerwood, but owing to its greater thickness the 
 area occupied appears darker on the whole than normal wood. 
 
 Cross section of log- containing compression wood 
 
Cross sections of wood measuring about 3A& inch along the 
 grain when held toward a strong light are translucent except 
 in areas containing compression wood, which are practically 
 opaque . 
 
 2. Abnormally light-weight wood . Wood below the average 
 weight shrinks lengthwise more than normally dense wood of a 
 given species; this is exactly the reverse of the rule for 
 transverse shrinkage. In lightweight wood the shrinkage along 
 the grain may in exceptional cases be as high as 1.5 percent. 
 In lumber that is graded for density the lightweight mate- 
 rial is eliminated from the better grades. 
 
 3. Springwood , the lighter-weight, lighter-colored part 
 of the annual growth layer, invariably shrinks more along 
 the grain in drying than the summerwood. Hence, any piece 
 of wood that has a large proportion of springwood will be 
 likely to show excessive end shrinkage. 
 
 The difference in lengthwise shrinkage tendency as be- 
 tween springwood and summerwood undoubtedly sets up shearing 
 stresses along the grain, which probably contribute to ease 
 of splitting or sliveringo In flat-grain flooring the 
 slivering nuisance can be largely avoided by dressing the 
 stock so that the "bark" side forms the face or wearing sur- 
 face. In that case the summerwood of each annual layer is 
 uppermost, and the tendency of emerging layers is to curve 
 down at the surface instead of up. Rotary-cut veneer, wher- 
 ever it happens to include a single annual growth ring, will 
 be especially subject to bowing in that area. 
 
 k-. Wood taken -from near the pith of the tree in some of 
 the softwoods shrinks lengthwise in drying more than the sur- 
 rounding wood. For this reason short cross breaks are often 
 seen in boards or timbers sawed lengthwise through the oith. 
 For the same reason a narrow piece so cut that the pith runs 
 along an edge will crook as it dries. On the other hand, it 
 has been reported that wood on the bark si de of some hard- 
 woods shrinks more than the wood farther in, thereby causing 
 crook or bowing in the reverse direction to the above. This 
 is probably an effect of density decreasing toward the out- 
 side of the tree, as frequently happens . 
 
 5- Very fast growing softwoods with wide annual growth 
 rings containing relatively soft and light-colored summer- 
 
wood bands have been found to yield wood of high longitudi- 
 nal shrinkage characteristic. Material showing the most 
 pronounced cases of this type has been found in the butt 
 portion of the trunk. 
 
 6. Cross-grained wood . Wood with spiral, diagonal 
 interlocked, wavy, or curly grain may show excessive appar- 
 ent longitudinal shrinkage on account of having a trans- 
 verse shrinkage component effective along the length of the 
 piece. Thus, if all the fibers ran through a board at an 
 angle of k-^>° » ^ would shrink in the same proportion end- 
 wise as edgewise. Even a small knot at the edge of a narrow 
 strip will cause bowing of the strip on account of the cross 
 grain introduced by fibers running around the knot. 
 
 7« Wood in long pieces concentrates all its lengthwise 
 shrinkage effect into large gaps at the ends; the longer 
 the piece the wider the gap. The present tendency toward 
 the use of short lengths will help to break up the longi- 
 tudinal shrinkage into smaller and less conspicuous units, 
 thereby improving the appearance and service of floors and 
 
 Siding. UNIVERSITY OF FLORIDA 
 
 3 1262 09216 7625 
 
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