THE LCNCITUCINAL Stil^lNrAGE 
 or REDWOOD 
 
 Jantdry 1931 
 
 ®s[ Of \mm \m 
 
 
 Z^7 
 
 UNITED STATES DEPARTMENT OF AGRICULTURE 
 
 FOREST SERVICE 
 
 FOREST PRODUCTS LABORATORY 
 
 Madison, Wisconsin 
 
 In Cooperation with the University of Wisconsin 
 
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THE LOIIGITUDIML SHRIMAGE OF EEDWOOIT 
 
 SCIEMTPIC RESEARCH FAILS TO BEAR OUT EEFJTATIOH 
 OF SPECIES FOR EXCESSIVE SHRIMAGE AlOFG TEE GRAIN 
 
 By 
 ARTHUR ZOEHLER, in Charge 
 Department of Silvicultural Relations 
 
 and 
 R. F. LUXPORD, Associate Engineer 
 
 Redwood and one or tiiro other native species of wood iiave the reputation of 
 shrinkage excessively along the grain. Seemingly extravagant statements of 
 "butt joints in redwood structures opening up to an unusual degree have been 
 made, yet measurements made at the Forest Products La'ooratory on several 
 hundred pieces of redwooa from different trees, indicate that the great bulk 
 of redwood does not shrink excessively along the grain. On the other hand, 
 an examination of redwood siding on a large number of houses in Los Angeles 
 revealed frequent instances of abnormally ^vlde openings in butt joints. In 
 some instances the openings had "been filled in with putty, and in others 
 sheet metal had been nailed over them. Only some of the butt joints had 
 opened up, however; others viere as tight as could be desired. Small slivers 
 for microscopic examination were taken from 50 pairs of pieces of siding 
 adjoining as many butt joints that had opened from l/S to 9/1^ inch, to see 
 if the wood vtere abnormal in any way. 
 
 A microscopic examination of 100 pieces of redwood siding forming open butt 
 joints, already mentioned, showed that all but four of the 50 pairs had com- 
 pression wood (Fig. l) on either one or both sides of the joints. Of the 
 four pairs that did not have compression wood on either side of the open joint, 
 one member of a pair was wavy grained, one member of another pair v;as found 
 to have compression wood some distance from the joint, and the other two 
 pairs shov;ed indications of having been put on in the first place vath open 
 joints, since the nails at the end showed no indication of pulling toward the 
 end as those at practically all of the other open butt joints did. Compres- 
 sion wood, therefore, was almost the universal cause of open butt joints in 
 the siding examined. It should be mentioned that the samples were talcen 
 almost entirely from joints in v;hich the gap was especially large. 
 
 Recently a large number of measurements of the longitudinal shrinkage of 
 redwood havo been made at the Forest Products Laboratory on both virgin and 
 
 ""Published in The Timberman, January 1931' 
 
 R1297 
 
Cr 
 
 second groi^th. The longitudinal shrinkage of some bOO strips of virgin aaid 
 second-grov/th redwood was plotted against their specific gravity (Pig. 2). 
 Some of the specimens had abnormally large shrinkage. A careful examination 
 of the 59 specimens having a shrinkage of three-tenths of 1 percent and over 
 shoiired tliat Ul consisted of compression vrood, and of the remainder tv/o were 
 cross grained and the others had either very little or no summerwood. Ex- 
 clusive of the points above 0.300 percent shrinkage, which for the most part 
 represent abnormal wood, as jvist stated, there is a general downward trend in 
 shrinkage with increasing specific gravity. Pieces of the soft, light spring- 
 wood whittled out by themselves showed the most longitudinal shrinkage and 
 pieces consisting entirely of the hard, dense suramerwood showed the least 
 shrinlcage from the green to the oven-dry condition, in fact, some of the 
 summerwood specimens elongated during the tests in the laboratory. 
 
 Measurements on second-growth redvrood indicate that it has a larger percentage 
 of specimens with relatively high longitudinal shrinkage than virgin growth. 
 This is especially true of the more openly grown materia].. Of the specimens 
 of vrood from trees that grew in close proximity to one another , 6U,5 percent 
 shrank less than two-tenths of 1 percent, and of those that grei-; more openly 
 56.0 percent shrank less than this limit. This difference betweaa virgin and 
 aecond-grov/th wood v;as largely due to the presence, in the second-growth 
 specimens mea-sured, of a certain anount of the wide-ringed wood with non-dense 
 summerwood haviiig different structure, already referred to. 
 
 The question as to whether redi^ood contains appreciably more compression v/ood, 
 wood light for the species, and spiral, wavy or curly .i:rain, cannot be defin- 
 itely answered at the present time, since it would require the examination of 
 a large number of representative trees of each species to give a reasonable 
 answer, sn.d that has not been done so far. Of 56 old growth trees examined 
 in connection with a specific gravity survey of redvrood by the Forest Products 
 Laboratory, 62 percent contained compression \rood. This may be a larger per- 
 centage of trees than in most other species, but that fact does not necessar- 
 ily mean tliat a larger percentage of pieces of redwood lumber contain com- 
 pression vrood than occurs in other commercial species. Compression vrood is 
 peculiar in tliat frequently it is formed only at irregularly occurring times 
 in a tree's life, A large tree like redwood, from 5OO to over 1000 years old, 
 would, therefore, have a greater chance of developing compression wood during 
 some time in its life than a tree of a smaller size and lower life limit, 
 
 Redvrood may possibly contain a greater percentage of v/ood light in weight for 
 the species than is found in several other trees, since the very narrow 
 annual growth rings found in many of the old trees and 'the very vride rings 
 in some of the second-grorvth trees contain a high percentage of springvrood, 
 which on account of its different cellular structure has a greater potential 
 longitudinal shrinkage than the summerwood. 
 
 There is no reason to believe that redvrood has more spiral grain than other 
 conifers. On the other hand, less diagonal grain is produced artificially in 
 sawing redi^rood than in cutting up some of the smaller, more crooked trees of 
 
 111297 
 
other species. Hence spiral and diagonal grain do not seem to be any more 
 responsible for abnormal shortening of pieces of redwood than of other species, 
 
 Redwood may perhaps have a little more wavy grain than many other conifers, 
 since wavywgrained boards are found rather frequently in red\'rood lumber. 
 
 The Ion,/; length in which redv/ood lumber is usually cut and frequently used 
 does cause greater absolute shortening of the members during drying than 
 would occur in shorter lengths of the same or other species, other things 
 being equal. 
 
 Since the bulk of redwood does not shrink a great deal along the grain, 
 where did redv/ood get its reputation for excessive longitudinal shrinka<ge? 
 
 The opening of butt joints, no matter what the species or type of wood, 
 shows that excessive shrinkage occurred after the wood was put in place, 
 since butt joints, are usually made tight when constructed. The fact that 
 the v/ood at the ends of siding next to the open butt joints previovisly re- 
 ferred to frequently had sheared along the grain at the nails, causing 
 noticeable displacement, is further evidence that shrinkage took place after 
 the siding had been nailed on and that the open joints in general were not 
 due to faultj'- carpentry. Widely open butt joints, therefore, mean in general 
 that the v;ood dried in place , which in turn means that it was not dry enough 
 when fastened in position. Even if it is dried, some openings in butt joints 
 may result from using lumber with a high potential shrinkage. Such lumber 
 would expand in damp weather, crushing its ends or displacing abutting pieces, 
 and when it subsequently shrank in dry weather it would leave open joints. 
 Much redwood lumber has been marketed and used in an insufficiently dry con- 
 dition; this is knovrn definitely. If the timber had been properly dried 
 before use much direct end shrinkage could have been avoided and many of the 
 pieces with abnormal shrinkage would have been thrown out on account of 
 crookedness and would not have been used for the better class of work, and 
 thereby trouble in use caused by end shrinkage would have been further min- 
 imized. 
 
 Siding of other species, if inadequately dried before uso, would also show 
 occasional open butt joints caused by compression wood and other abnormal 
 typos. Most other species of wood, however, could not be marketed in a wet 
 condition without quickly deteriorating from stain and decay. Redwood, on 
 accoxint of its high decay resistance and natural dark color, which does not 
 show stain readily, can be shipped and stored in a wet condition for longer 
 periods of time* 
 
 If all virgin redwood had been properly dried before it was put into use and 
 if the crooked pieces had been thrown out, most of the stock that has given 
 redwood its reputation for excessive longitudinal shrinkage would have been 
 eliminated and the size of the open joints caused by the abnormal pieces 
 that were not eliminated would have been greatly reduced. 
 
 RI297 -3. 
 
Figure 1. — Cross section of an IS- inch second- growth redwood log 
 
 showing compression wood on the lower side. The wide rings 
 with wide summerwood constitute the compression wood. 
 
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 """'"' jpfcir/c CRaviTY-ensto on ovcn-ory wcicht akb volumc ■ 
 
 Figure 2, — Longitudinal shrinkage of redwood in relation to its 
 z M if2i36 F specific gravity. 
 
UNIVERSITY OF FLORIDA 
 
 3 1262 08866 6218 
 
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