*■* y \ -i ' ■ / ( \ SOME EFFECTS Cf STORAGE ON SEASONED LUMBER Revised August 1949 No. LQ1C71 UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE FOREST PRODUCTS LABORATORY Madison 5, Wisconsin In Cooperation with the University of Wisconsin Digitized by the Internet Archive in 2013 - http://archive.org/details/efffstoOOfore SOIffi EFFECTS OF STORAGE ON SEASONED LUMBER By J. S. KATESWSON, Engineer and ". . P. FASMUSSEN, Engineer Forest Products Laboratory ,— Forest Service U. S. Department of Agriculture The- fullest efficiency and satisfaction to be derived from the use of wood in any construction are in a large part dependent upon its moisture content at the time of installation. If users are supplied with lumber suit- ably dried for the job, there is no apparent reason why they should be con- cerned about its serviceability so far as shrinking and swelling are concerned. Some mills machine and ship thoroughly seasoned lumber to wholesale and retail yards ] bo find that it is stored in piles unprotected from the weath er. Such storage largely offsets the benefits of proper seasoning, because the :'^od may absorb moisture to an undesirable extent before it is used. This i Lsture regain followed by a moisture loss during fabrication or use may result in warping and end checking. Correct storage practice is necessary for properly kiln-dried lumber, therefore, to maintain the lumber at the desired moisture content. By so doing, defects as well as changes in moisture content occurring during storage will be minimized, if not entirely eliminated, thereby reducing loss of material during fabrication and sub- secuent use. Effect of Storage on Moisture Content In order to obtain definite information upon the effect of storage on moisture content of lumber, the Forest Products Laboratory surveyed storage methods at sawmills, wholesale and retail distributing yards, end wood-using plants in various parts of the United States. Illustrative of the results obtained is figure 1, based on a study at a Louisiana sawmill. Somewhat similar results were obtained at a Florida sawmill and at several distributing yards in the vicinity of Chicago. The stock at the Louisiana mill was southern yellow pine 1- by 4-inch by 12-foot flooring and 1- by 8-inch by 12 -foot board? surfaced four sides and solid piled -- that is, piled without stickers. A group of boards selected for uniformity of moisture content by means of a moisturi m tor was piled in each ained at Madison e, V.is., in cooperation with the University of Wisconsin, rt K . R1071, revised. -1- of the following locations: (1) In a yard where the pile was 7/elI protected by a tight roof; (2) in a partly open shed with metal roof; (3) in an open shed with -.vcod roof; and (ty-) in a closed shed with a wood roof. Each curve in figure 1 represents the changes in average moisture content of SO boards. The average moisture content values, which were initial ly about 7«5 percent, ranged between 10 and 11 percent after Xp . ao nths in the three sheds. In I 1 the average moisture content reached 13*5 percent during the same period. .'. ..se are average values; the ends and ether surfaces of the boards irectly to the air would have moisture content values higher than the average. The relatively large absorption of moisture at the ends is objectionable either in the rough lumber or in a finished product such as FlooTia . .."her. the rough lumber is surfaced, ^he boards will have a uniform bh only at the time of machining; subsequently the ends will shrink and will be narrower than the remaining portions of the boards. In a finished product, assuming that the boards are fairly uniform in moisture content when machined and subsequently absorb moisture during storage, the ends will become wider and thicker than they should be. -. e effect of temperature and relative humidity on equilibrium moisture content is shown in figure 2. For example, at the average indoor temperature of 70° P« an - relative humidity of ^2 percent, the equilibrium moisture content of wood is S percent. It has been found that, for the most satis- factory service, such products as flooring and interior finishing woodwork conventionally heated dwellings in most parts of the United States should be installed at about 6 percent moisture content. Assume that in an unheated shed the temperature and relative humidity respectively 3~° P« an d 75 percent. According to figure 2, the corre- sponding equilibrium moisture content is 15 percent. If the shed is tightly constructed, so that no extraneous sources of moisture are present, the equilibrium moisture content can be reduced to 3 percent by merely heating the air in the shed to about H5 F. (See dotted line in figure 2.) In this . sprays, water sprays, desiccants, or refrigeration are not required. Further, the boat loss under this condition is less than if a common temper- ature of, say, 60° or 7^° F« were maintained. ndesirable changes in the moisture content of lumber within closed storage sheds can be largely prevented.— t ing storage, all th e lur. ads to come to the same moisture content. In other words, the ran. 1 - ': ire content between individual pieces becomes loss with storage (fig* 3)» this is advantageous if storage conditions are properly maintained, particularly if the moisture content of the lumber varies considerably after kiln drying. 2 -Forest Products Laboratory Report No. R1140, "Moisture Fluctuations in Lumber " ;hin Closed Storage Sheds Controlled With Electrical Equipment." . R1071, revised. -2- E f fect cf Stcra.-e or. Moisture C~rad.ie:. and Dryi • Stress- s A •' •- .- vantage of storage is that it givss time for the r^isture content to eq ualize throughout the thickness cf each piece. With some woods, especially "sofir.voods , the storage period and equalization of moisture content fficient for the relief cf drying stresses. Sone species, partic- y hardwoods, on the other hand, may shcv/ severe drying stresses even -r extendeH periods of storage. *Jnd-r poor storage conditions, stresses maybe induced in the lumber • Outside storage of kiln-dried lumber, for example, r;ay cause reverse case- ling in boards that are exposed to snow cr rain. Further, the absorption at drying ause old : ■ to open up --. . v ones to develop. Sfi b of S ra - ; litting cf Lurbor ;ly, rather heavy losses fron splitting occur during storage of lumber or partial] products, especially in workshops heated duri the winter. It is net _.:n, during winter months, f'or the equilibrium moisture sontgnt '.:..-. Is iops to be as lov; as h percent -while " : as high as 1 r . t or more ' bed storage s Hie ends of lumber stored in these shedc consequently -.ay have a very high moisture : t , and upon exposure tc bh« iry • as cf the workshop will end dry ra i I ■ cause end ch cks splits. Exposure : f freshly cut ends cf lumt - a comparatively high moisture content to cry conditions cay also : I . ■ i ST libs . Tae faces of par ricated pieces may split if -.. pieces are stacked so that ay r only partially cposc 10 tb equilibrium conditions. Shrink ige of i - ; - . d face aav also be sufficient to . opening of glue joints. Splits and openings of thi re are -ore common in stock 7ith a high :crs mc re content, one face : teen rtially cut away, thereby expos: ;he zone re content. A good exam] Storage of Green, .'•.'k---~;ried, and hllr.-.lr: i Lumber to score green lumber for an appreciable period of t ui tout loss frcic stain, a . or ins - ok. It car. be done, rever, by under-water storage or storage at low temperatur s, that is below 3f° f» ger of stain a::d leca; - also be minimized by treat r in . mber with some oi boxic chemicals used to • : blue .— i ~ 1 . 7 . S. Forest Products Laboratory. "Cause ar.d Prevention of Blue Stain in Wood. Technical ITote No. 225* (in cooperation with Div. of Jcr. Path., Bur. 11 Inc., Soils, and Agr. Engr.) Re;:-.- ;, R10J1, revised. -3- kiln The best practice, however, is to start/drying as soon as possible after cutting the log into lumber. If the season of the year is advantageous, air-dried lumber may be stored by leaving it stickered in the air-drying pile. The best method of storage, however, v/ould be to pile it in open, partially closed, or unheated closed sheds. It could be sticker piled or, if the average moisture content is below 20 percent, solid piled. Storage in this manner will best maintain the final air-dried moisture content without causing losses or decay from exposure to the elements. Kiln-dried lumber should be given the best of care in storage. There is no advantage in kiln drying lumber to a low moisture content if during storage it eventually attains the moisture content of air-dried stock. Preferably, kiln-dried lumber should be stored in closed heated sheds in which the equilibrium moisture content is controlled. If such sheds are not available, storage in the upper part of a closed or partially closed shed in which the temperature is slightly higher than the outside temperature is desirable. 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