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Abstract 

This report describes how lumber can be air-dried most 

effectively under outdoor conditions and illustrates the 

principles and procedures of air-drying lumber that were 

developed through field investigations and observations of 

industrial practices. Particular emphasis is placed on the 

yarding of lumber in unit packages. Included are topics such 

as why lumber is dried, advantages and limitations of the 

drying process, properties of wood in relation to drying, 

layout of the drying yard, piling methods, causes and remedies of air-drying defects, and protection of air-dried lumber. 

Keywords: drying lumber, air dry, wood structure, wood 

shrinkage, drying rate, wood defects 

October 1999 

Forest Products Laboratory. 1999. Air drying of lumber. Gen. Tech. Rep. 

FPL–GTR–117. Madison, WI: U.S. Department of Agriculture, Forest 

Service, Forest Products Laboratory. 62 p. 

A limited number of free copies of this publication are available to the 

public from the Forest Products Laboratory, One Gifford Pinchot Drive, 

Madison, WI 53705–2398. Laboratory publications are sent to hundreds 

of libraries in the United States and elsewhere. 

The Forest Products Laboratory is maintained in cooperation with the 

University of Wisconsin. 

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Preface

This manual is a revision of the 1971 edition of Air Drying of 

Lumber: A Guide to Industry Practice by Raymond C. Rietz 

and Rufus H. Page. A major contributor to the 1971 edition 

was Edward C. Peck, formerly a wood drying specialist at the 

Forest Products Laboratory. 

The major reason for this revison is the continued interest in 

and requests for the manual, even almost 30 years after initial 

publication. Because of this continued interest, we maintained the same basic framework in the revision. Our main 

objective was to update with new or more current information where necessary, keeping in mind that our audience will 

range from small industry (perhaps new to lumber drying) to 

larger industry (perhaps having substantial experience in air 

drying). Involved in this revision were 

 William T. Simpson, Research Forest Technologist, 

 John L. Tschernitz, Chemical Engineer (retired), and 

 James J. Fuller, Research Forest Products Technologist, 

of the Forest Products Laboratory. 

Acknowledgments 

Special recognition is given to Edward C. Peck, formerly a 

Wood Drying Specialist at the USDA Forest Service, Forest 

Products Laboratory, for his research and related contributions that formed the basis for most of this publication. We 

also appreciate the cooperation of lumber-producing industries in providing photographs of their operations. 
Air Drying of Lumber 

USDA Forest Service 

Forest Products Laboratory 

Madison, Wisconsin 
Contents

Page

Introduction ......................................................................... 1 

1. Why Dry Lumber?.......................................................... 3 

Drying Methods .............................................................. 3 

Objectives ....................................................................... 4 

Advantages and Limitations............................................ 6 

2. Wood Properties and Moisture....................................... 7 

Structure.......................................................................... 7 

Moisture Content .......................................................... 10 

Methods to Determine Moisture Content...................... 10 

Moisture Movement...................................................... 12 

Equilibrium Moisture Content ...................................... 14 

Fiber Saturation Point................................................... 14 

Shrinkage ...................................................................... 14 

Weight .......................................................................... 18 

Color............................................................................. 18 

3. Air-Drying Process....................................................... 21 

Utilizing Air Movement................................................ 21 

Factors That Influence Drying Rate.............................. 21 

Drying Time and Final Moisture Content..................... 23 

Deterioration of Lumber ............................................... 25 

4. Air-Drying Yard ........................................................... 27 

Yard Layout and Orientation ........................................ 27 

Yard Transportation Methods....................................... 29 

Pile Foundations ........................................................... 29 

Shed Drying.................................................................. 31 

Yard Operation and Maintenance ................................. 32 

Page

5. Piling Methods For Air Drying..................................... 33 

Sorting Lumber............................................................. 33 

Sorting Equipment ........................................................ 34 

Stacking Lumber in Packages....................................... 34 

Stacking Random Length Lumber ................................ 38 

Special Piling Methods................................................. 38 

Stickering Lumber ........................................................ 39 

6. Air-Drying Defects—Causes and Remedies................. 43 

Chemical Reaction........................................................ 43 

Fungal Infection............................................................ 43 

Insect Infestation........................................................... 44 

Shrinkage...................................................................... 44 

Warp Reduction............................................................ 48 

Stacking, Package Piling, and Pile Protection .............. 49 

7. Protection of Air-Dried Lumber ................................... 51 

Outdoors ....................................................................... 51 

Indoors.......................................................................... 51 

In Transit ...................................................................... 53 

Appendix. Summarized Guide for Air-Drying Practices .. 55 

Glossary............................................................................. 58 
Introduction

Trees contain a considerable amount of water. Most of this 

water must be evaporated before the lumber obtained from 

a tree can be converted into consumer products. The lumber 

from which most wood products are manufactured must 

be dried. 

Of all the methods to remove large quantities of water from 

wood, air drying has the least capital costs, especially in the 

early stages of drying. However, air drying offers little 

opportunity to control the drying process. 

Drying wood by allowing natural forces to evaporate the 

water is not a new idea. Over the years, people learned how 

to utilize these forces effectively. However, this information 

accumulated slowly, and many individuals are aware of only 

part of the technology. This publication assembles information on air drying learned through research and information 

that has evolved from many years of experience. 

This report gives plant managers, yard supervisors, lumber 

handlers, and others in related areas an overview of the 

principles of air drying. It will also assist them in analyzing 

their air-drying practices. Drying wood by any means is 

expensive, so operators are constantly on the alert to reduce 

costs. Application of general air-drying principles may lead 

to changes in yarding methods that will result in faster drying 

of lumber and improved quality. 

The results of efficient air drying benefit consumers of forest 

products and are of key importance in utilization of the 

Nation’s forest resource. Efficient air drying helps to conserve supplies of wood, thereby the timber resource, by 

reducing loss of product from drying degrade. Air drying 

also reduces the need to burn fuels for energy to dry lumber, 

thus conserving these fuels and reducing atmospheric emissions. In addition, air drying helps to ensure continued markets for wood products by contributing to customer satisfaction. Both conservation of the timber resource and customer 

satisfaction contribute to the wise use of timber, which has 

long been an accepted tenet of the USDA Forest Service’s 

conservation policy. 

The emphasis of this publication is practicality. After touching on the variety of reasons for drying lumber and the wood 

properties that relate to drying, the basics of the air-drying 

process are presented. Then, descriptions are given of the 

drying yard, methods of piling lumber for air drying, defects 

that occur in air drying, and protection of the air-dried lumber. Finally, an appendix summarizes the information in this 

report and serves as a convenient means of checking specific 

points in an individual operation. As in any industry, specific 

terminology has evolved and shades of meaning show up 

geographically, between hardwood and softwood production 

or between a plant operator and a researcher. For this reason, 

a glossary is included to delineate how words are used in this 

publication. Wood species are generally listed in this report 

by their common names. However, different common names 

are often used for the same species; therefore, both common 

and botanical names are given in Table 1. 
2

Table 1—Average moisture content of green wood, by species

 Moisture contenta (%) Moisture contenta

 (%) 

Species Botanical name 

Heartwood Sapwood 

Mixed 

heartwood 

and

sapwood Species Botanical name Heartwood Sapwood 

Mixed 

heartwood 

and

sapwood 

Hardwoods Softwoods 

Alder, red Alnus rubra — 97 — Baldcypress Taxodium distichum. 121 171 — 

Ash, black Fraxinus pigra 95 — — Cedar, Alaska` Chamaecyparis nootkatensis. 

32 166 — 

Ash, green F. pennsylvanica — 58 — Cedar, Atlantic white Chamaecyparis thyoides. 

— — 35 

Ash, white F. americana 46 44 — Cedar, eastern red Juniperus virginiana 33 — — 

Aspen, bigtooth Populus grandidentata 95 113 — Cedar, incense Libocedrus decurrens. 40 213 — 

Aspen, quaking P. tremuloides 95 113 — Cedar, Northern white Thuja occidentalis 32 240 93 

Basswood, American Tilia americana 81 133 — Cedar, Port-Orford Chamaecyparis lawsoniana. 

50 98 — 

Beech, American Fagus grandifolia 55 72 — Cedar, western red Thuja plicata 58 249 62 

Birch, paper Betula papyrifera 89 72 — Douglas-fir, coast 

typeb Pseudotsuga menziesii 37 115 45 

Birch, sweet B. lenta 75 70 — Douglas-fir, intermediate type 

P. menziesii 34 154 

Birch, yellow B. alleghaniensis 74 72 — Douglas-fir, Rocky 

Mountain type 

P. menziesii 30 112 43 

Butternut Juglans cinerea — — 104 Fir, balsam Abies balsamea 88 173 117 

Cherry, black Prunus serotina 58 — 65 Fir,, California red A. magnifica — — 108 

Cottonwood, black Populus trichocarpa 162 146 — Fir, grand A. grandis 91 136 — 

Cottonwood, eastern P. deltoides 160 145 — Fir, noble A. procera 34 115 — 

Elm, American Ulmus americana 95 92 — Fir, Pacific silver A. amabilis 55 164 — 

Elm, rock U. thomasii 44 57 — Fir, subalpine A. lasiocarpa — — 47 

Hackberry Celtis occidentalis 61 65 — Fir, white A. concolor 98 160 — 

Hickory, Carya spp 71 51 — Hemlock, eastern Tsuga canadensis 97 119 — 

Magnolia, southern Magnolia grandiflora. 80 104 — Hemlock, western T. heterophylla 85 170 — 

Maple, bigleaf Acer macrophyllum 77 138 — Larch, western Larix occidentalis. 54 119 — 

Maple red A. rubrum — — 70 Pine, eastern white Pinus strobus 50 175 90 

Maple, silver A. saccharinum 58 97 — Pine, jack P. banksiana — — 70 

Maple, sugar A. saccharum 65 72 — Pine, lodgepole P. contorta 41 120 — 

Oak, northern red Quercus rubra 80 69 — Pine, ponderosa P. ponderosa 40 148 — 

Oak, northern white Q. alba 64 78 — Pine, red P. resinosa 32 134 — 

Oak, southern red Q. falcata 83 75 — Pine, Southern, 

loblolly 

P. taeda 33 110 — 

Oak, southern white 

(chestnut)

Q. prinus 72 — — Pine, Southern, 

longleaf 

P. palustris 31 106 — 

Pecan Carya illinoensis 71 62 — Pine, Southern, 

shortleaf 

P. echinata 32 122 — 

Sweetgum Liquidambar styraciflua 79 137 — Pine, Southern, slash P. elliottii 30 100 — 

Sycamore, American Platanus occidentalis 114 130 — Pine, sugar P. lambertiania 98 219 — 

Tanoak Lithocarpus densiflorus — — 89 Pine, western white P. monticola 62 148 — 

Tupelo, black Nyssa sylvatica 87 115 — Redwood, old growth Sequoia sempervirens. 86 210 — 

Tupelo, water N. aquatica 150 116 — Spruce, black P. moriana 52 113 77 

Walnut, black Juglans nigra 90 73 — Spruce, Engelmann Picea engelmannii 51 173 — 

Willow, black Salix nigra — — 139 Spruce, red P. rubens — — 55 

Yellow-poplar Liriodendron tulipifera. 83 106 — Spruce, Sitka P. sitchensis 41 142 43 

 Spruce, white P. glauca — — 55 

a

Based on weight when ovendried. 

b

Coast Douglas-fir is defined as Douglas-fir growing in the states of Oregon and Washington west of the summit of the Cascade Mountains. Interior West includes 

 the California and all counties in Oregon and Washington east of but adjacent to the Cascade summit. Interior North includes the remainder of Oregon and 

 Washington and the States of Idaho, Montana, and Wyoming. Interior South is made up of Utah, Colorado, Arizona, and New Mexico.
3

Chapter 1

Why Dry Lumber? 

Most water in the cut tree must be removed before useful 

products can be made from the wood. The rough, green 

lumber sawn from the log must be dried before it is processed into most end products. Drying the lumber at this stage 

has a number of distinct and important advantages: 

• Drying reduces weight, thereby reducing shipping and 

handling costs. 

• The shrinkage that accompanies drying takes place before 

the wood is used as a product. 

• As wood dries, most strength properties increase. 

• The strength of joints made with nails and screws is greater 

in dry wood than in green wood. 

• Wood must be relatively dry before it can be glued or 

treated with preservatives and fire-retardant chemicals. 

• Drying reduces the likelihood of mold, stain, or decay. 

• Drying increases thermal insulating properties and improves finishing characteristics. 

Drying Methods 

Several methods can be used to dry lumber, ranging from air 

and kiln drying to special seasoning processes. Basically, all 

methods involve moving moisture from the inside of the 

wood to the surface, where it is evaporated into the air. Heat 

and air movement speed up the drying process. Although this 

publication deals with air drying only, a brief description of 

other major methods is included to clarify how they differ 

from air drying. 

Air Drying 

To air dry, the lumber is arranged in layers, or courses, with 

separating stickers, and built up into unit packages and piles 

outdoors so that atmospheric air can circulate through the 

piles and carry away moisture (Fig. 1). One modification of 

air drying is shed drying, where the lumber to be dried is 

placed in a shed having open sides (Fig. 2). The roofed 

structure protects the lumber from rain and direct solar radiation but allows outdoor air to circulate through the stickered 

lumber to dry it. 

Fan Shed Air Drying 

To accelerate air drying, stickered unit packages of lumber 

are placed in an unheated shed or building that has fans on 

one side and is open on the other. The fans create air movement through the spaces between the courses of wood

(Fig. 3). 

Forced Air Drying and Predrying 

In more complex drying processes, stickered packages of 

lumber are placed in closed buildings that have fans to recirculate heated air through the lumber piles. Both forced air 

dryers and predryers are commonly considered low temperature, forced-air circulation, ventilated dry kilns (Fig. 4). 

Kiln Drying 

To kiln dry, lumber is dried in a closed chamber by controlling the temperature, relative humidity, and air circulation 

until the wood reaches a predetermined moisture content 

(Fig. 5). 

Special Drying Processes 

To reduce drying time and degrade, several processes for 

drying lumber have been investigated. Solar energy has 

gained popularity with small drying operations. Vacuum 

drying, especially when combined with radio frequency or 

microwave delivery of energy to evaporate water, has also 

gained attention in recent years. The main advantage of 

vacuum drying is reduced drying time without increased 

drying defects. Other special drying processes in limited use 

are press drying between heated platens, solvent seasoning, 

and boiling in oil. 

Choice of Methods 

Factors that determine the lumber drying process used at a 

plant are generally related in one way or another to economics. A sawmill that produces a considerable volume of a 

rather slow-drying wood (such as oak) often selects air drying followed by kiln drying. However, softwoods are often 

kiln dried green from the saw. 
4

Figure 2—Shed for drying lumber protects the 

lumber from the weather. 

Objectives 

The main purpose for air drying lumber is to evaporate as 

much water as possible while minimizing capital expenditures for dry-kiln capacity and without incurring a cost for 

the energy required. In air drying, lumber is usually left on 

stickers in the yard until it reaches a moisture content between 20% and 25%. The lumber may then be ready for 

further processing, depending upon its end use. If it must be 

dried to lower moisture content levels, such as for use in 

furniture factories, the lumber will be kiln dried. 

When lumber use does not require a low moisture content, 

air drying is usually sufficient. Lumber used for outdoor 

furniture and other outdoor exposures, or for building structures such as barns, pole sheds, and garages that are not 

heated, can usually be air dried to a low enough moisture 

content. Rough sawn hardwoods are often air dried at the 

producing sawmills to reduce weight so that shipping costs 

are reduced. In addition, air drying reduces subsequent kilndrying time. 

The main benefit of the air-drying process, particularly for 

hardwoods, is that it offers a way to add value while the 

inventory of lumber is being held. To meet production of 

shipping schedules during periods of the year when the sawmill cannot be operated to capacity, the yard inventory is 

built up when sawing conditions are favorable and the lumber is air dried while being held. Air drying is sometimes 

used to reduce the moisture content in wood, such as railroad 

ties, to a level suitable for preservative treatment. 

Air drying further reduces the chance that mold and decay 

may develop in transit, storage, or subsequent use. Blue stain 

and wood-destroying fungi cannot grow in wood with a 

moisture content of 20% or less. However, green lumber may 

have to be treated with a fungicide to protect it from these 

fungi in the early stages of the air-drying process. Drying is 

also a protective measure against damage from most insects 

that bore holes in wood. 

Figure 1—An air-drying yard arranged for good circulation of air around the piles of packaged lumber. 
5

Figure 3—Fan shed air dryer draws air through the stickered packages of lumber. 

Figure 4—Predryers are large structures that provide some means of controlling the drying 

conditions. The forklift in the background provides size comparison. 
6

Advantages and Limitations 

The greatest advantage of air drying lumber when compared 

with drying by other processes is low capital costs. However, 

as the value of the wood increases, kiln drying green wood 

becomes more feasible. Species such as beech, birch, and 

maple are often kiln dried green from the saw. The limitations of air drying are associated with the uncontrollable 

nature of the process. The drying rate is very slow during the 

cold winter months in the northern sector of the country. At 

other times, hot, dry winds may increase degrade and volume 

losses as a result of severe surface checking and end splitting. 

Production schedules depend on changing climatic conditions of temperature, relative humidity, rainfall, sunshine, 

and winds. Warm, humid, or sultry periods with little air 

movement encourage the growth of blue stain and aggravate 

chemical brown and gray stain. 

Figure 5—Typical package-loaded dry kiln. 
7

Chapter 2

Wood Properties and Moisture 

The structure of wood, the location and amount of moisture 

contained in green wood, and the physical properties of wood 

greatly influence its drying characteristics and reactions to 

air-drying conditions. 

Structure

Bark, Wood, and Pith 

A cross section of a tree shows well-defined features from 

the outside to the center (Fig. 6). The bark is divided into two 

layers—the corky, outer dead portion and the inner living 

portion. The light-colored zone of wood next to the bark is 

called sapwood and the darker inner zone of wood is called 

heartwood. In the center of the tree is a very small, soft core 

known as pith. 

Hardwoods and Softwoods 

Typically, hardwoods are trees with broad leaves, and softwoods are trees with needle-like or scale-like leaves. These 

terms do not apply to the hardness or density of the woods. 

Some softwoods, such as southern yellow pine, are harder 

than some hardwoods, such as basswood or cottonwood. 

The structure of hardwoods is generally more complex than 

that of softwoods. Figure 7 shows the vessels and other cells 

in a hardwood cube highly magnified. Figure 8 shows a 

similar cube of softwood. Many hardwoods contain relatively 

large wood rays, and some softwoods contain resin ducts. 

Both rays and resin ducts are related to increased susceptibility to surface checking during drying. 

Cellular Structure

Wood is composed of hollow, tube-like cells called fibers, 

usually closed at both ends. Thin spots or pit membranes are 

located in the walls of cells, through which the sap flows in 

the living tree or moisture moves during the drying of lumber. Most cells lie nearly parallel to the long axis of the tree 

trunk, but some lie perpendicular to the long axis of the tree 

on radial lines from the pith to the bark. These cells are 

called wood rays. Each year, several layers of new cells are 

produced on the outside of the sapwood by the thin living 

layer called the cambium. 

Earlywood and Latewood 

A cross section of a tree grown in a temperate climate shows 

well-defined concentric layers of wood, which correspond 

closely to yearly increments of growth. For that reason, they 

are commonly called annual growth rings. Earlywood, sometimes called springwood, is formed during the early part of 

each growing season. The cells of the earlywood are generally larger in cross section and thinner walled than those 

formed later in the season. Consequently, earlywood is 

softer, weaker, and generally lighter in color than latewood 

or summerwood. 

Figure 6—Cross section of an oak tree: A, Outer bark 

or corky layer is composed of dry dead tissue, gives 

general protection against external injuries. B, Inner

bark is moist and soft, carries prepared food from leaves 

to all growing parts of the tree. C, Cambium layer is 

inside of the inner bark and forms wood and bark cells. 

D, Sapwood is the light-colored wood beneath the bark; 

it carries sap from the roots to leaves. D, Heartwood 

(inactive. is formed by a gradual change in the sapwood; 

it gives the tree strength. F, Pith is the soft tissue about 

which the first wood growth takes place in the newly 

formed twigs. G, Wood rays are strips of cells that 

extend radially within the tree and serve primarily to 

store and transport food. 
8

Sapwood and Heartwood 

In the living tree, the sapwood layer, which is near the bark, 

contains many living cells that serve mainly in the transfer 

and storage of food. Some of the cells that store food are 

associated with stain. These cells remain alive after the tree is 

cut, and they release enzymes that convert the sugars into a 

brown chemical. However, most sapwood cells are dead and 

serve only as channels for sap to move upward in the tree and 

to help support the tree. 

The central part of the trunk is called heartwood. All heartwood cells are dead, and their principal function is to supply 

strength to the trunk and store extraneous materials. As a tree 

increases in diameter by adding new layers of sapwood under 

the bark, the zone of heartwood also enlarges at substantially 

the same rate. The living cells of the sapwood die and become infiltrated with gums, resins, coloring matter, and other 

materials. The circumference of the heartwood may be irregular and does not necessarily follow the annual growth 

rings closely. 

The relative amounts of sapwood and heartwood vary considerably, both between species and in trees of the same 

species. A tree of small diameter has more sapwood proportionately than a similar tree of larger diameter. Within 

a species, sapwood is thickest in the most vigorously 

growing trees. 

Heartwood, as a rule, is less permeable to liquids than is 

sapwood. For this reason, heartwood dries more slowly than 

does sapwood. In resinous woods, the heartwood usually 

contains more resin than does the sapwood. 

Juvenile Wood 

The wood adjacent to the pith is called juvenile wood. This 

wood shrinks more parallel to the grain than does the surrounding wood and may contribute to warping longitudinally 

(Fig. 9). 

Reaction Wood 

The growing tree develops wood with distinctive properties 

in parts of leaning or crooked trunks and in branches. This 

wood is called compression wood in softwoods and tension 

wood in hardwoods. 

Figure 7—A section of hardwood highly magnified: 

rr, radial surface; tg, tangential surface; ar, annual 

growth ring; ew, earlywood; lw, latewood; wr, wood 

ray; wf, wood fiber; v, vessels or pores. 

Figure 8—A section of a softwood highly magnified: 

rr, radial surface; tg, tangential surface; ar, annual 

growth ring; ew, earlywood; lw, latewood; wr, wood ray; 

tr, trachied or fiber; vrd, vertical resin ducts. The large 

hole near the center of the top section and the passage 

along the right edge (vrd) are vertical resin ducts. 
9

Compression wood (Fig. 10) occurs on the underside of the 

trunks of leaning softwood trees and on the underside of 

limbs. Annual growth rings in compression wood are usually 

wider than normal rings; latewood rings are unusually wide 

but do not appear as dense as normal latewood. A lack of 

color contrast between earlywood and latewood gives a 

lifeless appearance to compression wood. It is usually yellowish or brownish in color and may also have a reddish 

tinge. Streaks of compression wood are frequently interspersed with normal wood. Compression wood shrinks more 

longitudinally than does normal wood and may cause warping (Fig. 11) or develop cross breaks (Fig. 12) during drying. 

Tension wood occurs in hardwoods, on the upper side of 

leaning tree trunks and on the upper side of limbs. It can 

cause a stressed condition in the log and later contributes to 

splitting in sawn products. Tension wood also shrinks abnormally, thus possibly contributing to warp during drying. 

When lumber is machined, zones of tension wood are 

indicated by torn grain (Fig. 13). 

Structural Irregularities 

The length of the wood cells is usually parallel to the length 

of the tree trunk. Sometimes the length of the wood cells 

forms an angle with the length of the tree trunk. If this orientation of the fibers continues around the circumference and 

upward in the trunk, spiral grain results. Spiral grain shows 

up on the surface of boards as grain that is at an angle to the 

edge of the board. Another cause for disoriented grain is a 

log that is sawn parallel to the pith rather than to the bark, 

and these boards will have a diagonal grain. Both spiral and 

diagonal grain are termed cross grain or slope of grain in 

boards or other sawn products. The principal drying defect 

resulting from cross grain is warp. Cross grain also causes 

mechanical weakness. 

Figure 9—When the board was ripped in two, each piece crooked because of longitudinal shrinkage of juvenile 

wood in the center of the board. 

Figure 10—Eccentric growth around the pith in a cross 

section containing compression wood. The dark area 

in the lower third of the cross section is compression 

wood. 

Figure 11—Crook resulted when the band of compression wood (darker strip running length of piece) 

shrank more longitudinally than did the lighter colored 

normal wood. 

Figure 12—Board with dark band of compression wood 

with cross breaks caused by longitudinal shrinkage. 
10

Knots

A knot is revealed when lumber is cut from the portion of a 

tree containing an embedded branch. Normally, a knot starts 

at the pith and grows outward through the bark. Knots are 

generally objectionable because the distortion and the 

discontinuity of the grain around knots weakens the wood. 

Furthermore, knots cause irregular shrinkage and warping. 

When lumber dries, knots and the wood adjacent to them 

tend to check. Knots can also become loose because of their 

change in size. In addition, loose knots are formed when a 

tree grows around a dead branch. When the lumber is sawn, 

the dead knot may fall out because it is not connected to the 

rest of the tree. 

Moisture Content 

The liquid in freshly cut lumber is often called sap. Sap is 

composed primarily of water, with varying amounts of other 

dissolved materials. Moisture in wood exists in two forms: as 

free water in the cell cavities and as bound moisture held 

within the cell walls. Sapwood usually contains more 

moisture than does heartwood, particularly in softwoods. 

Table 1 lists moisture content values of green wood for 

several species. 

The moisture content of wood can vary at different heights in 

the tree; species and growing conditions are involved. Butt 

logs of sugar pine, western larch, redwood, and western red 

cedar sometimes sink in water because their density at these 

high moisture content levels exceed that of water, although 

the upper logs from the same trees float. In addition to having a higher moisture content, these “sinker” logs can have a 

higher specific gravity or more wood substance per unit 

volume. 

Methods to Determine

Moisture Content 

The performance of wood is influenced by the amount of 

moisture it contains; for many uses, wood serves best at 

specific levels of moisture content. Therefore, knowing the 

moisture content is an essential part of determining the readiness of wood for use. The amount of moisture in wood is 

expressed as a percentage of the weight of the dry wood 

substance. Moisture content of wood can be determined by 

the oven-drying method, a distillation method, or the use of 

electric moisture meters (Figs.14,15). 

Oven-Drying Method 

The oven-drying method is the standard way of determining 

the moisture content of wood. A cross section is cut from a 

board, then completely dried in a heated oven. The method 

consists of the five following steps: 

1. Cut a cross section from a board about 25 mm (1 in.) thick 

along the grain. 

2. Immediately after sawing, remove all loose splinters and 

weigh the section. 

3. Put the section in an oven maintained at 105o

C (220oF) and 

dry until constant weight is attained, usually about 24 h. 

4. Weigh the dried section to obtain the ovendry weight. 

5. Subtract the ovendry weight from the initial weight and 

divide the difference by the ovendry weight, multiplying 

the result by 100 to obtain the percentage of moisture in 

the section: 

 moisture content (%) = 100 × (initial weight − ovendry 

 weight)/ ovendry weight 

A short-cut formula convenient to use is 

 moisture content (%) = 100 × (initial weight/ovendry 

 weight −1)

Several types of balances are used in weighing specimens to 

determine moisture content. One inexpensive type is the 

triple beam balance (Fig 16). A direct reading automatic 

balance is very convenient (Fig. 16) when several specimens 

are weighed in and out of the drying oven. Self-calculating 

Figure 13—Area of tension wood is indicated by grain 

torn during machining. 
11

balances are also available (Fig. 17). The moisture content 

calculation is carried out on the balance by following the 

prescribed sequence of operations supplied by the manufacturer. The ovens used for drying the moisture sections should 

be large enough to accommodate several specimens with 

space between them. Ovens should also be well ventilated to 

allow the evaporated moisture to escape. The temperature of 

the oven must be controlled with a reliable thermostat. Excessive temperatures will char the specimens, introducing 

errors in the moisture analysis. 

Electrical Methods 

Electric moisture meters permit the determination of moisture content without cutting or seriously marring the board. 

Such meters are rapid and reasonably accurate through the 

range from 7% to 25% moisture content. Several tests can be 

made with the electric moisture meter on a board or a lot of 

lumber to arrive at a better overall average, canceling out the 

inaccuracies that might exist in individual measurements. 

With some types of electrodes, a minor consideration is that 

small needlepoint holes are made in the lumber being tested. 

Electric moisture meters are extensively used, particularly the 

portable or hand meters. Two types, each based on a different fundamental relationship, have been developed: (1) the 

resistance, or pin type, which uses the relationship between 

moisture content and electrical resistance (Fig. 14); and 

(2) the capacitance type, which uses the relationship between 

moisture content and the dielectric properties of wood and its 

included water (Fig. 15). 

Portable, battery-operated, resistance-type moisture meters 

are wide range ohmmeters. Most models have a direct reading meter, calibrated in percentage for one species; the 

manufacturer provides corrections for other species, either 

Figure 14—Conductance-type electrical moisture meters. 
12

built into the meter or in the form of tables. The manufacturer also provides a temperature correction chart or table for 

correcting the meter reading when tests are made on wood 

warmer than 32o

C (90°F) or cooler than 21oC (70°F). 

Resistance-type meters are generally supplied with two pintype electrodes that are driven into the wood being tested. 

Lengthy, two-pin electrodes that are insulated except for the 

tip are also available for use on lumber thicker than 41 mm 

(1-5/8 in.). Valid estimates of the average moisture content 

of the drying board are obtained by driving the pins deep 

enough in the lumber so that the tip reaches a fifth to a fourth 

of the thickness of the board. If the surface of the lumber has 

been wetted by rain, the meter indications are likely to be 

much greater than the actual average moisture content unless 

insulated shank pins are used. 

Capacitance-type hand meters use surface contact-type electrodes. The electric field radiating from the electrode penetrates about 19 mm (3/4-in.) into the wood so that lumber 

thicknesses to about 38 mm (1-1/2 in.) can be tested. However, the moisture content of the surface layers of the lumber 

has a predominant effect on the meter readings, simply because the electric field is stronger near the surface in contact 

with the electrode. 

Temperature correction charts or tables are not provided by 

the manufacturers of capacitance-type meters. However, they 

do provide species correction tables for converting the meter 

scale reading of the instrument to moisture content for 

individual species. 

Moisture Movement 

During air drying, water at or near the lumber surface evaporates first. Then, water deeper in the wood moves from zones 

of high moisture content to zones of lower moisture content 

in an effort to reach a moisture equilibrium throughout the 

board. This means that moisture from the interior of a wet 

board moves to the drier surface zones. Outdoor air must not 

be restricted from circulating over the green lumber to dry 

the surfaces and draw moisture from the interior of the board. 

Moisture moves through several kinds of passageways in the 

wood. The principal ones are the cavities in the cells, the pit 

chambers, and pit membrane openings in the cell walls. 

Movement of moisture in these passageways occurs not only 

lengthwise in the cells but also sideways from cell to cell 

through pit membranes toward the drier surfaces of the wood.

When wood dries, several moisture-driving forces may be 

operating to reduce its moisture content. These forces, which 

may be acting at the same time, include the following: 

• Capillary action that causes the free water to move through 

the cell cavities, pit chambers, and pit membrane openings 

(the same action that causes a liquid to wet a wick) 

• Differences in relative humidity that cause moisture in the 

vapor state to flow through cell cavities, pit chambers, pit 

membrane openings, and intercellular spaces to regions of 

lower humidity 

• Moisture content differences that cause movement of 

moisture to regions of lower moisture content through the 

passageways within the cell walls 

Figure 15—Capacitance-type electrical moisture meter. 
13

When green wood starts to dry, evaporation of water from 

openings in the surface cells creates a capillary pull on the 

free water in the cell cavity and in adjacent cells. Free water 

moves from one tubular cell to another toward the wood 

surface by capillary action. When the free water has evaporated in the cells, the moisture remaining is in the form of 

vapor in the cell cavities or bound water in the cell walls. 

The movement of water vapor through void spaces in wood

depends on how much water vapor is contained in the air in 

the voids or in the air surrounding the wood. If the air surrounding the wood has a low relative humidity, water vapor 

will move from the wet wood to the air. Thus, rapid drying 

depends on the surface moisture content of the wood being 

dried and whether a difference in moisture content can be 

developed between the surface and the interior of a board. 

The higher the temperature to which the drying wood is 

subjected, the faster the combined vapor and moisture 

movement. This is why the air-drying rate is faster in summer 

than in winter. 

Figure 16—Three types of balances that can be used to weigh moisture sections and kiln samples: 

(a) triple beam balance for moisture sections, (b) electronic top loading balance for moisture sections,

(c) electronic top loading balance for kiln samples, (d) non-electronic balance for kiln samples. 

Figure 17—Self-calculating moisture content balance.

The triple beam balance is provided with a special scale 

on the specimen pan that is used to calculate the 

moisture content after the section is ovendried. 
14

Equilibrium Moisture Content

Wood is a hygroscopic material. It gives off or takes on 

moisture until it is in equilibrium with the relative humidity 

of the surrounding air. When the moisture content of the 

wood has reached equilibrium, it is said to have reached its 

equilibrium moisture content (EMC). The EMC of wood can 

be predicted by knowing the temperature and relative humidity conditions of the air in contact with the surface of the 

drying lumber; this relationship is shown in Table 2. Table 3 

shows the average monthly EMC values for various cities in 

the United States. 

The air temperature in a lumber drying yard is determined 

with a dry-bulb thermometer. To determine the relative 

humidity of the air, a wet-bulb thermometer can also be used. 

If the two thermometers are mounted on a single base, the 

instrument is called a hygrometer (Fig. 18). The wet-bulb 

thermometer has the bulb covered with a clean, soft cloth 

wick that dips into a reservoir of pure, clean water. Water in 

the reservoir wets the wick by capillary action. As a result of 

the cooling caused by evaporation from the wet surface of the 

wick, the wet-bulb thermometer will give a lower reading 

than the dry-bulb thermometer. This difference in readings, 

called the wet-bulb depression, is a measure of the relative 

humidity of the air. To obtain a true reading with the wetbulb thermometer, the hygrometer must be placed in a strong 

current of air. Table 4 gives relative humidity and EMC 

values for various dry-bulb temperatures and wet-bulb depressions. In addition to wet- and dry-bulb thermometers, 

direct reading hygrometers are also available, although they 

may not be as accurate as the measurement using wet- and 

dry-bulb thermometers. 

Fiber Saturation Point

As previously stated, the moisture in the cell cavity is called 

free water, and that held in the cell walls is called bound 

water. Conceptually, the moisture content when all the freewater in the cell cavity is removed but the cell wall remains 

saturated is called the fiber saturation point (fsp). For most 

practical purposes, this condition exists at a cell moisture 

content of about 30%. Note that the definition of fsp applies 

to cells or localized regions but not to the moisture content of 

an entire board. During drying, the center of a board may be 

at a relatively high moisture content (for example, 40%), and 

the surface fibers may be much dryer (for example, 12%). 

Overall, this board may be at an average moisture content of 

30%, but it is not correct to say that the entire board is at fsp. 

Comparatively large changes in the physical and mechanical 

properties of wood occur with changes of moisture content 

around the fsp. The moisture removed below this point 

comes from the cell wall; therefore, the fsp is the moisture 

content level at which shrinkage begins. More energy is 

required to evaporate the bound water, because the attraction 

between the wood and water must be overcome. 

Shrinkage

When the cells in the surface layers of a board dry below the 

fsp, about 30%, the cell walls shrink. Shrinkage of cells in 

the surface region of the board can be sufficient to squeeze 

the core of the board and cause a slight overall shrinkage of 

the board. For most practical purposes, the shrinkage of 

wood is considered as being directly proportional to the 

amount of moisture lost below 30%. Shrinkage varies with 

the species and the orientation of the fibers in the piece. 

Normally, shrinkage is expressed as a percentage of the 

green dimension. The reduction in size parallel to the growth 

ring, or circumferentially, is called tangential shrinkage. The 

reduction in size parallel to the wood rays, or radially, is 

called radial shrinkage. Tangential shrinkage is about twice 

as great as radial shrinkage in most species. This explains the 

characteristic shrinkage and distortion of several wood 

shapes shown in Figure 19. 

Table 2—Moisture content of wood in equilibrium with stated dry-bulb temperature and relative humidity 

Dry-bulb Moisture content (%) for the following relative humidity values 

temperature 

(°C (°F)) 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95

–1.1 (30) 1.4 2.6 3.7 4.6 5.5 6.3 7.1 7.9 8.7 9.5 10.4 11.3 12.4 13.5 14.9 16.5 18.5 21.0 24.3 

4.4 (40) 1.4 2.6 3.7 4.6 5.5 6.3 7.1 7.9 8.7 9.5 10.4 11.3 12.3 13.5 14.9 16.5 18.5 21.0 24.3 

10.0 (50) 1.4 2.6 3.6 4.6 5.5 6.3 7.1 7.9 8.7 9.5 10.3 11.2 12.3 13.4 14.8 16.4 18.4 20.9 24.3 

15.6 (60) 1.3 2.5 3.6 4.6 5.4 6.2 7.0 7.8 8.6 9.4 10.2 11.1 12.1 13.3 14.6 16.2 18.2 20.7 24.1 

21.1 (70) 1.3 2.5 3.5 4.5 5.4 6.2 6.9 7.7 8.5 9.2 10.1 11.0 12.0 13.1 14.4 16.0 17.9 20.5 23.9 

26.7 (80) 1.3 2.4 3.5 4.4 5.3 6.1 6.8 7.6 8.3 9.1 9.9 10.8 11.7 12.9 14.2 15.7 17.7 20.2 23.6 

32.2 (90) 1.2 2.3 3.4 4.3 5.1 5.9 6.7 7.4 8.1 8.9 9.7 10.5 11.5 12.6 13.9 15.4 17.3 19.8 23.3 

37.8 (100) 1.2 2.3 3.3 4.2 5.0 5.8 6.5 7.2 7.9 8.7 9.5 10.3 11.2 12.3 13.6 15.1 17.0 19.5 22.9 

43.3 (110) 1.1 2.2 3.2 4.0 4.9 5.6 6.3 7.0 7.7 8.4 9.2 10.0 11.0 12.0 13.2 14.7 16.6 19.1 22.4 

48.9 (120) 1.1 2.1 3.0 3.9 4.7 5.4 6.1 6.8 7.5 8.2 8.9 9.7 10.6 11.7 12.9 14.4 16.2 18.6 22.0 
15

Table 3—Average monthly EMC values for various U.S. citiesa

Equilibrium moisture content (%) 

State City Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. 

AK Juneau 16.5 16.0 15.1 13.9 13.6 13.9 15.1 16.5 18.1 18.0 17.7 18.1 

AL Mobile 13.8 13.1 13.1 13.3 13.4 13.3 14.2 14.4 13.9 13.0 13.7 14.0 

AZ Flagstaff 11.8 11.4 10.8 9.3 8.8 7.5 9.7 11.1 10.3 10.1 10.8 11.8 

AZ Phoenix 9.4 8.4 7.9 6.1 5.1 4.6 6.2 6.9 6.9 7.0 8.2 9.5 

AR Little Rock 13.8 13.2 12.8 13.1 13.7 13.1 13.3 13.5 13.9 13.1 13.5 13.9 

CA Fresno 16.4 14.1 12.6 10.6 9.1 8.2 7.8 8.4 9.2 10.3 13.4 16.6 

CA Los Angeles 12.2 13.0 13.8 13.8 14.4 14.8 15.0 15.1 14.5 13.8 12.4 12.1 

CO Denver 10.7 10.5 10.2 9.6 10.2 9.6 9.4 9.6 9.5 9.5 11.0 11.0 

DC Washington 11.8 11.5 11.3 11.1 11.6 11.7 11.7 12.3 12.6 12.5 12.2 12.2 

FL Miami 13.5 13.1 12.8 12.3 12.7 14.0 13.7 14.1 14.5 13.5 13.9 13.4 

GA Atlanta 13.3 12.3 12.0 11.8 12.5 13.0 13.8 14.2 13.9 13.0 12.9 13.2 

HI Honolulu 13.3 12.8 11.9 11.3 10.8 10.6 10.6 10.7 10.8 11.3 12.1 12.9 

ID Boise 15.2 13.5 11.1 10.0 9.7 9.0 7.3 7.3 8.4 10.0 13.3 15.2 

IL Chicago 14.2 13.7 13.4 12.5 12.2 12.4 12.8 13.3 13.3 12.9 14.0 14.9 

IN Indianapolis 15.1 14.6 13.8 12.8 13.0 12.8 13.9 14.5 14.2 13.7 14.8 15.7 

IA Des Moines 14.0 13.9 13.3 12.6 12.4 12.6 13.1 13.4 13.7 12.7 13.9 14.9 

KS Wichita 13.8 13.4 12.4 12.4 13.2 12.5 11.5 11.8 12.6 12.4 13.2 13.9 

KY Louisville 13.7 13.3 12.6 12.0 12.8 13.0 13.3 13.7 14.1 13.3 13.5 13.9 

LA New Orleans 14.9 14.3 14.0 14.2 14.1 14.6 15.2 15.3 14.8 14.0 14.2 15.0 

ME Portland 13.1 12.7 12.7 12.1 12.6 13.0 13.0 13.4 13.9 13.8 14.0 13.5 

MA Boston 11.8 11.6 11.9 11.7 12.2 12.1 11.9 12.5 13.1 12.8 12.6 12.2 

MI Detroit 14.7 14.1 13.5 12.6 12.3 12.3 12.6 13.3 13.7 13.5 14.4 15.1 

MN Minneapolis–

St.Paul 

13.7 13.6 13.3 12.0 11.9 12.3 12.5 13.2 13.8 13.3 14.3 14.6 

MS Jackson 15.1 14.4 13.7 13.8 14.1 13.9 14.6 14.6 14.1 14.1 14.3 14.9 

MO St. Louis 14.5 14.1 13.2 12.4 12.8 12.6 12.9 13.3 13.7 13.1 14.0 14.9 

MT Missoula 16.7 15.1 12.8 11.4 11.6 11.7 10.1 9.8 11.3 12.9 16.2 17.6 

NE Omaha 14.0 13.8 13.0 12.1 12.6 12.9 13.3 13.8 14.0 13.0 13.9 14.8 

NV Las Vegas 8.5 7.7 7.0 5.5 5.0 4.0 4.5 5.2 5.3 5.9 7.2 8.4 

NV Reno 12.3 10.7 9.7 8.8 8.8 8.2 7.7 7.9 8.4 9.4 10.9 12.3 

NM Albuquerque 10.4 9.3 8.0 6.9 6.8 6.4 8.0 8.9 8.7 8.6 9.6 10.7 

NY New York 12.2 11.9 11.5 11.0 11.5 11.8 11.8 12.4 12.6 12.3 12.5 12.3 

NC Raleigh 12.8 12.1 12.2 11.7 13.1 13.4 13.8 14.5 14.5 13.7 12.9 12.8 

ND Fargo 14.2 14.6 15.2 12.9 11.9 12.9 13.2 13.2 13.7 13.5 15.2 15.2 

OH Cleveland 14.6 14.2 13.7 12.6 12.7 12.7 12.8 13.7 13.8 13.3 13.8 14.6 

OK Oklahoma City 13.2 12.9 12.2 12.1 13.4 13.1 11.7 11.8 12.9 12.3 12.8 13.2 

OR Pendleton 15.8 14.0 11.6 10.6 9.9 9.1 7.4 7.7 8.8 11.0 14.6 16.5 

OR Portland 16.5 15.3 14.2 13.5 13.1 12.4 11.7 11.9 12.6 15.0 16.8 17.4 

PA Philadelphia 12.6 11.9 11.7 11.2 11.8 11.9 12.1 12.4 13.0 13.0 12.7 12.7 

SC Charleston 13.3 12.6 12.5 12.4 12.8 13.5 14.1 14.6 14.5 13.7 13.2 13.2 

SD Sioux Falls 14.2 14.6 14.2 12.9 12.6 12.8 12.6 13.3 13.6 13.0 14.6 15.3 

TN Memphis 13.8 13.1 12.4 12.2 12.7 12.8 13.0 13.1 13.2 12.5 12.9 13.6 

TX Dallas–Ft.Worth 13.6 13.1 12.9 13.2 13.9 13.0 11.6 11.7 12.9 12.8 13.1 13.5 

TX El Paso 9.6 8.2 7.0 5.8 6.1 6.3 8.3 9.1 9.3 8.8 9.0 9.8 

UT Salt Lake City 14.6 13.2 11.1 10.0 9.4 8.2 7.1 7.4 8.5 10.3 12.8 14.9 

VA Richmond 13.2 12.5 12.0 11.3 12.1 12.4 13.0 13.7 13.8 13.5 12.8 13.0 

WA Seattle–Tacoma 15.6 14.6 15.4 13.7 13.0 12.7 12.2 12.5 13.5 15.3 16.3 16.5 

WI Madison 14.5 14.3 14.1 12.8 12.5 12.8 13.4 14.4 14.9 14.1 15.2 15.7 

WV Charleston 13.7 13.0 12.1 11.4 12.5 13.3 14.1 14.3 14.0 13.6 13.0 13.5 

WY Cheyenne 10.2 10.4 10.7 10.4 10.8 10.5 9.9 9.9 9.7 9.7 10.6 10.6 

a

EMC values were determined from the average of 30 or more years of relative humidity and temperature data available 

 from the National Climatic Data Center of the National Oceanic and Atmospheric Administration. 
16

A flatsawn (or plainsawn) board (Fig. 20) shrinks tangentially in width and radially in thickness. A quartersawn (or 

vertical grain) board (Fig. 20) shrinks radially in width, 

tangentially in thickness. Table 5 gives tangential and radial 

shrinkage values for the wood of many species. The longitudinal shrinkage of wood is generally slight, 0.1% to 0.2% of 

the green dimension. If reaction wood and juvenile wood are 

in the board, the longitudinal shrinkage may be appreciably 

increased. 

The values in Table 5 can be converted into dimensional 

changes by using the following formula: 

I

T R

I F

30

or

30

( )

M

S S

M M D S

+













 −

− =

where S is shrinkage or swelling (mm, in., or any linear unit); 

MI is initial moisture content (%); MF is final moisture content (%); D is dimension at initial moisture content (linear 

units); 30 is fiber saturation point (%); ST and SR are tangential and radial shrinkage divided by 100. Neither the initial 

nor the final moisture content can be greater than 30%. Examples using this formula to determine dimensional changes 

are given in the following: 

Example 1: Determine the shrinkage in width of a 152-mm 

(6-in.), flatsawn, sugar maple board in drying from the green 

condition to a moisture content of 15%. The average green 

moisture content of sugar maple, sapwood and heartwood 

combined, is about 69% (Table 1). This board is flat grained; 

therefore, use a total tangential shrinkage value of 9.9% 

(Table 5). Substituting in the formula: 

7.5 mm

303

2280

30 30

0.099

30

(30 15)152 = =

 +



 



 −

− S =

0.30 in. 303

90

30 30

0.099

30

(30 15)6 = =

 +



 



 −

− S =

Example 2: Determine the shrinkage in width of a vertical 

grained, old-growth redwood board, 8.5 in. (216 mm) wide, 

in drying from a moisture content of 24% to 5%. Table 5 

gives the radial shrinkage of redwood as 2.6%. 

3.6 mm

1147.8

4104

30 24

0.026

30

(24 5)216 = =

 +



 



 −

− S =

0.14 in. 1147.8

161.5

30 24

0.026

30

(24 5)8.5 = =

 +



 



 −

− S =

Figure 18—(a) Stationary dry- and wet-bulb thermometers indicating 24o

C (75oF) dry-bulb and 17oC (63oF)

wet-bulb, for a relative humidity of 66% and EMC of 

12.0% (Table 3). (b) A sling psychrometer to enhance 

air velocity over the wet-bulb wick for greater accuracy. 
Table 4—Relative humiditya and equilibrium moisture contentb at various dry-bulb and wet-bulb temperatures 

Dry bulb Wet-bulb depression 

temperature 0.6 1.1 1.7 2.2 2.8 3.3 3.9 4.4 5.0 5.6 6.1 6.7 7.2 7.8 8.3 8.9 9.4 10.0 10.6 11.1 11.7 12.2 12.2 13.3 13.9 14.4 15.0 15.6 16.1 16.7 17.8 18.9 20.0 21.1 22.2 25.0 27.8 °C

(°F (°C)) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 32 34 36 38 49 45 50 °F

30 (–1.1) 89 78 67 57 46 36 27 17 6 — — — — — — — — — — — — — — — — — — — — — — — — — — — — 

 — 15.9 12.9 10.8 9.0 7.4 5.7 3.9 1.6 — — — — — — — — — — — — — — — — — — — — — — — — — — — — 

35 (1.7) 90 81 72 63 54 45 37 28 19 11 3 — — — — — — — — — — — — — — — — — — — — — — — — — — 

 — 16.8 13.9 11.9 10.3 8.8 7.4 6.0 4.5 2.9 0.8 — — — — — — — — — — — — — — — — — — — — — — — — — — 

40 (4.4) 92 83 75 68 60 52 45 37 29 22 15 8 — — — — — — — — — — — — — — — — — — — — — — — — — 

 — 17.6 14.8 12.9 11.2 9.9 8.6 7.4 6.2 5.0 3.5 1.9 — — — — — — — — — — — — — — — — — — — — — — — — — 

45 (7.2) 93 85 78 72 64 58 51 44 37 31 25 19 12 6 — — — — — — — — — — — — — — — — — — — — — — — 

 — 18.3 15.6 13.7 12.0 10.7 9.5 8.5 7.5 6.5 5.3 4.2 2.9 1.5 — — — — — — — — — — — — — — — — — — — — — — — 

50 (10.0) 93 86 80 74 68 62 56 50 44 38 32 27 21 16 10 5 — — — — — — — — — — — — — — — — — — — — — 

 — 19.0 16.3 14.4 12.7 11.5 10.3 9.4 8.5 7.6 6.7 5.7 4.8 3.9 2.8 1.5 — — — — — — — — — — — — — — — — — — — — 

—

55 (12.8) 94 88 82 76 70 65 60 54 49 44 39 34 28 24 19 14 9 5 — — — — — — — — — — — — — — — — — — — 

 — 19.5 16.9 15.1 13.4 12.2 11.0 10.1 9.3 8.4 7.6 6.8 6.0 5.3 4.5 3.6 2.5 1.3 — — — — — — — — — — — — — — — — — — — 

60 (21.2) 94 89 83 78 73 68 63 58 53 48 43 39 34 30 26 21 17 13 9 5 1 — — — — — — — — — — — — — — — — 

 — 19.9 17.4 15.6 13.9 12.7 11.6 10.7 9.9 9.1 8.3 7.6 6.9 6.3 5.6 4.9 4.1 3.2 2.3 1.3 0.2 — — — — — — — — — — — — — — — — 

65 (18.3) 95 90 84 80 75 70 66 61 56 52 48 44 39 36 32 27 24 20 16 13 8 6 2 — — — — — — — — — — — — — — 

 — 20.3 17.8 16.1 14.4 13.3 12.1 11.2 10.4 9.7 8.9 8.3 7.7 7.1 6.5 5.8 5.2 4.5 3.8 3.0 2.3 1.4 0.4 — — — — — — — — — — — — — — 

70 (21.1) 95 90 86 81 77 72 68 64 59 55 51 48 44 40 36 33 29 25 22 19 15 12 9 6 3 — — — — — — — — — — — — 

 — 20.6 18.2 16.5 14.9 13.7 12.5 11.6 10.9 10.1 9.4 8.8 8.3 7.7 7.2 6.6 6.0 5.5 4.9 4.3 3.7 2.9 2.3 1.5 0.7 — — — — — — — — — — — — 

75 (25.9) 95 91 86 82 78 74 70 66 62 58 54 51 47 44 41 37 34 31 28 24 21 18 15 12 10 7 4 1 — — — — — — — — — 

 — 20.9 18.5 16.8 15.2 14.0 12.9 12.0 11.2 10.5 9.8 9.3 8.7 8.2 7.7 7.2 6.7 6.2 5.6 5.1 4.7 4.1 3.5 2.9 2.3 1.7 0.9 0.2 — — — — — — — — — 

80 (26.7) 96 91 87 83 79 75 72 68 64 61 57 54 50 47 44 41 38 35 32 29 26 23 20 18 15 12 10 7 5 3 — — — — — — — 

 — 21.0 18.7 17.0 15.5 14.3 13.2 12.3 11.5 10.9 10.1 9.7 9.1 8.6 8.1 7.7 7.2 6.8 6.3 5.8 5.4 5.0 4.5 4.0 3.5 3.0 2.4 1.8 1.1 0.3 — — — — — — — 

85 (29.9) 96 92 88 84 80 76 73 70 66 63 59 56 53 50 47 44 41 38 36 33 30 28 25 23 20 18 15 13 11 9 4 — — — — — — 

 — 21.2 18.8 17.2 15.7 14.5 13.5 12.5 11.8 11.2 10.5 10.0 9.5 9.0 8.5 8.1 7.6 7.2 6.7 6.3 6.0 5.6 5.2 4.8 4.3 3.9 3.4 3.0 2.4 1.7 0.9 — — — — — — 

90 (32.2) 96 92 89 85 81 78 74 71 68 65 61 58 55 52 49 47 44 41 39 36 34 31 29 26 24 22 19 17 15 13 9 5 1 — — — — 

 — 21.3 18.9 17.3 15.9 14.7 13.7 12.8 12.0 11.4 10.7 10.2 9.7 9.3 8.8 8.4 8.0 7.6 7.2 6.8 6.5 6.1 5.7 5.3 4.9 4.6 4.2 3.8 3.3 2.8 2.1 1.3 0.4 — — — — 

95 (35.0) 96 92 89 85 82 79 75 72 69 66 63 60 57 55 52 49 46 44 42 39 37 34 32 30 28 26 23 22 20 17 14 10 6 2 — — — 

 — 21.3 19.0 17.4 16.1 14.9 13.9 12.9 12.2 11.6 11.0 10.5 10.0 9.5 9.1 8.7 8.2 7.9 7.5 7.1 6.8 6.4 6.1 5.7 5.3 5.1 4.8 4.4 4.0 3.6 3.0 2.3 1.5 0.6 — — — 

100 (37.8) 96 93 89 86 83 80 77 73 70 68 65 62 59 56 54 51 49 46 44 41 39 37 35 33 30 28 26 24 22 21 17 13 10 7 4 — — 

 — 21.3 19.0 17.5 16.1 15.0 13.9 13.1 12.4 11.8 11.2 10.6 10.1 9.6 9.2 8.9 8.5 8.1 7.8 7.4 7.0 6.7 6.4 6.1 5.7 5.4 5.2 4.9 4.6 4.2 3.6 3.1 2.4 1.6 0.7 — — 

105 (40.6) 96 93 90 87 83 80 77 74 71 69 66 63 60 58 55 53 50 48 46 44 42 40 37 35 34 31 29 28 26 24 20 17 14 11 8 — — 

 — 21.4 19.0 17.5 16.2 15.1 14.0 13.3 12.6 11.9 11.3 10.8 10.3 9.8 9.4 9.0 8.7 8.3 7.9 7.6 7.3 6.9 6.7 6.4 6.1 5.7 5.4 5.2 4.8 4.6 4.2 3.6 3.1 2.4 1.8 — — 

110 (43.3) 97 93 90 87 84 81 78 75 73 70 67 65 62 60 57 55 52 50 48 46 44 42 40 38 36 34 32 30 28 26 23 20 17 14 11 4 — 

 — 21.4 19.0 17.5 16.2 15.1 14.1 13.3 12.6 12.0 11.4 10.8 10.4 9.9 9.5 9.2 8.8 8.4 81 7.7 7.5 7.2 6.8 6.6 6.3 6.0 5.7 5.4 5.2 4.8 4.5 4.0 3.5 3.0 2.5 1.1 —

115 (46.1) 97 93 90 88 85 82 79 76 74 71 68 66 63 61 58 56 54 52 50 48 45 43 41 40 38 36 34 32 31 29 26 23 20 17 14 8 2 

 — 21.4 19.0 17.5 16.2 15.1 14.1 13.4 12.7 12.1 11.5 0.9 10.4 10.0 9.6 9.3 8.9 8.6 8.2 7.8 7.6 7.3 7.0 6.7 6.5 6.2 5.9 5.6 5.4 5.2 4.7 4.3 3.9 3.4 2.9 1.7 0.4 

120 (48.9) 97 94 91 88 85 82 80 77 74 72 69 67 65 62 60 58 55 53 51 49 47 45 43 41 40 38 36 34 33 31 28 25 22 19 17 10 5 

 — 21.3 19.0 17.4 16.2 15.1 14.1 13.4 12.7 12.1 11.5 11.0 10.5 10.0 9.7 9.4 9.0 8.7 8.3 7.9 7.7 7.4 7.2 6.8 6.6 6.3 6.1 5.8 5.6 5.4 5.0 4.6 4.2 3.7 3.3 2.3 1.1 

aRelative humidity values not italic (first row). bEquilibrium moisture content values italic (second row). 
18

Example 3: Determine the swelling in width of a flatsawn, 

sugar maple flooring strip machined to 57 mm (2.25 in.), in 

changing from 5% to 13% moisture content. 

1.6 mm

278

456

30 5

0.099

30

(5 13)57 = − − =

 +



 



 −

− S =

0.065 in.

278

18.0

30 5

0.099

30

(5 13)2.25 = − − =

 +



 



 −

− S =

The negative shrinkage obtained denotes swelling. Using the 

swelling of 1.644 mm per 57.15 mm (0.0647 in. per 2.25 in). 

of width, a 12.2-m- (40-ft-) wide floor will swell 

12.2 1000 351 mm

57.15

1.644

× × =

40 12 13.8 in.

2.25

0.0647

× × =

Weight

The weight of lumber decreases as its moisture content is 

reduced. One benefit of air drying is a reduction in shipping 

weight. All wood substance, containing no air spaces and 

regardless of the species from which it comes, has an ovendry specific gravity of approximately 1.54. This means that 

0.0283 m3 (1 ft3

) of ovendry wood substance weighs 

1.54 times the weight of 0.0283 m3

 (1 ft3) of water, or 

28.4 ×1.54 = 43.7 kg (62.5 × 1.54 = 96.3 lb). Usually, the 

specific gravity of wood is based on the volume of the wood 

when green and its weight when ovendry. Thus, if the specific gravity of a specimen of green wood is listed as being 

0.5, the ovendry weight of the wood substance in 0.0283 m3

(1 ft3

) of green wood is half the weight of a cubic foot of 

water, or 14 kg (31.3 lb). 

The differences in specific gravity between species are due to 

differences in the size of the cells and the thickness of the 

cell walls. The average specific gravity, based on ovendry 

weight and green volume of commercial woods, varies from 

0.31 for western red cedar and black cottonwood to 0.64 for 

the hickories, and can be as high as 1.1 for some tropical 

species. The moisture in the wood adds to the weight of a 

given volume. Most species of wood, even when green, float 

in water because of air entrapped in the cells. 

Table 6 gives the specific gravity and lumber weight of 

commercial species per 2.36 m3 (1,000 board feet) at a moisture content of 25%. 

Color

The sapwood of most species is light colored and ranges in 

color from white to yellowish white. The heartwood of most 

species is distinctly darker than the sapwood. Considerable 

variation exists in the heartwood color of various species. 

The colors are generally a combination of yellow, brown, and 

red. Sometimes a greenish or purplish tint is present. The 

color of wood exposed to air and light becomes duller and 

darker. In air-drying yards, wood also becomes discolored by 

ultraviolet degradation, wetting, and airborne dust. These 

naturally caused surface discolorations do not penetrate far 

into the wood, except into surface checks, and can be removed by planing the wood. 

Figure 19—Characteristic shrinkage and distortion of

flats, squares, and rounds as affected by the direction 

of annual growth rings. 

Figure 20—Shrinkage direction in (a) flatsawn 

(plainsawn) and (b) quartersawn (vertical grain) boards. 
19

Table 5—Shrinkage values of domestic woodsa

 Shrinkagea (%) from green 

to ovendry moisture content 

 Shrinkagea (%) from green 

to ovendry moisture content 

Species Radial Tangential Species Radial Tangential 

Hardwoods Softwoods 

Alder, red 4.4 7.3 Baldcypress 3.8 6.2 

Ash Cedar 

Black 5.0 7.8 Alaska 2.8 6.0 

Green 4.6 7.1 Atlantic white 2.9 5.4 

White 4.9 7.8 Eastern redcedar 3.1 4.7 

Aspen Incense 3.3 5.2 

Bigtooth 3.3 7.9 Northern white 2.2 4.9 

Quaking 3.5 6.7 Port-Orford 4.6 6.9 

Basswood, American 6.6 9.3 Western redcedar 2.4 5.0 

Beech, American 5.5 11.9 Douglas-fir 

Birch Coastb 4.8 7.6 

Paper 6.3 8.6 Interior northb

 3.8 6.9 

Sweet 6.5 9.0 Interior westb

 4.8 7.5 

Yellow 7.3 9.5 Fir 

Butternut 3.4 6.4 Balsam 2.9 6.9 

Cherry, black 3.7 7.1 California red 4.5 7.9 

Cottonwood Grand 3.4 7.5 

Black 3.6 8.6 Noble 4.3 8.3 

Eastern 3.9 9.2 Pacific silver 4.4 9.2 

Elm Subalpine 2.6 7.4 

American 4.2 9.5 White 3.3 7.0 

Rock 4.8 8.1 Hemlock 

Hackberry 4.8 8.9 Eastern 3.0 6.8 

Hickory 7.4 11.4 Western 4.2 7.8 

Magnolia, southern 5.4 6.6 Larch, western 4.5 9.1 

Maple Pine 

Bigleaf 3.7 7.1 Eastern pine 2.1 6.1 

Red 4.0 8.2 Jack 3.7 6.6 

Silver 3.0 7.2 Lodgepole 4.3 6.7 

Sugar 4.8 9.9 Ponderosa 3.9 6.2 

Oak Pine 

Northern red 4.0 8.6 Red 3.8 7.2 

Northern white 5.6 10.5 Southern 

Southern red 4.7 11.3 Loblolly 4.8 7.4 

Southern white (chestnut)5.3 10.8 Longleaf 5.1 7.5 

Pecan 4.9 8.9 Shortleaf 4.6 7.7 

Sweetgum 5.3 10.2 Slash 5.4 7.6 

Sycamore, American 5.0 8.4 Sugar 2.9 5.6 

Tanoak 4.9 11.7 Western white 4.1 7.4 

Tupelo Redwood 

Black 5.1 8.7 Old growth 2.6 4.4 

Water 4.2 7.6 Young growth 2.2 4.9 

Walnut, black 5.5 7.8 Spruce 

Willow, black 3.3 8.7 Engelmann 3.8 7.1 

Yellow-poplar 4.6 8.2 Red 3.8 7.8 

 Sitka 4.3 7.5 

 White 4.7 8.2 

a

Expressed as a percentage of the green dimension. 
20

Table 6—Average specific gravity and approximate weight of 2.36 m3 (1,000 board feet) of various 

species of lumber at 25% moisture content 

Species

Average 

specific

gravitya

Average weight 

of 2.36 m3

(1,000 board 

feet) (kg (lb)) Species

Average 

specific

gravitya

Average weight 

of 2.36 m3

(1,000 board 

feet) (kg (lb)) 

Hardwoods Softwoods 

Alder, red 0.37 1,114 (2,456) Baldcypress 0.42 1,260 (2,778) 

Ash Cedar 

Black 0.45 1,361 (3,001) Alaska 0.42 1,257 (2,772) 

Green 0.53 1,595 (3,518) Atlantic white 0.31 927 (2,045) 

White 0.55 1,658 (3,656) Eastern redcedar 0.44 1,315 (2,899) 

Aspen Incense 0.35 1,045 (2,305) 

Bigtooth 0.38 1,083 (2,387) Northern white 0.29 865 (1,908) 

Quaking 0.35 1,053 (2,321) Port-Orford 0.40 1,169 (2,578) 

Basswood, American 0.32 972 (2,143) Western redcedar 0.31 925 (2,039) 

Beech, American 0.56 1,700 (3,748) Douglas-fir 

Birch Coastb 0.45 1,355 (2,987) 

Paper 0.48 1,458 (3,216) Interior northb

 0.45 1,351 (2,978) 

Sweet 0.60 1,816 (4,005) Interior westb

 0.46 1,383 (3,050) 

Yellow 0.55 1,668 (3,678) Fir 

Butternut 0.36 1,081 (2,383) Balsam 0.34 991 (2,186) 

Cherry, black 0.47 1,413 (3,115) California red 0.36 1,081 (2,384) 

Cottonwood Grand 0.35 1,051 (2,317) 

Black 0.31 934 (2,059) Noble 0.37 1,114 (2,456) 

Eastern 0.37 1,111 (2,450) Pacific silver 0.40 1,206 (2,659) 

Elm Subalpine 0.31 929 (2,048) 

American 0.46 1,390 (3,065) White 0.37 1,109 (2,445) 

Rock 0.57 1,723 (3,799) Hemlock 

Hackberry 0.49 1,478 (3,260) Eastern 0.38 1,138 (2,510) 

Hickory 0.64 1,944 (4,287) Western 0.42 1,265 (2,788) 

Maple Larch, western 0.48 1,448 (3,194) 

Bigleaf 0.44 1,322 (2,916) Pine 

Red 0.49 1,475 (3,253) Eastern white 0.34 1,016 (2,241) 

Silver 0.44 1,324 (2,919) Lodgepole 0.38 1,142 (2,518) 

Sugar 0.56 1,692 (3,732) Ponderosa 0.38 1,138 (2,510) 

Oak Red 0.41 1,232 (2,716) 

Northern red 0.56 1,689 (3,725) Southern 

Northern white 0.60 1,818 (4,008) Loblolly 0.47 1,414 (3,118) 

Southern red 0.52 1,575 (3,472) Longleaf 0.54 1,625 (3,583) 

Southern white 

(chestnut)

0.57 1,818 (4,009) Shortleaf 0.46 1,414 (3,118) 

Pecan 0.60 1,809 (3,990) Sugar 0.35 1,015 (2,239) 

Sweetgum 0.46 1,393 (3,071) Western white 0.36 1,052 (2,320) 

Sycamore, American 0.46 1,388 (3,061) Redwood 

Tanoak 0.56 1,760 (3,881) Old growth 0.38 1,133 (2,498) 

Tupelo Young growth 0.33 1,014 (2,236) 

Black 0.46 1,390 (3,064) Spruce 

Water 0.46 1,385 (3,053) Engelmann 0.32 990 (2,184) 

Walnut, black 0.51 1,536 (3,387) Red 0.38 1,112 (2,453) 

Willow, black 0.34 1,086 (2,395) Sitka 0.37 1,112 (2,451) 

Yellow-poplar 0.40 1,204 (2,655) 

a

Based on weight when ovendried and green volume. 
21

Chapter 3

Air-Drying Process 

The air drying of lumber involves exposing piles of stickered 

lumber to the outdoor air. The dry-bulb temperature of the 

air, its relative humidity as indicated by the wet-bulb depression, and the rate of air circulation are important factors in 

successful air drying. Lumber in the pile dries most rapidly 

when the temperature is high, the wet-bulb depression is 

large, and air movement is brisk through the stickered layers 

of lumber. The drying rate and the minimum moisture content attainable at any time in any place depend almost entirely on the weather. Therefore, air drying of lumber cannot 

be a closely controlled drying process. 

Utilizing Air Movement 

Green lumber dries because air conducts heat to the wood 

surface and carries away the evaporated moisture. Thus, the 

air must move within and through the lumber in the pile. As 

warm, dry air enters the lumber pile, it takes up moisture 

from the wood and the air temperature decreases. As the 

cool, damp air leaves the pile, fresh air enters and drying 

continues. The way air moves within a lumber pile depends 

on the construction of the pile, its location within the yard, 

and the yard layout and arrangement. 

Boards are usually placed as close to edge-to-edge as possible in each course of a package, and there is limited opportunity for the air to drop downward through the package as it 

cools and becomes heavier. Consequently, the air must move 

laterally across the boards. A downward movement of air can 

be encouraged in lumber piles by building vertical flues, 

chimneys, or spacing boards within the pile (Fig. 21). Air 

enters the pile near the top or from the sides, passes over the 

broad faces of the boards, cools, and drops downward 

through the pile openings. A high and open pile foundation 

permits the moist air to pass out readily from beneath the pile 

and promotes the general downward movement of air. Chimneys are not necessary in kiln drying and are not commonly 

used when packages go directly from the air-drying yard to 

the kiln without restacking. 

Boards in the upper parts of a pile dry quicker than do those 

in the lower parts because the top boards are more exposed 

to wind than are the lower boards. The air near the surface of 

the ground and the bottom of the pile is generally cooler and 

consequently has a higher relative humidity than the air near 

the top of the pile. This cool, moist air entering the sides of 

the lumber pile in the lower portions will not induce as rapid 

drying as will the hotter, drier air entering the upper parts of 

the lumber pile. 

Factors That Influence

Drying Rate 

The rate at which green lumber will dry after it is placed in 

the air-drying yard depends on factors that involve the wood 

itself, the pile, the yard, and climatic conditions. 

Species

Some woods dry quickly, others slowly. Softwoods and some 

lightweight hardwoods dry rapidly under favorable air-drying 

conditions. The heavier hardwoods require longer drying 

periods to reach the desired average moisture content. Specific gravity is a physical property of wood that can guide 

estimations of drying rates or overall drying time. Southern 

Pine will dry much faster in the yard than will southern oak. 

In contrast, the lower density hardwoods like willow, yellow 

poplar, and some of the gums will dry quickly. Sugar maple 

will usually dry faster in northern yards than will northern red 

oak. However, both species have about the same specific 

gravity. 

Thickness

One common rule of thumb is that drying time increases at a 

rate of approximately the thickness raised to the 1.5 power. 

This means that 50-mm- (2-in.-) thick lumber will require 

about three times longer to lose a given amount of moisture 

than will 25-mm- (1-in.-) thick lumber. 

Grain Patterns 

Quartersawn lumber dries more slowly than does flatsawn 

lumber. Wood rays aid the movement of moisture, and in 

quartersawn lumber, few wood rays are exposed on the broad 

surfaces of the boards. 

Sapwood and Heartwood 

In softwoods, the moisture content of sapwood is usually 

much greater than that of the heartwood (Table 1). However, 

sapwood air dries faster. Usually it will be just as dry as the 

heartwood, and perhaps drier, when the heartwood reaches 
22

the desired moisture content. In hardwoods, the sapwood 

moisture content is often not much greater than the heartwood and is generally lower when air drying is terminated. 

Piling Methods 

The drying rate of lumber is affected by the way the boards 

are stacked. For instance, lumber dries faster in air drying 

when air spaces are left between the boards of a package than 

when the boards are placed edge-to-edge. However, this is 

not true in forced air circulation such as a dry kiln. Chimneys 

built into packages promote better air flow through the lumber stack, thus speeding up drying. 

The drying yard can be opened up for faster drying by spacing the piles farther apart. Placing piles on the outer fringes 

of the yard will generally increase drying rate compared with 

piles placed in the central portion of the yard. 

Height and Type of Pile Foundation 

Although downward air circulation within a pile is created by 

natural convection, it is stimulated by winds carrying away 

the cool moisture laden air underneath the pile. The foundation under the pile should be reasonably high (460–610 mm; 

18–24 in.) to allow the prevailing winds to create a brisk air 

movement under the piles. Pile foundations should not obstruct air flow, and weeds or other debris should not be allowed to block the air passage. 

Yard Surface 

The drying efficiency of a yard depends to some extent on 

how well the ground surface is graded, paved, and drained 

(Fig. 22). If water stands in a yard after a rain, it will decrease the drying rate and increase the risk of developing 

stain. The yard should also be kept clean. Vegetation and 

debris in the form of broken stickers, boards, or pieces of 

timber from pile foundations interfere with the movement of 

air over the ground surface. Vegetation, in particular, may 

prevent air from passing out from beneath the bottom of the 

piles and interfere with the circulation of air through the 

lower courses of lumber. 

Climate

The climate of the region in which the air-drying yard is 

located can greatly influence the air-drying rate (Fig. 23). 

Perhaps the most influential factor is temperature, but relative humidity and rainfall are also important factors. In 

northern lumber-producing regions, the low temperature 

retards the drying rate during the winter months. In the 

southern part of the country, where the winter dry-bulb temperature is higher, better drying conditions are expected. 

Unfortunately, these higher temperatures may be offset,

"	


	




#	


"	


	






$%

Figure 21—(a) Random width boards in this hand-built 

pile are spaced on each layer in addition to the chimney 

to aid circulation within the pile; (b) shows air flow 

through lumber pile and chimney. 

Figure 22—Well-graded and surfaced main alleys aid 

drainage and restrict growth of weeds. 
23

for instance in the Southeast, by rains that wet the lumber and 

extend the drying time. In the Southwest, arid conditions can 

make it difficult to keep degrade losses within bounds. 

Typically, the drying rate does not change a great deal from 

week to week, but in some areas hot, dry winds can accelerate drying. Periods of high relative humidity may retard 

drying. Precipitation is particularly important in air drying 

lumber. Rewetting of the drying lumber results in a significant decrease in the drying rate, and the added moisture must 

be evaporated. 

In addition to the general climatic conditions for a region, 

local geographical features influence climatic conditions, 

thereby influencing air-drying results. Yard sites at low 

elevations, such as those near swamps or marshes or those 

bordering on bodies of water, are likely to be damper than 

surrounding areas. Such sites cause a decrease in drying rate 

and encourage the development of mold, stain, and decay. 

Elevated sites are more likely to be dry. Open sites, in contrast to those in valleys and those surrounded by tall trees, 

hills, or buildings, are conducive to rapid drying because of 

the greater movement of winds through them. Note that a 

high, dry site encourages rapid drying but may cause surface 

checking and end splitting in the lumber. Most importantly, 

yearly variation of the weather can create a significantly 

different drying rate compared with the expected average 

drying rate. 

Sunshine is also a factor. Solar radiation heats the land areas, 

exposed areas of the lumber piles, and surrounding buildings. 

Air moving over these warmed areas and structures is heated 

and its drying potential increases. Black material absorbs 

more solar energy and becomes hotter than light-colored 

material. Thus, black topping the roadways and sometimes 

the entire area can be advantageous in air-drying yards. 

Drying Time and Final 

Moisture Content 

The time required to air dry lumber to a predetermined average moisture content depends not only on yard site, yard 

layout, piling methods, and climatic conditions, but on species, grain angle, and thickness. As previously mentioned, 

lumber of low specific gravity will dry faster than will the 

heavier woods. The approximate air-drying times for 25-mm 

(1-in.) softwood and hardwood lumber (Table 7) are based 

on climatic conditions for the region in which the particular 

species is cut. These data are based on experience with handstacked piles that vary in width from 2 to 5 m (6 to 16 ft). 

Piles of lumber in packages, less than 2 m (6 ft wide), would 

presumably dry in shorter periods. The minimum period 

given applies to lumber piled during good drying weather, 

generally during spring and summer. Lumber piled too late in 

the period of good drying weather to reach 20% moisture 

content, or lumber that is piled during the fall or winter, 

usually will not reach a moisture content of 20% until the 

following spring. This accounts for the maximum periods 

given in Table 7. For example, 25-mm (1-in.) northern red 

oak is often air dried to an average moisture content of about

Figure 23—Eastern (a) and western (b) United States showing the length of good air-drying conditions each year. 

(Note: These maps are from different sources and do not agree exactly where they overlap. 
24

20% in southern Wisconsin in about 60 days when yarded in 

June or July. If piled in the late fall and winter, the stock 

must stay on sticks for 120 days or more. Local yard and 

weather conditions and yard layout should be considered as 

well as the seasonal weather pattern in estimating the time 

required for air drying. Table 8 gives the air-drying times of 

hardwood lumber in four regions of the United States. Drying 

time estimates are available for a few species in a few locations (Table 9), where the drying time is estimated for lumber 

stacked in each month of the year. 

Table 7—Approximate time to air dry green 25-mm (1-in.) lumber to 20% moisture content 

Species Time (days) Species Time (days) 

Hardwoods Softwoods 

Alder, red 20 to 180 Baldcypress 100 to 300 

Ash Cedara

Black 60 to 200 Douglas-fir 

Green 60 to 200 Coast 20 to 200 

White 60 to 200 Interior north 20 to 180 

Aspen Interior south 10 to 100 

Bigtooth 50 to 150 Interior west 20 to 120 

Quaking 50 to 150 Fira

Basswood, American 40 to 150 Hemlock 

Beech, American 70 to 200 Eastern 90 to 200 

Birch Western 60 to 200 

Paper 40 to 200 Larch, western 60 to 120 

Sweet 70 to 200 Pine 

Yellow 70 to 200 Eastern white 60 to 200 

Butternut 60 to 200 Jack 40 to 200 

Cherry, Black 70 to 200 Lodgepole 15 to 150 

Cottonwood Ponderosa 15 to 150 

Black 60 to 150 Red 40 to 200 

Eastern 50 to 150 Southern 

Elm Loblolly 30 to 150 

American 50 to 150 Longleaf 30 to 150 

Rock 80 to 180 Shortleaf 30 to 150 

Hackberry 30 to 150 Slash 30 to 150 

Hickory 60 to 200 Sugar 

Magnolia, Southern 40 to 150 Light 15 to 90 

Maple Sinker 45 to 200 

Bigleaf 60 to 180 Western white 15 to 150 

Red 30 to 120 Redwood 

Silver 30 to 120 Light 60 to 185 

Sugar 50 to 200 Sinker 200 to 365 

Oak Spruce 

Northern red 70 to 200 Engelmann 20 to 120 

Northern white 80 to 250 Red 30 to 120 

Southern red 100 to 300 Sitka 40 to 150 

Southern white (chestnut) 120 to 320 White 30 to 120 

Pecan 60 to 200 

Sweetgum 

Heartwood 70 to 300 

Sapwood 60 to 200 

Sycamore, American 30 to 150 

Tanoak 180 to 365 

Tupelo 

Black 70 to 200 

Water 70 to 200 

Walnut, black 70 to 200 

Willow, black 30 to 150 

Yellow-poplar 40 to 150 

a

These species are usually kiln dried. 
25

Deterioration of Lumber

Losses in lumber value resulting from degrade that develops 

during air drying are reflected in the overall cost of drying. 

Air-drying degrade may be caused by shrinkage, fungal 

infection, insect infestation, or chemical action. Shrinkage 

causes surface checking, end checking and splitting, honeycomb, and warp. Fungus infection causes blue or sap stain, 

mold, and decay. Insect infestation results in damage as a 

result of pith flecks, pinholes, and grub holes left in the 

wood. Chemical reactions cause brown stain, gray stain, and 

sticker marking. Ultraviolet radiation degrades the surface of 

lumber and causes it to turn gray. However, this discoloration 

is limited to a thin layer and can be planed off. If kept in the 

yard for an extended time, the lumber may appear excessively weathered because of an accumulation of sawdust and 

windborne dirt. 

Table 8—Estimated time to air dry green 25- and 50-mm (1- and 2-in.) eastern hardwood lumber to approximately 

20% average moisture content 

Estimated time by region (days) 

Species 

Size 

(mm) South Mid-South Central Mid-North 

Ash 25 45 to 70 45 to 75 45 to 80 60a to 165 

50 180 to 210 180 to 220 180 to 230 No data 

Aspen 25 — — — 50a to 120 

50 — — — No data 

Basswood, American 25 40 to 65 40 to 70 40 to 75 40a to 120 

50 170 to 200 170 to 210 170 to 220 No data 

Beech, American 25 45 to 70 45 to 75 45 to 80 60a to 165 

50 180 to 210 180 to 220 180 to 230 No data 

Birch, paper 25 — — — 40a to 120 

50 — — — 170 to 220 

Birch, sweet; yellow 25 — 50 to 85 50 to 90 70a to 165 

50 — 190 to 240 190 to 250 No data 

Butternut 25 — 40 to 70 40 to 75 60a to 165 

50 — 170 to 220 170 to 220 No data 

Cherry 25 45 to 70 45 to 75 45 to 80 60a to 165 

50 180 to 210 180 to 220 180 to 230 No data 

Cottonwood, eastern 25 40 to 65 40 to 70 40 to 75 50a to 120 

50 170 to 200 170 to 210 170 to 220 No data 

Elm, American; slippery 25 40 to 65 40 to 70 40 to 75 50a to 120 

50 170 to 200 170 to 210 170 to 220 No data 

Elm, rock; cedar; winged 25 50 to 80 50 to 85 50 to 90 80a to 150 

50 190 to 230 190 to 240 190 to 250 No data 

Hackberry, sugarberry 25 40 to 65 40 to 70 40 to 75 30a to 120 

50 170 to 200 170 to 210 170 to 220 No data 

Hickory 25 50 to 80 50 to 95 50 to 90 60a to 165 

50 190 to 230 190 to 240 190 to 250 No data 

Magnolia 25 40 to 75 — — —

50 170 to 220 — — —

Maple, red; silver 25 40 to 65 40 to 70 40 to 75 30a to 120 

50 170 to 200 170 to 210 170 to 220 No data 

Maple, sugar; black 25 45 to 70 45 to 75 45 to 80 50a to 165 

50 180 to 210 180 to 220 180 to 230 No data 

Oak, lowland 25 100a to 280 — — —

50 No data — — —

Oak, red (upland) 25 60 to 120 55 to 100 50 to 90 60a to 165 

50 240 to 360 215 to 300 190 to 250 No data 

Oak, white (upland) 25 60 to 120 55 to 100 50 to 90 70a to 200 

50 240 to 360 215 to 300 190 to 250 No data 

Pecan 25 60 to 120 65 to 100 50 to 90 60a to 165 

50 240 to 360 215 to 300 190 to 250 No data 

Sweetgum, heartwood (red gum) 25 50 to 80 50 to 95 50 to 90 70a to 200 

50 190 to 230 180 to 240 190 to 250 No data 

Sweetgum, sapwood (sap gum) 25 40 to 65 40 to 70 40 to 75 60a to 165 

50 170 to 200 170 to 210 170 to 220 No data 

Sycamore 25 40 to 65 40 to 70 40 to 75 30a to 120 

50 170 to 200 170 to 210 170 to 220 No data 

Tupelo (and blackgum) 25 60 to 110 45 to 90 45 to 80 70a to 165 

50 210 to 300 180 to 220 180 to 230 No data 

Walnut, black 25 45 to 70 45 to 75 45 to 80 70a to 165 

50 180 to 210 180 to 220 180 to 230 No data 

Willow, black 25 30 to 65 35 to 70 40 to 75 30a to 120 

50 150 to 200 160 to 210 170 to 220 No data 

Yellow-poplar 25 40 to 65 40 to 70 40 to 75 40a to 120 

50 170 to 200 170 to 210 170 to 220 No data 

a

To an average moisture content of 25%. 
Table 9—Approximate days of air drying for lumber stacked in various locations at various times of the year to specified final moisture content 

Air drying time (days)

Stack date 

Northern 

red oak 

25 mm 

(1 in) 

Madison, 

WI 

20% MC 

Northern 

red oak 

25 mm 

(1 in) 

Roanoke, 

VAa

Hard

maple 

25 mm 

(1 in) 

Amasa, 

 MI 

20% MC 

Beech 

25 mm 

(1 in) 

Philadelphia, PA 

20% MC 

Yellow 

poplar 

25 mm 

(1 in) 

Roanoke, 

VA

20% MC 

Ponderosa 

pine 

25 mm 

(1 in) 

Flagstaff, 

AZ

15% MC 

Ponderosa 

pine 

38 mm 

(1 in) 

Snowflake, 

AZ

15% MC 

Ponderosa 

pine 

38 mm 

(1-1/2 in.) 

Flagstaff, 

AZ

15% MC 

Ponderosa 

pine 

50 mm 

(2 in.) 

Snowflake, 

AZ

15% MC 

Sitka 

spruce 

25 mm 

(1 in) 

Vancouver, 

BC 

20% MC 

Douglas-fir 

25 mm 

(1 in) 

Everett, 

WA 

20% MC 

Douglas-fir 

50 mm 

(2 in.) 

Everett, 

WA 

20% MC 

Western hemlock 

25 mm 

(1 in) 

Vancouver, 

BC 

20% MC 

January 115 90 158 133 

February 107 68 133 96 March 91 35 102 67 

April 75 60 (21) 50 35 18 10 10 43 19 31 33 90 32 May 65 60 (21) 39 35 18 10 6 25 10 18 31 79 30 June 70 60 (20) 40 35 15 7 7 25 10 20 27 67 31 July 72 60 (21) 42 35 14 41 32 52 55 28 22 54 38 August 86 70 (25) 45 35 15 34 26 40 34 35 18 43 51 September 212 60 (28) 205 35 20 9 9 130 17 40 206 227 61 October 188 60 (30) 170 125 43 94 83 110 104 167 179 199 197 November 173 160 (22) 155 105 141 79 59 97 87 151 151 167 171 December 151 130 (22) 126 85 111 62 44 95 75 146 125 162 146 

aFinal moisture content in parentheses. 
27

Chapter 4

Air-Drying Yard 

The air-drying yard is generally located near the sawmill that 

produces the lumber or close to the factory that uses the 

wood for manufacturing finished products, and the yard is 

usually laid out for convenient transport. The main roadways 

or alleys of the air-drying yard are wide enough for the type 

of lumber handling equipment being used (Fig. 24). Most 

yarding operations build-up piles of lumber for air drying 

with prestacked packages handled by a forklift truck or other 

mechanized equipment. In some yards, piles have been built 

by hand, either from the ground level or elevated trams or 

docks. 

Yard Layout and Orientation 

An efficient yard layout should provide good drainage of rain 

and melting snow, free movement of air in and out of the 

yard, and easy transportation and piling of lumber. A yard 

laid out for rapid drying potential should be on high, welldrained ground with no obstructions to prevailing winds. 

However, the need to keep the yard close to the plant limits 

site selection, and convenient areas are not always favorable 

to rapidly air drying lumber and providing a minimum of 

degrade. Yard sites bounded by buildings or with standing 

water or streams nearby should be avoided because this 

retards lumber drying. 

Most yards are laid out in a rectangular scheme. The alleys or 

roadways cross each other at right angles, and the areas 

occupied by the piles are rectangular. Specific areas may be 

designated for certain species, grades, or thicknesses of 

lumber. The alleys serve as routes for transporting the lumber, as pathways for the movement of air through the yard, 

and as protection against the spread of fire. The alleys in an 

air-drying yard are classified as main and cross alleys 

(Figs. 25, 26). Main alleys are for access to the lumber stacks 

and cross alleys are for access to the main alleys. In large airdrying yards, blocks or areas are often separated by still 

wider roadways or strips of land to protect the lumber from 

the spread of fire and to meet insurance requirements. These 

wide alleys are sometimes called fire alleys. 

The sides of the lumber piles are parallel to the main alleys, 

which means air flows through the lumber stacks from one 

main alley to an adjacent main alley. Spaces between the 

sides and ends of the piles are also part of the yard layout. 

These spaces form additional passageways for air movement. 

Yards can be oriented in either of two ways. The main alleys 

run north and south to obtain faster roadway drying after rain 

or faster snow melt. With this orientation, the roadways are 

exposed to solar radiation more than when the main alleys 

are oriented east and west, where the lumber piles shade the 

main alley roadways and retard roadway drying and snow 

melt. This orientation might be best suited for areas of high 

precipitation, especially snowfall. It is also desirable to orient 

the main alleys as close as possible to parallel with the prevailing winds. With this orientation, the wind can blow unimpeded through the main alleys. As a result of complex 

patterns of air velocities through adjacent main alleys, air 

pressure varies within, which causes air to flow through the 

lumber packages from one main alley to the next. If the 

prevailing winds are directly west–east, it is impossible to 

satisfy both the advantages of solar radiation and prevailing 

winds. However, there is often a north–south component to 

prevailing winds that does allow some air flow parallel to the 

main alleys. The problem with north–south main alley orientation and direct west–east winds is that drying from the first 

row of lumber stacks upwind increases the relative humidity 

of the air exiting that row, thus slowing the drying in rows 

Figure 24—In an air-drying yard, the alleys must be wide 

enough to enable the forklift to travel down the roadway 

and maneuver the package onto the pile. 
28

further downwind. This orientation also results in upwind 

rows blocking air flow to packages downwind. 

Alley Size 

Lumber piles are arranged either in rows of approximately 

six piles (Fig. 25) or in lines of two piles (Fig. 26). The line 

arrangement is used when the maximum air-drying rate is 

desired because air only has to flow through two piles rather 

than six as in the row arrangement. The main alleys are 

generally 7 to 9 m (24 to 30 ft) wide. Cross alleys intersect 

the main alleys at right angles and provide access to the main 

alleys. Cross alleys also afford protection against the spread 

of fire and may be 18 m (60 ft) or more in width and spaced 

every 61 or 91 m (200 or 300 ft). In addition to providing 

ample room for the forklift truck to maneuver in and out of 

the rows, alleys must be wide enough to allow 

clearance for the longest lumber being handled. 

Row Spacing 

Spaces between the ends of lumber piles aligned in rows 

should be large enough so that a lift truck can operate easily. 

The rows should be a minimum of about 1 m (3 ft) apart 

(Fig. 25). In a line-type yard, the spacing between lines is 

usually 0.6 m (2 ft). 

Lateral Pile Spacing 

The spacing between piles varies with differences in climate, 

yard site, and the character of the lumber. Pile spacing also 

varies with the different specific drying defects to be 

avoided. Where surface checking is the defect most likely to 

occur, the width of the spaces should be reduced. Where 

staining is likely to occur, it is desirable to increase the spacing. In arid regions, during the hot, dry season, piles should 

be placed closer together than they are during the cool, moist 

season. In a row-type yard, the space between the piles 

within the rows should be about 0.6 m (2 ft) but may be as 

!&'

$'(%

#


	

#


	 )		

 *

$+,%

,&+ 	
	
	 $'% 	-
-
 ,&. $/%

 0

$0,%

(0

$ 0,%

Figure 25—General arrangement of a row-type 

air-drying yard. 

!&'

$'(%

#


	

#


	 )		

,&+ 	
	
	 $'% 
	-


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$+,%

Figure 26—General arrangement of 

a line-type air-drying yard. 
29

much as 2.4 m (8 ft) in the middle of the row when variable 

spacing is practiced. The spaces between the ends of piles in 

a line-type yard are usually 0.3 to 0.6 m (1 to 2 ft). 

If the lift truck has a side shifter, the space can be minimized 

if air movement needs to be reduced. 

Pile Width 

For air drying, the width of the lumber pile varies from about 

1 to 2.4 m (3-1/2 to 8 ft). Piles only 1 m (3-1/2 ft) wide 

usually consist of several packages of lumber that are to be 

placed on kiln trucks after air drying. 

Pile Height 

Pile height usually ranges from about 1.2 to 4.6 m 

(4 to 15 ft). Sometimes 9-m- (30-ft-) high piles are built 

using forklift trucks by stacking packages separated by 

bolsters. High piles of narrow packages are often tied together with long bolsters for stability. 

Yard Transportation Methods 

In a forklift yard, the unit packages are already stickered 

when they are carried out to the yard for piling. Bumping, 

jarring, and rough handling will displace stickers. The most 

practical remedial measure is to improve the roadways so 

that transport can be rapid without jolting the stickered package. Some companies have found it advantageous to blacktop 

the entire yard surface. Blacktop absorbs and stores solar 

radiation and acts as a moisture vapor barrier against ground 

water. 

In yards where only the main and cross alleys are improved 

roadways, the initial arrangement of the rows of piles within 

blocks becomes permanent. The surface in the rows may be 

graded and graveled. 

Pile Foundations 

A pile foundation, or pile bottom, supports the lumber pile 

and provides clearance (Fig. 27) between the lumber and the 

ground. However, pile foundations must allow the air that 

has moved downward through the pile to be readily exhausted. Fixed pile foundations are an integral part of the 

yard layout, because they determine the location of the piles 

or rows or lines of piles. Pile foundations represent a considerable capital investment, and they should be well designed 

and made of materials that will contribute to a long life and 

low maintenance. 

A pile foundation can consist of the following parts: mud 

sills or sleepers, posts or piers, stringers, and crossbeams 

(Fig. 28). Mud sills or sleepers rest on the surface of the 

ground or slightly below the surface and support the piers or 

posts. Wood mud sills should be pressure treated with a 

preservative or be of the heartwood of a decay-resistant 

species. When the entire surface of the yard is paved with 

concrete or blacktop, mud sills are unnecessary, and the posts 

or piers may rest directly on the pavement. 

Posts or piers may be made from round or square pieces of 

wood, concrete, cement building blocks, or masonry. Square 

wood posts should be about 150 by 150 mm (6 by 6 in.) in 

cross section, and round wood posts should be 150 to 

200 mm (6 to 8 in.) in diameter. Diagonal bracing may be 

fastened between the posts to prevent lateral tipping. When 

posts or piers are set in the ground, they should extend 

beneath the frost line and be supported by footings designed 

to carry the estimated load. This arrangement requires 

no bracing. 

Cross beams may consist of wood timbers, steel angle irons, 

or old rails and must be aligned with the stickers. If they are 

made of wood, they should be 100 by 150 mm (4 by 6 in.) in 

dimension and placed on edge. Stringers and cross beams are 

not as susceptible to attack by decay fungi as are mud sills 

and posts. However, stringers can decay where they come in 

contact with the tops of the posts or with the cross beams. A 

thorough brush treatment with a wood preservative at these 

areas will reduce the decay hazard. 

Pile foundations should not obstruct or block air movement 

in any direction. An arrangement that restricts air movement 

over the surface of the yard and from beneath the lumber 

pile, such as laying several long planks or timbers on top of 

one another to form foundations, is to be avoided. In this 

instance, air beneath the pile is permitted to move only in the 

direction parallel to the foundation timbers. 

Forklift Yard 

Pile foundations should be high enough to allow air drift 

under the bottom package. Foundations for piles in a forklift 

yard are usually level and often are movable or temporary. 

These foundations frequently consist of short timbers, 150 by 

150 mm (6 by 6 in.) in dimension, placed directly upon the 

ground immediately before building the pile (Fig. 28). 

Figure 27—The main features of well-constructed pile 

foundations and lumber piles in an air-drying yard. 
30

Sometimes 150- by 230-mm (6- by 9-in.) preservative-treated 

timbers are placed with the broad face on the ground. All 

timbers except the outer ones are moved aside when the 

forklift truck travels up and down the space for the row 

of piles. 

Lumber Pile Protection 

One disadvantage of yard air drying is exposing lumber to 

the weather. Exposure to direct sunshine and rain or melted 

snow causes alternate drying and absorption of moisture, 

which in turn causes checking and splitting. Roofs and covers 

of various types can protect lumber while it dries and reduce 

degrade and loss in value that will occur when exposed 

alternately to liquid water and sunshine. 

Pile Roofs or Covers 

An effective pile roof is an essential feature of a good airdrying practice. A roof protects the upper courses of lumber 

and, to a lesser extent, the lower parts of the pile from direct 

sunshine and precipitation. Without a roof, the lumber in the 

upper courses, and particularly the top course, will warp, 

check, and split. Rain or snow penetrating the pile may retard 

drying, contribute to the development of fungus stains and 

chemical sticker stain, and cause surface checks to increase 

in size. 

To afford maximum protection, a roof should project beyond 

the ends and sides of the pile. For a level pile of packages, 

the roof should project about 0.3 m (1 ft) at both ends. The 

roof on a package does not usually project on the sides, 

because the roof would be in the way of the forklift. A sloped 

roof will permit the water to drain (Fig. 28). 

Piles of lumber may be roofed with waterproof paper, building paper, or roll roofing laid directly on the top course of 

lumber and weighted. These materials may also be combined 

with boards to form a panel roof. A pile roof may consist of a 

single panel or a pair. A single panel is usually designed to 

slope from one end of the pile to the other. When two panels 

are used, they should overlap at the center. Paper or roofing 

felt provides water tightness, while the boards support the 

paper or roofing in a flat sheet and permit the panel roof to 

be anchored to the pile by clamps (Fig. 29) or other forms of 

tie-downs. The narrowness of package piles encourages the 

use of numerous materials in the design of pile roofs. Materials such as exterior grade plywood, or hardboard, may be 

used. Sheets of galvanized corrugated iron and corrugated 

aluminum are also satisfactory; several roofing materials are 

available. The choice of a suitable roofing material should be 

based on the cost and expected life of the material. Pile roofs 

can be reused many times, and their cost is small when

Figure 28—Features of a level pile of packages and the design of all wood movable foundations. 
31

compared with the cost of lumber degrade that would occur 

if no pile covers were used. 

Shade Cloth 

Refractory woods, such as the oaks and beech, exposed to 

hot, dry winds may surface check badly. By baffling the air 

movement through the stickered lumber with shade cloth, the 

surface EMC conditions are modified enough to reduce 

surface checks and improve lumber brightness (Fig. 30). A 

shade cloth also prevents rain from blowing into the pile. 

End Coatings 

Moisture-resistant coatings should be applied to the end 

grain surfaces of green lumber to retard end drying and 

minimize the formation of end checks and end splits. The 

wood beneath the coating is maintained at a higher moisture 

content than if the coating were not used. To be effective, the 

coatings must be applied to the freshly trimmed green lumber 

before any checking has started. 

Many kinds of end coatings are commercially available, but 

there are two basic types: those that are applied cold and 

those that are applied hot. Cold coatings are most widely 

used for lumber products. They are applied by swab, brush, 

or spray and are often wax emulsions. It is important to 

obtain a thick coating. If the end coating material is thinned 

for easier application, additional coatings may be necessary. 

The costs of an end coating and its application are often 

justified by the value of the lumber saved. 

Shed Drying 

Lumber, particularly the more valuable grades, is sometimes 

air dried in open sheds (Fig. 31). Drying sheds are usually 

pole-type structures, although other structural materials such 

as steel posts and metal trusses and roofing may be used. The 

air-drying shed has a permanent roof so that the lumber is not 

rewetted by rain. In areas or regions where rain wetting 

unduly extends the drying time in a conventional air-drying 

yard, shed drying reduces the time required to attain the 

desired moisture content and maintain quality. 

Sheds are generally open so that the sides and ends do not 

obstruct air movement. Under certain conditions, shade cloth 

may be used to form walls. It keeps the lumber clean and 

reduces surface checking of refractory woods. 

Shed-dried lumber is usually brighter in appearance than is 

lumber air dried in a yard because weathering from ultraviolet radiation or discoloration caused by rewetting is prevented (Fig. 32). The shed roof usually extends beyond the 

piles and protects the lumber from sunshine; therefore, end 

checking and splitting are also greatly reduced. 

Drying sheds are usually loaded by forklift trucks. Pile foundation requirements are the same as for the forklift yard. The 

shed floor may or may not be paved. If not, grading may be 

required, depending upon soil conditions, and perhaps gravel 

or crushed stone application can be justified. 

The drying shed can be fairly wide to make up long rows of 

piles within the bays. Entry to the rows would usually be 

from both ends of the rows or both sides of the shed. In 

contrast, the drying shed may be long and narrow with two 

lines similar to a line-type forklift yard. 

Figure 29—Loose sheets of corrugated metal are tied to

the package with a “C” clamp. A sticker is laid across 

the top of the package on which the clamp is placed. 

Figure 30—Shade cloth or any loose weave fabric can 

be used in air drying and shed drying to reduce 

degrade resulting from uncontrolled rapid drying 

and exposure to the sun and rewetting. 
32

Yard Operation and Maintenance 

Supervision of an air-drying yard involves considerable 

responsibility and skill. The yard superintendent is often 

required to maintain existing yards while laying out new airdrying yards and expand or extend old ones. Site, orientation 

of main alleys, sizes of main and cross alleys, length of lines 

or rows, row and pile spacing within rows and lines, and size 

of blocks all require consideration for the inventory of lumber anticipated. 

Roadways in an air-drying yard are often busy highways for 

several kinds of transport trucks. Grading, graveling, or 

crushed stone fill or paving are investments to maintain the 

brightness of the lumber and improve the mobility of 

wheeled equipment. Frequent inspections of road conditions 

are a responsibility of the yard superintendent. In northern 

areas, snow removal equipment is needed to keep the alleys 

open for yard traffic. 

The design, installation, and repair of pile foundations also 

require considerable attention. Sagging foundations cause 

warped lumber and pile tipping. 

Sometimes the yard superintendent is responsible for sorting 

and package stacking at the sawmill. Sorted length stacking 

or box piling involves decisions about sticker spacing and 

alignment. Hand-built piles may require use of a frame for 

sticker guides. Automatic or semiautomatic stacking equipment requires routine inspection and maintenance. 

As a result of the large number of stickers required in drying, 

the total investment can be expensive. Therefore, investment 

in stickers justifies careful attention to their fabrication and 

storage. Dry storage of stickers is recommended to minimize 

infecting green lumber with stain and decay fungi. Excessive 

sticker breakage requires a determination of causes and 

corrective measures. Sticker replacement costs may justify 

salvage operations. 

The design, fabrication, handling, and storage of pile roofs or 

covers and sun shields are a yarding operational task. Reduced degrade and footage losses can make pile protection 

pay off. 

Weed control and preventing the accumulation of debris that 

creates fire hazards are other functions of yard operational 

supervision. Fire prevention policies and regulations must be 

formulated and routine inspections made for compliance. 

Air drying is a moisture content reduction process; therefore, 

check tests with portable electric moisture meters every few 

weeks enable the yard supervisor to determine when lumber 

in certain blocks or areas in the yard is ready for removal. 

Yarding costs not only involve time in the yard but grade and 

volume losses, thus the yard supervisor should conduct degrade studies to determine how and where yarding practices 

should be changed to significantly reduce these losses. 

Records of the lumber placed in an air-drying yard must be 

kept. An inventory tracking system best suited for the operation is essential. The location of lumber in the yard by species, thickness, grade, volume, and placement time must be 

readily available. Packages or hand-built piles should be 

ticketed to show date of piling, species, thickness, length, 

grade, and volume of the lumber; duplicates should be systematically filed in the yard supervisor’s office. Package or 

pile tickets are sometimes placed on a large panel in which 

the air-drying yard is outlined, showing where the lumber is 

located. Plants that use computerized cost accounting systems or automatic data processing equipment would probably 

also use this equipment for inventory control purposes. 

Figure 31—Packages are protected from the sun and

rewetting in this open drying shed. 

Figure 32—The protection provided by a drying shed 

can be seen by comparing the air-dried (on top) with 

the shed-dried package (bottom). 
33

Chapter 5

Piling Methods For Air Drying 

To prepare lumber for air drying, it is usually laid up into 

courses or layers with separating stickers (Fig. 33). The 

objective is to expose the board surfaces throughout the pile 

to air circulation. The lumber packages are often made of 

lumber previously sorted by species, size, and grade. 

Sorting Lumber

Lumber coming from the sawmill is generally sorted into 

classifications based on similar drying characteristics before 

it is sent to the stacker. Sorting facilitates the stacking 

operations. 

Species

Although several softwoods and hardwoods have similar 

drying characteristics, the species sort is usually made for 

merchandising purposes. Those woods that dry rapidly without serious degrade can be yarded in areas where drying 

conditions are favorable. Species that dry slowly and are 

likely to surface check can be air dried in yard areas where 

drying conditions are less severe. 

Thickness

The time required to air dry lumber to a predetermined moisture content is greatly influenced by board thickness. 

For example, 50-mm- (2-in.-) thick lumber may require about 

three times longer to dry than 25-mm- (1-in.-) thick lumber 

of the same species. Therefore, it is usual practice to segregate the rough sawn lumber by thickness classes for air drying. Miscut lumber is difficult to stack, and thinner parts of 

the board cannot be held flat. One thick board in a course 

may allow adjacent well-manufactured boards to warp because of lack of restraint. Presurfacing miscut lumber to a 

uniform thickness facilitates stacking and reduces warp, 

sticker breakage, and sticker deformation. 

Width

Some softwood lumber is sawn in the mill to produce certain 

final dry dressed sizes. These width classes are kept together 

for drying to reduce sorting and rehandling costs after drying. 

Hardwoods are most often sawn to random widths in the 

sawmill, and width segregation is seldom practical. 

Length

Good package buildup by hand or mechanical stackers is best 

accomplished by sorted length stacking. The main objective 

is to gain as much restraint to distortion as possible and 

maintain uniform air flow by good stickering practices. The 

advantages of sorting for length apply to both softwoods and 

hardwoods. 

Grain

A flatsawn board dries faster than a quartersawn board, and 

mills specializing in producing quartersawn lumber may find 

it advantageous to sort for this characteristic. Flatsawn lumber is more susceptible to surface checking than is quartersawn lumber and is often yarded in areas where the drying 

potential is less severe. When relatively few quartersawn 

boards are produced along with the major production of 

flatsawn boards, they are seldom segregated. 

Grade

The separation of green lumber by grade is generally a matter 

of keeping like-value lumber together. Higher grades of 

lumber may be given better protection in the piles or may be 

Figure 33—Good arrangement of stickers, foundations shed dried. 

and piles, which allow for air flow through the stack. 
34

Sorting Equipment 

Several types of sorting equipment have been developed to 

segregate lumber into various classifications for air drying. 

Conventional Green Chain 

In conventional green chain, sawmill lumber items are separated into the predetermined sorts by pulling the pieces from 

a conveyor chain that carries the lumber out of the mill 

(Fig. 34). The lumber is bulk piled in units for transportation 

to the stacker by carrier, forklift truck, or transfer chains. 

Sometimes the lumber is stacked and stickered in bins alongside the green chain, particularly if the packages are small, 

such as those for forklift truck operation. Most often, however, the units on the green chain are solid-piled handling 

units that are transported elsewhere for stacking. 

Edge Sorter 

The edge sorter usually consists of a long continuous row of 

live rolls with a number of slots into which the operator feeds 

the boards for the various length, grade, and width combinations. An improved sorter, in which the number of slots is 

reduced, separates the lumber by 0.6- (2-ft) increments using 

electrical or mechanical devices. A two-slot edge sorter, for 

example, may have 25-mm (1-in.) lumber fed into one slot 

and 50-mm (2-in.) lumber put down on the other side with 

the boards electrically or mechanically separated into all 

length and width combinations. Lumber is manually stacked 

at the sorter or transported to a mechanical stacker. In manual operations, two people move as a team from bay to bay to 

remove the lumber from the bin and place it on the stack. 

Mechanization can also be provided; when a full package of 

lumber has been collected, an operator can empty the bay by 

pushing a button to activate the chains in the bottom of the 

bay. The lumber is automatically fed to a transfer belt that 

carries the stock to the stacker. 

Pocket Drop Sorter 

In the pocket drop sorter, lumber is transferred crosswise and 

carried on an overhead lug chain. The boards are dropped 

into dump buggies according to predetermined length, width, 

thickness, and grade. The dump buggies move on rails; full 

buggies are transferred to a buggy dumper that feeds a 

mechanical stacker. 

The boards may also be dropped into slings (Fig. 35) rather 

than buggies. When a sling is full, the operator feeds the 

boards onto a transfer chain located under the slings. This 

transfer chain feeds the mechanical stacker. 

One factor that limits sorting is the time required to accumulate enough of a particular item. If the sorting is carried too 

far, the lumber of each kind accumulates so slowly that it 

takes too long to complete a pile. Lumber in the lower parts 

may become quite dry before the pile is completed. This 

problem is generally more critical when lumber is stacked in 

hand-built piles, because the piles usually are wider and 

contain more lumber than do package piles. Consequently, it 

is possible to sort to a finer degree when lumber is made into 

packages and piled by forklift than by hand-built piles. In 

addition, piles of packages do not necessarily have to be 

composed of a single item. For example, three packages of a 

pile may all contain the same species and thickness but 

different grades. 

Stacking Lumber in Packages 

Packages are sometimes stacked by hand, using a crew of 

one or two people (Fig. 36). Packages are made up beside 

Figure 34—Hardwood lumber is graded on a 

conventional green chain and sorted on trucks before 

being stacked or sent to the automated stacker. 

Figure 35—Sling sorter for holding lumber prior to being

sent to the stacker. 
35

the sorter or at a special stacking location to which the sorted 

lumber is delivered. Because the stacking of packages can 

be done at one place, in contrast to stacking lumber onto 

hand-built piles in the yard, a stacker building is often constructed to shelter the workers. The stacker operators stand 

on the ground or on a platform that is approximately at the 

same level as the bottom of the package of lumber. Because 

the stickered packages are seldom more than 1.2 m (4 ft) 

high, the whole package can be constructed without much 

lifting of the boards. The packages are usually built up on a 

stacking rack or jig equipped with sticker guides. The sorted 

lumber is delivered to the stackers in bulk units, which are 

placed alongside the stacker frame. The completed stickered 

package is lifted out of the sticker guide framework by forklift truck, transported to the yard, and placed on a designated 

pile. 

When the volume of lumber being stacked for air drying is 

sufficient to justify their operation, automatic or semiautomatic stackers are sometimes installed. An automatic stacker 

is identified by the magazines that hold the stickers. The 

stickers are mechanically fed out the bottom of the magazine 

onto the lumber layers. A sticker magazine is located at each 

sticker tier, and the magazines are manually kept filled. The 

semiautomatic stacker lays up the courses of lumber onto the 

package, but the stickers are laid manually into sticker guides 

on both sides of the stacker frame (Fig. 37). 

The lumber is transferred from the sorter as bulk units that 

are set on a conveyor, either roller or chain, which moves the 

unit to a breakdown hoist. The boards move from the breakdown hoist to an “unscrambler” where they are separated into 

a single layer and moved by conveyor to the stacker lay-up 

table. A grading and tally station might be located between 

the breakdown hoist and the stacker lay-up table. 

When a course of lumber has been laid on the package by the 

stacker arms and the stickers laid automatically or manually, 

the package is lowered mechanically so that it is in a position 

to receive the next course. When the package is completely 

stacked, it is removed from the stacker and conveyed on 

rollers or chains to a station where a forklift truck can pick it 

up for transport to the air-drying yard. 

Stacking Method 

The stickered packages can be built up from length-sorted 

lumber, box piled (Fig. 38), or stacked with both ends ragged. Sorted length stacking is rapid, and with close sticker 

spacing and good sticker alignment, warp is restrained. 

Sticker tiers are at the ends of trimmed stock and uniformly 

spaced in between. In stacking untrimmed sorted lumber, one 

end of the package is made square at the bumper board end 

of the stacking stall or by the “even ender” on the automatic 

stacker. The sticker tier is at or very near the end. The other 

end of the package is ragged to the extent that the board 

Figure 36—Lumber is sometimes manually stacked. 

Figure 37—Semiautomatic stacker; (a) crews lay the 

stickers in place (b) at which time the stacker places a 

layer of lumber on the stickers. 
36

length varies from the nominal length. The sticker tier is back 

far enough from this end to support most of the boards. 

In box piling random length lumber, the long pieces are 

placed at the edges of the package and the shorter lengths are 

placed in between. By alternating the shorter boards from 

one end to the other, both ends of the package can be made 

square. Sticker tiers can be located so that most of the ends 

of the shorter boards are supported. Random length lumber 

can be box piled to produce unit packages with both ends 

square (Fig. 38) or with one end square and the other end 

ragged. Box-piled packages with a ragged end are difficult to 

transport without stickers being jolted out of alignment at the 

ragged end; therefore, the package should be built up with 

both ends square if possible. Bumper boards at both ends of 

the stacking stall can be used to produce square ends for both 

Figure 38—To improve the air flow through the stack of box-piled random length lumber, short boards are 

staggered in the center and full-length boards are placed on the edges. 
37

trimmed and untrimmed lumber. It is desirable to have long 

boards at the edges of each lumber layer. This may require 

manual replacement of boards on the lay-up table as the 

lumber moves toward the stacker. 

Package Width and Board Spacing 

The width of the stacked package is determined by the subsequent drying process and the load capacity of the forklift 

truck. Packages for yard drying only are likely to vary between 1.0 and 1.8 m (3 and 6 ft), with 1.2 m (4 ft) the most 

common width. When the lumber is ready to go from the 

yard to the dry kiln, the packages vary from 1.0 to 2.4 m 

(3-1/2 to 8 ft) wide, depending on the width of the kiln truckloads and whether the loads are to be one or two packages 

wide. The packages are usually 1.2 m (4 ft) wide for air 

drying when subsequent drying in package-loaded kilns is 

anticipated. Forklift trucks can handle packages up to 2.4 m 

(8 ft) wide, but package height is controlled to keep the total 

weight within the safe loading capacity of the machine. 

The boards making up a course in a package may be stacked 

with or without spaces between them. Boards in packages 

that are 1.2 m (4 ft) or less in width are usually stacked edge 

to edge. There is generally one space in each course, because 

the sum of the board widths does not equal the exact width of 

the package. If the packages are to be placed later on kiln 

trucks or loaded into a package-loaded dry kiln, it is desirable that both sides of the package be square and plumb and 

that all courses have boards extending to the sides. This 

means that the space in the course should be within the 

course and not at the side. In mechanical stacking, this space 

is usually developed between the first two boards of a course 

that is laid down by the stacker arms. The people stickering 

or the stacker operator makes sure the first board is against 

the sticker guide frame. 

If the packages are wider than 1.2 m (4 ft) and the species 

being stacked is relatively slow drying or inclined to blue 

stain, the boards on each course may be spaced to promote 

downward movement of air. The board spacing is generally 

about 25 mm (1 in.) but can be increased if the package 

needs to be “opened” up. In hand-stacking packages, the 

board spacing is done manually, but stackers acquire a skill 

in placing boards on the stickers as they lay up the course so 

that getting reasonably good board spacing is not a problem 

(Fig. 39). Some mechanical stackers can lay down the boards 

of a course with uniform spacing between boards. The 

equipment can be adjusted to make the board spacing larger 

or smaller. Wide unit packages for forklift yards are sometimes stacked with flues or chimneys. 

Package Height 

The height of the package or the number of layers in the 

package is determined by the thickness of the lumber being 

stacked, the thickness of the stickers being used, the loading 

or carrying capacity of the forklift truck or carrier, and the 

height of the kiln door. A standard height is 1.2 m (4 ft), and 

most forklift trucks can transport two packages, one above 

the other. Packages that are 1.8 to 2.4 m (6 to 8 ft) wide, 

when carried two packages high, may require lift truck 

equipment designed to carry the heavier loads. 

Pile Width and Height 

Pile widths in a forklift yard are determined by the width of 

the package. Sometimes the air-drying pile of packages of 

softwoods is made two units wide with a relatively small 

space between the two tiers on the pile foundation. Pile 

height in a forklift yard depends upon the equipment used for 

placing packages in the pile. Piles of packages built up with 

forklift trucks are generally not more than about 4.6 m (15 ft) 

high and consist of four packages. 

Use of Bolsters 

Bolsters separate the packages to permit ready entrance and 

exit of the lift truck forks. The bolsters are usually 100- by 

100-mm (4- by 4-in.) wood members and as long as the 

stickers. Bolster placement is critical as out-of-line bolsters 

between packages can cause warp (Fig. 40). The bolsters 

should be placed immediately above the tier of stickers of the 

bottom package in the pile. The packages on top should also 

be lined up so that the sticker tiers and all bolsters between 

packages are in vertical alignment. Thus, distortion or warp 

is restrained. 

Figure 39—Sticker guides are used to improve sticker 

alignment.
38

A “side shifter” carriage on the forks of the lift truck aids in 

placing the packages so stickers are aligned with the bolster. 

Bolsters must still be carefully placed, and the lift truck 

operator must avoid jolting the bolsters out of place as the 

upper packages in the pile are moved into position. The 

bolsters are placed on the packages when the package is at 

ground level. Although bolsters between packages at every 

sticker tier would produce maximum influence of superimposed loading to restrain warp, it is not considered practical, 

and five bolsters are usually used on 4.9-m- (16-ft-) long 

lumber. 

Both softwood and hardwood species are used for bolsters. 

Because they are generally used over and over again, bolsters 

should be a durable hardwood species that is being processed 

at the sawmill. Bolsters are generally rough sawn and are 

dried the first time out. When not in use, bolsters should be 

stored under cover and kept dry. Bolsters are usually not 

treated to prevent deterioration caused by stain, decay, and 

insects, yet should not infect or infest the green lumber on 

which they will be used. 

Piling Modifications 

The yarding arrangement of main alleys, cross alleys, rows, 

lines, and pile foundations within a row or line is generally 

well fixed. In completely paved yards with portable foundations, however, areas of the yard can be rearranged to either 

hasten or retard the drying rate. Even in line-type yards 

having fixed foundations, lumber might be piled in every 

other line when drying conditions are poor. An alternate 

practice would be to stagger the piles between the two foundations in the line. Yards with long rows between main alleys 

may have a foundation system that will permit changing pile 

spacing, increasing the spacing when drying weather is expected to be poor and reducing the pile spacing when the 

anticipated drying potential is good. 

The makeup of the package is usually fixed as to size and 

method of course lay-up. But within a course, edge- to-edge 

stacking can be changed to spaced board stacking. Also, the 

spaces between boards can be increased or decreased as airdrying conditions warrant. 

Stacking Random Length 

Lumber

Sloped and pitched hand-built piles of lumber are usually 

constructed of lumber sorted for length. If random length 

lumber is stacked, box piles are made with the boards flush 

with the front or pitched end of the pile. The other end of the 

pile is ragged, and good sticker support for the various 

lengths is lacking. Lumber is sometimes grouped according 

to length; for example, 1.8, 3.0, and 3.7 m (6, 10, and 12 ft) 

and 2.4, 4.3, and 4.9 m (8, 14, and 16 ft). The shortest boards 

are half as long as the longest boards; therefore, the scheme 

aids stacking and provides a good means for supporting 

ragged ends with a sticker tier. 

Level, hand-built piles can be box piled with both ends 

square to incorporate good sticker restraint to warp. Grouping 1.8-, 3.0-, and 3.7-m (6-, 10-, and 12-ft) lengths and 2.4-, 

4.3-, and 4.9-m (8-, 14-, and 16-ft) lengths into separate piles 

will result in less void space and better stickering. Level, 

hand-built piles are usually provided with pile roofs to keep 

rain entry to a minimum. 

Special Piling Methods 

The packages piled for air drying or the hand-built piles are 

usually considered flat piles. Other methods developed previously that have more or less limited use are crib piling, end 

racking, and end piling. These are methods of stacking lumber for air drying without stickers. These methods enable one 

person to stack lumber and require more area in the drying 

yard. These methods are not generally recommended and are 

not currently common in the United States, but are used 

sometimes in other countries, especially in the tropics. 

Crib piling may be single, double, or triple (Fig. 41a). The 

first plank rests on a block at each end. One end of the second plank crosses the first plank with the other end resting on 

a third block. The third plank closes the triangle. Successive 

courses are built similar and on top of the first course. Crib 

piles are not provided with roofs. 

Figure 40—Improper bolster-sticker alignment will 

increase the defect of bow or warp. 
39

In end racking, the boards are placed upright and cross each 

other to form an “X” or an inverted “V” (Fig. 41b). End 

piling is approximately equivalent to tilting a flat pile on end 

until the boards are nearly vertical (Fig. 41c). In both end 

racking and end piling, the rows of boards are supported by 

frameworks. Neither of these types of pile is roofed. With 

end piling and end racking, it is important to support the 

lower ends of the boards above the ground surface. The 

lower ends of end-piled boards may rest on a spaced board 

platform or on boards laid directly on the ground. In end 

racking, the lower ends of the boards usually rest on timbers 

or planks placed directly on the ground. 

All these piling methods have disadvantages and advantages 

compared with flat piling. All three methods can be done 

easily by one person; flat piling methods are more difficult 

for one person to do efficiently. These methods are also more 

susceptible to end and surface checking and warping than are 

flat piled. The ends of the boards in crib piling are likely to 

stain because of the retarded drying where they come in 

contact with each other. End piling and end racking promote 

rapid surface drying and consequently reduce the likelihood 

of staining. But treating lumber with a fungicide is common 

today, so this piling method is rarely used as the primary 

method of stain control. The boards are often wetter at the 

bottom than at the top ends. 

Stickering Lumber 

Stickers perform several important functions in air drying. 

They separate the courses or layers of lumber, permitting air 

to circulate over the broad faces of the boards (Fig. 42). Each 

sticker carries its proportionate share of the weight of the pile 

above it. Stickers, in combination with the loads that are 

imposed on them, tend to keep the boards flat, thus restraining warp. Finally, the position of the stickers at the two ends 

of a pile may reduce end checking, splitting, and warping 

near the board ends. In carrying out these roles, stickers must 

contact wet boards and the contact area that is somewhat 

retarded from drying may develop stain (Fig. 43). 

Sticker Manufacture 

Stickers are often made from lumber that is available at the 

plant where the air drying is performed, but sometimes they 

are purchased. In any case, stickers should be straight 

grained, durable, and strong so they can be used over and 

over again. The oaks, beech, and hickory make good stickers 

for hardwood operations. Douglas-fir and larch are used for 

stickers in the Pacific Northwest. Southern Pine and cypress 

heartwood stickers are used on softwoods in the South. It is 

preferable to make stickers from heartwood rather than sapwood. Stickers made from sapwood can become infected 

with stain or decay fungi, thus infecting freshly cut lumber on 

which they are laid. Green edgings from lumber have been 

used for stickers but do not perform as well as stickers made 

from thoroughly air-dried or kiln-dried lumber. 

Figure 41—Alternative manual stacking arrangements 

that have been used in the past: (a) crib pile, 

(b) end racking, (c) end piling. 
40

Sticker Size 

For piling hardwoods, the stickers are usually made from 

25-mm- (1-in.-) thick lumber, either rough or dressed to 

about 19 mm (0.75 in.), and from 25 to 50 mm (1 to 2 in.) 

wide, usually 32 mm (1.25 in.) wide. For piling softwoods, 

the 25-mm- (1-in.-) thick stickers are generally wider, as 

much as 100 mm (4 in.). 

Sticker length is determined by the width of the package or 

pile being constructed. Four 1.2-m (4-ft) packages being 

stacked by hand in a framework of sticker guides or on an 

automatic stacker will use stickers about 1.3 m (50 in.) long. 

Hand-built piles in the yard will use stickers that are 1.8 m 

(6 ft) long on a 1.8-m (6-ft) pile. The pile is usually a few 

millimeters (inches) narrower than 1.8 mm (6 ft) so that the 

stickers project enough for the stacker to see the sticker tier 

location as the pile is constructed. 

All stickers should be of uniform thickness to minimize 

warping. Miscut lumber from which stickers are being ripped 

should be surfaced on one or both sides prior to ripping in 

order to produce stickers of uniform thickness. 

Rough Compared With

Dressed Stickers 

Whether the sticker surface contacting the lumber is rough or 

dressed probably does not greatly influence the drying rate of 

the wood under the sticker, although some yard supervisors 

prefer to use stickers with rough-sawn surfaces. In contrast, 

the operation of automatic stackers seems to be less troublesome when stickers are made from surfaced lumber. 

Sticker Alignment 

Stickers should be in good vertical alignment in both packages and hand-built piles (Fig. 44). If the stickers are not 

aligned, the superimposed weight will not be transmitted 

downward from one sticker, through the lumber, and directly 

down to the next sticker by direct compression, but will act 

on the span of the boards to cause warp (Fig 40). Sticker 

guides not only ensure good sticker alignment within a package, but they regulate the sticker spacing so that it will be the 

same from package to package. Uniform spacing is necessary 

if good alignment of stickers, bolsters, and foundation cross 

beams is to be obtained in a pile consisting of several packages. In stacking hand-built piles, sticker guides are seldom 

used and sticker alignment depends on the care and skill of 

the lumber stacker. 

Sticker Spacing 

The space between stickers in a course of lumber depends on 

the thickness of the lumber. Standard 19-mm (nominal 1-in.) 

lumber requires closer sticker spacing to control warp than 

does standard 38-mm (nominal 2-in.) material. Stickers 

Figure 42—So that air can circulate over the board 

surfaces, wood stickers separate the layers or courses 

of lumber. 

Figure 43—The area under the sticker is retarded from

drying and stain sometimes results. 
41

retard drying to some extent at the areas where they contact 

the boards, so the fewest number of stickers that will produce 

the desired warp control is best. Fewer stickers are used with 

softwoods than with hardwoods, so softwood stickers are 

wider to provide sufficient bearing surface and avoid crushing the sticker or the lumber. Five stickers are generally used 

on 25-mm by 5-m (1-in. by 16-ft) softwoods. From five to 

nine stickers are used on 25-mm by 5-m (1 in. by 16 ft) 

hardwood lumber. Spacing stickers 400 to 600 mm (16 in. to 

2 ft) apart is common for hardwood lumber, particularly for 

packages where sticker guides keep them in place when the 

courses of lumber are laid down. 

Special Sticker Designs 

Stickers used in stacking lumber for air drying are usually 

rectangular in shape. The broad faces of the sticker contact 

the stacked lumber. Where sticker marking is associated with 

chemical brown stain, grooved stickers have lessened the 

problem (Fig. 45). Here, 50-mm- (2-in.-) wide by 22-mm- 

(7/8-in.-) thick hardwood stickers have a groove approximately 35 mm (1-3/8 in.) wide machined out on the two sides 

to provide 8-mm (5/16-in.) bearing areas at each edge of the 

sticker. The grooved sticker presumably allows some air 

circulation under the stickers, reducing the likelihood of 

chemical reaction causing the stain. 

Sticker Requirements 

Automatic stackers eject stickers from the bottom of the 

sticker magazine. If the stickers are not straight and uniform 

in cross sectional dimension and length, the stacker will not 

function properly. Warped and end-battered stickers create 

problems in automatic stacker operation, so poor stickers 

must be replaced. 

In semiautomatic stacking, the stickers are laid on the courses 

of lumber by hand. Sticker guides on both sides of the package are usually provided. Sometimes the sticker guide slots 

are considerably wider than necessary, resulting in some 

offset stickers in the tiers. As the stickers are hand laid, some 

sticker warpage can be tolerated, but very crooked stickers 

should be discarded. Stickers that are extremely bowed may 

require quite a few lumber layers on top to flatten them. 

Twisted stickers should be rejected because they cause the 

same problems as do excessively thick stickers mixed in with 

others of uniform thickness. Thin stickers should be rejected 

because they restrict the volume of air flow through the 

sticker openings. 

Handling and Storage of Stickers 

Stickers used in air drying lumber last longer than those used 

in kiln drying. In air drying, stickers are not subjected to 

repeated high temperatures and are not handled so frequently. However, sticker life can be shortened by rough 

Figure 44—Good sticker alignment. 

Figure 45—To avoid sticker stain, grooved stickers 

sometimes replace rectangular sticker. 
42

handling. Stickers that have dropped out of the package may 

be run over by transport machines and broken. 

Wetting may cause the moisture content of the stickers to 

increase to the point where stain or decay fungi can grow. 

This can shorten the life of the stickers, and fresh lumber 

piled with them may become infected. Poor sticker storage 

and handling and exposure to wetting and redrying may also 

cause the stickers to warp, which contributes to their breakage when they are used. 

Forklift yards usually develop a sticker handling system. The 

packages are generally built at some central location. This 

requires that the stickers be returned from where the packages are taken down to the place where the stacking is done. 

If the packages of lumber are kiln dried after air drying, the 

stickers are returned to the stacker after breakdown of the 

kiln-dried package in the planer mill, the rough mill in a 

factory. At the package breakdown location, the stickers are 

often loaded into racks or bins that can be carried to the 

stacker by the lift truck or carrier (Fig. 46). In a forklift yard, 

it is possible to provide protection for the stickers from the 

weather by keeping the carrier racks or bins under cover. 

Sticker Salvage 

Stickers represent a considerable investment. Although the 

cost of each individual sticker is not great, the piles on the 

yard and in the kiln contain a considerable number of stickers. To what extent stickers are salvaged depends on their 

value. Today, some plants salvage broken stickers by trimming long stickers to the shorter lengths used in package 

operation. When only short or long stickers are used, the 

broken stickers with some salvage value are trimmed, scarfed 

or finger jointed, glued, and cut to length. Used stickers have 

re-sale value. 

Figure 46—The handling of stickers is facilitated if they 

are racked for transport by forklift. 
43

Chapter 6

Air-Drying Defects—Causes and Remedies 

Losses in value and volume resulting from defects that develop in lumber during air drying increase the cost of air 

drying. If known, such losses should be assessed directly 

against the air-drying process. Air-drying defects may be 

caused by chemical reaction, fungal infection, insect infestation, or shrinkage. Chemical reactions cause chemical brown 

and gray stain, and sticker marking is one form of this discoloration. Fungal infection causes blue stain in the sapwood

as well as decay and mold. Insect infestation results in pith 

flecks, pinholes, and grub holes. Shrinkage causes end 

checks and end splitting, surface checks, honeycomb, and 

warp. Exposure of lumber directly to weathering conditions 

aggravates these shrinkage defects, and extended yarding 

after the lumber is air dried accelerates the rate of grade 

deterioration or volume losses. Drying defects that do not 

degrade rough lumber may cause volume losses during machining. Warp may cause skips or splits in the dressing operation. Knots that are loosened in softwoods during drying 

may be knocked out during planing, and knots that are 

checked may be broken. 

Chemical Reaction 

Certain stains or discolorations can develop in lumber during 

air drying in addition to those caused by fungi or by general 

weathering. These stains result from chemical changes that 

occur in the wood. Called chemical stains, they darken the 

wood to colors ranging from gray to buff to dark brown. 

Species that are subject to objectionable chemical stains are 

ponderosa pine, the true white pines, western hemlock, noble 

fir, redwood, and several hardwoods including ash, maple, 

birch, hickory, and magnolia. 

In pine, chemical brown stain develops in either sapwood or 

heartwood. It is more prevalent in lumber sawn from old logs 

that have been cut for some time than in lumber from newly 

cut logs. It is also more prevalent in boards that have been 

solid piled for 2 or more days immediately after sawing than 

in boards stacked for drying immediately after sawing. The 

stain develops during hot, humid months and usually is observed on the surfaces of the boards, but it can penetrate 

deeply into the wood. In some cases, it occurs inside a board 

and does not show on the surface. The stain results from a 

concentration of extractives that are transported by the water 

and deposited at the point where the water is vaporized or 

absorbed as bound water. These extractives are believed to 

be sugars and amino acids, which are present within the free 

water or are formed through enzymatic action immediately 

after felling or during solid piling of the lumber. The enzymatic action can be slowed down by dipping the green lumber in enzyme-inhibiting chemicals. 

Deep grayish-brown chemical stains may occur in the sapwood of lumber in air-drying yards and predryers. These 

stains can be a problem in oak, hickory, ash, maple, tupelo 

gum, magnolia, persimmon, birch, basswood, and Douglasfir. Stains develop during very slow drying. In this situation, 

enzymes are produced by slowly dying parenchyma cells, 

which darken when oxidized. To minimize these stains, green 

lumber should be stickered immediately after sawing, with 

good air circulation provided, and preferably at temperatures 

greater than 21o

C (70oF). 

Sticker marking, a form of chemical stain, develops in woods 

such as maple, white ash, and magnolia during the warm, 

humid summer months. The discoloration, which often does 

not surface out of the rough, dry boards, can be significantly 

reduced by using dry stickers and subjecting the lumber to 

good drying conditions immediately after stacking. Fan shed 

drying and forced air drying, or low temperature kiln drying, 

are very effective in preventing sticker marking in these 

woods during the warm months of the year. 

Chemical stains can be reduced by conditions that encourage 

rapid drying. Rapid air drying can be promoted by keeping 

the yard surface well drained, free from vegetation and debris, using high and open-pile foundations, increasing the 

spacing between piles in the rows, and opening the unit 

packages by increasing board spacing and constructing more 

chimneys in hand-built piles. 

Fungal Infection 

Stain

Stains in wood can be caused by fungi that grow in the wood

and use parts of it for food. These stains are confined largely 

to sapwood of both softwoods and hardwoods and are of 

various colors. The so-called “blue” stains, which vary in 

color from gray to bluish to bluish black, are most common. 

The blue color is caused by the thread-like hyphae of the 
44

fungus that invade the wood. Except for toughness, blue stain 

has little effect on the strength properties of wood, although

it does cause degrade where color is important. 

Blue stain is likely to develop where air drying is retarded. It 

is most likely to occur during the warm, damp seasons of the 

year. It occurs in flat piles of self-stickered lumber, where 

green boards are used for stickers, and in end racking and 

crib piling, where the boards themselves come in contact. 

The likelihood of blue staining can be reduced by using 

narrow, dry stickers and by opening up the yard and the piles 

to encourage rapid air drying. 

Fungal growth can be prevented in lumber by quickly drying 

it to a moisture content of 20% or less and keeping it dry. 

Air-drying conditions may not always be favorable to prevent 

the growth of staining fungi; therefore, chemical treatment of 

the freshly cut lumber may be necessary. The chemical 

treatment is usually accomplished by dipping the lumber or 

spraying it with a suitable fungicide. However, if the lumber 

has already become infected, the fungus may have penetrated 

so far below the surface that the organism is not completely 

killed by dipping. When such lumber is air dried slowly, the 

interior portions may blue stain, although the surfaces may 

remain bright. The effectiveness of the antistain chemicals 

may also be reduced by leaching if the lumber is exposed to 

wetting while in the yard. 

Successful stain control with chemicals not only depends 

upon immediate and adequate treatment but also upon proper 

handling of the lumber in the air-drying yard. The yard and 

lumber storage areas must be kept as well drained and sanitary as possible to reduce the chance of infection. The fungus 

is propagated by spores when the fungus has reached a certain stage. These spores are airborne, are practically always 

present, and infect freshly sawn lumber by coming to rest on 

the surfaces. If the conditions of air, moisture, and temperature are favorable, spores develop quickly into the fungus. 

Sap stain fungi can grow at temperatures from 1.7°C to 38°C

(35°F to 100°F ). Spores, although generally carried by the 

wind, are also carried by insects, and when the insect 

burrows into the sapwood, the spores are carried into the 

burrows. 

Mold 

Mold is also propagated by airborne spores. During warm, 

moist weather, mold grows on wood surfaces and penetrates 

the wood. In contrast to blue stain, the hyphae of molds are 

colorless and do not stain the wood. The discolorations of 

wood surfaces are caused by the fruiting bodies. Under exceptional conditions, mold may develop to a point where it 

restricts air circulation in certain portions of a pile, thereby 

retarding drying. The measures used to reduce or control 

blue stain apply also to mold. 

Decay 

Decay or rot is caused by fungi that not only discolor wood

but actually destroy it. Decay, blue stain, and mold organisms 

all thrive under similar conditions of moisture content, air, 

and temperature, but decay requires somewhat longer time to 

develop. Freshly sawn lumber may be infected by airborne 

spores or contact with decayed foundation timbers or stickers. The best way to combat decay is to dry the lumber to a 

moisture content of 20% or less as quickly as possible. In 

some cases, it may be necessary to treat decay with a suitable 

fungicide. Decay is frequently present in the living tree, and 

lumber sawn from the logs will contain the organisms. Some 

decay fungi may continue to develop in the lumber as it dries. 

Insect Infestation 

In all stages of drying—from the green condition to completely dry—wood can be subject to attack by insects. Piles 

of lumber in an air-drying yard are sometimes infested. Debris, in the form of broken timbers or stickers, provides 

breeding places for insects that may spread to the lumber. 

Powder post beetles attack hardwoods and softwoods, freshly 

cut and air-dried lumber. The sapwood of hickory, ash, and 

oak are particularly susceptible to attack. Damage is indicated by holes left in the surface of the lumber by the winged 

adults as they emerge and the fine powder that may fall from 

the wood. This is evidence that the insect has departed and 

that subsequent treatments to sterilize may serve no purpose. 

Sterilization of green lumber in saturated steam at 54°C

(130°F) or at a lower relative humidity at 82°C (180°F) for 

2°h is effective for 25-mm (1-in.) lumber. Thicker lumber 

requires a longer time to sterilize. Heat sterilized wood will 

not prevent subsequent infestation; therefore, good yard 

sanitation is essential to check infestation by these insects. 

Shrinkage

When a log is sawn into lumber, the drying process starts and 

soon after stacking in the air-drying yard, shrinkage of the 

board begins. The stresses set up in the surface zones of the 

lumber by shrinkage may cause deformation or failure. Because the amount of shrinkage varies with the species of 

wood and the grain patterns of the lumber, a change in shape 

usually results. If the drying stresses exceed the strength of 

the wood, failures can develop, such as various types of splits 

and cracks. 

Checks

Checks are failures of the wood that develop along the grain 

because of drying stresses. Checks are of three types: end, 

surface, and honeycomb. Some woods are inclined to check 

more readily than others (Table 10). 
45

End checks originate on end-grain surfaces and appear as 

radiating lines pointing toward the pith or heart center of the 

tree (Fig. 47). They occur at the junction of the wood rays 

and the remainder of the wood cells, or within the wood rays. 

When started, end checks become wider and, by extending 

radially and longitudinally, develop into splits. Surface 

checks are similar separations of the wood under stress, but 

they occur on tangential or flatgrain faces (Fig. 48). End 

checks become longer by extending in the longitudinal direction of the grain of the wood and deeper by extending in the 

radial direction. As drying progresses, the surface checks 

may close up on the surface but widen and deepen internally. 

These are called bottleneck checks (Fig. 49). 

Exposure to the sun and breeze during sawing and hot, dry 

weather immediately after piling is likely to cause checking. 

End and surface checks will probably be more severe in 

those parts of the pile that are fully exposed, that is, the ends, 

sides, and top parts of the pile. Checks will be particularly 

severe in the upper surfaces of the boards in the top course if 

the pile is not roofed. 

Table 10—Checking tendency of various woods

Low Intermediate High 

Hardwoods 

Alder Ash Beech

Aspen Birch, yellow Oaks 

Basswood Butternut Sycamore 

Birch, paper, sweet Elm, rock Tanoak 

Cherry Hackberry 

Cottonwood Hickory 

Elm, American Maple, sugar, bigleaf 

Magnolia, southern Pecan

Maple, red, silver Sweetgum 

Tupelo Walnut 

Yellow-poplar Willow 

Softwoods 

Baldcypress Firs, true Douglas-fir 

Cedar Hemlocks Larch, western 

Pine, sugar Pine, jack 

Pine, loblolly Pine, lodgepole 

Pine, shortleaf Pine, longleaf 

Redwood Pine, ponderosa 

Spruce Pine, red 

Pine, slash 

Pine, white 

Figure 47—End checks in lumber can extend to

develop splits. 

Figure 48—These oak boards surface checked in the

early stages of air drying. These surface checks can 

penetrate deep into the wood, and when they close at 

the surface, the internal failure appears to be 

honeycomb. 

Figure 49—Bottleneck checks, which develop from 

closed surface checks, can look like honeycomb. 
46

Volume losses as a result of end checking and end splitting 

can be serious. End coating the freshly sawn lumber immediately after trimming to length retards the end drying that 

causes these failures. Several proprietary end coatings are 

available. The emulsified waxes that can be applied by either 

brushing or spraying are effective for use on lumber being 

air dried. 

End checking frequently begins in the log yard. It may pay to 

end coat both ends with emulsified wax to minimize end 

checking. An alternative method would be to sprinkle the dry 

log deck with water. 

Honeycomb 

Although not so common in air drying as end and surface 

checks, honeycomb or internal checking can occur (Fig. 50). 

This results from tensile failures entirely within the interior 

of the piece and usually occurs during kiln drying caused by 

too high a temperature too early in the schedule. Sometimes, 

surface depressions and grooves are indicated by the presence of honeycomb, but sometimes it cannot be detected 

until a piece is planed or sawn. 

Honeycomb and bottleneck checks look similar but result 

from different causes; therefore, it is beneficial to distinguish 

between the two. In doing so, elimination of the problem may 

be possible. Honeycomb begins internally and usually does 

not extend to the surface, thus is rarely exposed to the air. In 

contrast, bottleneck checks begin on the surface, and at one 

time were exposed to the air. With the surface open, sometimes dirt collects in the check. If the defect in question 

extends to the surface or contains dirt, it is considered a 

bottleneck check. 

After determining which type of defect is present, measures 

can be taken to avoid the defect in the future. Honeycomb is 

generally a kiln-drying problem caused by too high a temperature too early in the drying schedule. By waiting to 

increase the dry-bulb temperature or using a different drying 

schedule, this risk is reduced. Bottleneck checks can be an 

air- or a kiln-drying problem. If the check contains dirt, it 

most likely was initiated before entering the kiln and uncontrolled air drying was probably the cause. By reducing the air 

flow and/or providing more protection, the risk of checking 

is reduced. This is accomplished by using a protective cloth 

(Fig. 30), closer stacking of piles, or converting to shed 

drying. If there is no debris in the check, severe conditions 

early in the kiln schedule could have caused the checks. In 

which case, either reducing the initial wet-bulb depression or 

not advancing to the second kiln schedule step as quickly can 

help avoid the defect. 

Shakes

Failures that do not conform with the usual definition of 

checks sometimes occur under shrinkage stresses. These 

failures are caused by abnormally weak wood and are called 

shakes. They are check-like openings located within or at the 

junction of the growth rings. Shakes may originate on endgrain surfaces and look like end checks, except that they 

follow the growth rings rather than the wood rays. These ring 

failures are not considered a drying defect and cannot be 

avoided by altering the drying method (Fig. 51). 

Splits

Splits are longitudinal and radial separations of the wood. 

Usually they occur radially. Splits are generally located at a 

board end, but occasionally occur along the length of a 

board. A split along the length of the board may or may not 

extend completely through the thickness of the piece. End 

splits often result from extension of end checks further into a 

board. Splits are sometimes associated with longitudinal 

stresses that were in the log and the board when it was 

freshly sawn. When a split originates, the longitudinal 

stresses cause it to open wide and extend along the length of 

the piece. 

Cracks

Cracks have the appearance of surface checks or splits but 

are formed differently. Cracks occur in pieces containing the 

pith or heart center of the tree. They generally develop from 

surface checks or end checks, but their characteristically 

large width is caused by the difference between tangential 

Figure 50—Honeycomb is failure initiated internally 

along the rays. 

Figure 51—Shake is failure that follows the growth rings. 
47

and radial shrinkage. Cracks are common in poles, posts, and 

boxed-heart timbers. On a transverse section, a crack appears 

as a wedge-shaped opening extending from a face to the pith. 

Cross Breaks 

Boards containing abnormal wood (e.g., compression wood) 

adjacent to normal wood, when dry, can develop checks 

crosswise of the board (Fig. 12). These checks result mainly 

because the abnormal wood shrinks more longitudinally than 

does the adjacent normal wood. The restraint by the normal 

wood results in tension stresses parallel to the grain in the 

abnormal wood; finally, the stress exceeds its strength and 

causes failure. What looks like small cross breaks in normal 

wood are sometimes observed and are generally identified as 

impact failures that may have developed in felling the tree. 

Collapse

Some woods with a high green moisture content tend to take 

on a corrugated or washboard appearance when dry. The 

excessive shrinkage is termed collapse (Fig. 52). It is observed more in quartersawn lumber than in flatsawn lumber. 

Normal shrinkage of wood causes little change in the size of 

the cell cavity, whereas when wood is collapsed, the cell 

cavities are much smaller and often appear caved in. Collapse is not caused by shrinkage of the wood material but by 

the capillary forces in the free water in the cell lumen. Collapse is aggravated by kiln-drying temperatures. Air drying 

green lumber cut from butt or sinker logs of redwood and 

western red cedar and the heartwood of sweetgum and 

swamp oaks reduces the amount of collapse compared with 

what would be found if the material were kiln dried. Collapsed eucalyptus lumber that is air dried or predried can be 

reconditioned with steam to restore normal thickness prior to 

kiln drying the lumber to lower moisture content values. This 

process has not been adapted for woods of the United States, 

probably because the volume of collapsed lumber involved 

does not justify the added costs. 

Casehardening

In air drying green lumber, the fibers in the outer zones of a 

board dry below the fiber saturation point, but their shrinkage 

is restrained by the core. A tension set, that is, a permanently 

stretched condition, develops in the outer zones. Some compression set—a permanently compressed condition—can 

develop in the core. Later when the board is uniformly dry 

through its cross section, the drying stresses have reversed. 

The outer zones are in compression, and the inner fibers are 

in tension. This is a condition known as casehardening. It is 

the normal behavior of lumber to undergo stresses as it dries 

and to develop tension set. In air drying, a considerable 

amount of tension set can be developed in the outer zones of 

a board. In air drying, the development of casehardening is 

normal and is not considered a defect because it is normally 

relieved after kiln drying. 

Knots

Knots that are not intergrown with surrounding wood generally loosen during air drying. Both the shrinkage of the wood 

surrounding the knot and the shrinkage of the knot contribute 

to the loosening. A knot does not shrink as much lengthwise 

as the board shrinks in thickness, and as a result, the knot 

tends to project above the surfaces of the dried board. Encased knots, formed when a branch becomes embedded in a 

tree trunk when the branch dies, are surrounded by bark or 

pitch that may become hard and brittle when dry, loosening 

the bond between the knot and the wood. This, combined 

with shrinkage, tends to cause encased knots to be knocked 

out during machining. 

Warp

The differences in longitudinal, tangential, and radial shrinkage characteristics of wood result in distortions of the cross 

section of a board. These distortions are termed warp and in 

lumber items, are classified as cup, bow, crook, twist 

(Fig. 53), and kink. Normal longitudinal shrinkage of wood 

does not greatly influence lengthwise distortions, but if juvenile and reaction wood are present, the resulting changes can 

cause degrade. A classification of the warping tendencies of 

some woods is given in Table 11. The general kinds of warp 

are defined as follows: 

Cup: Deviation of the face from being a straight line 

across the width of the board 

Bow: Deviation of the face from being a straight line 

along the length of the board 

Crook: Deviation of the edge from being a straight line 

along the length of the board 

Twist: A distortion caused by a turning or winding of 

the edges of a board so that the four corners of either 

face are no longer in the same plane 

Kink: An edgewise deviation, similar to crook, caused 

by a knot or severe localized grain distortion to develop 

two straight portions in the piece at a large obtuse angle; 

also, a flatwise deviation, similar to bow, caused by 

physical bending and restraint such as out-of-line 

stickers or bolsters 

Figure 52—The irregular surface is evidence of collapse 

in this section from an air-dried western redcedar board. 
48

Cup is common in flatsawn boards, and all such boards tend 

to cup if permitted to dry and shrink without restraint. In a 

flatsawn board, because of the position of the growth rings, 

the face toward the bark of the tree has a greater shrinkage 

potential than does the face toward the pith. Consequently, 

when the board shrinks, the face toward the bark tends to 

become concave and the face towards the pith tends to become convex. 

For a given species, the tendency to cup varies inversely with 

the distance the annual rings in the board are from the pith of 

the tree. The other forms of warp, bow, crook, and twist may 

be caused by spiral or diagonal grain, or localized distortions 

of grain, as around knots, for example. Spiral grain is present 

in the log, and diagonal grain is developed by sawing 

crooked logs or sawing parallel to the pith rather than to 

the bark. 

The amount and character of warp depends on the slope of 

the grain and the location and size of the areas of cross grain. 

Bow, crook, or twist may also be caused by the presence of 

bands of compression, tension, or juvenile wood that has 

abnormal longitudinal shrinkage. Wood near the pith, which 

may contain juvenile or compression wood, often has 

abnormal longitudinal shrinkage. This is a good reason for 

boxing the heart in sawing so that the pith does not fall on 

one edge or face. Juvenile wood in the inner portions of 

mature trees is lighter in weight than the more mature wood 

nearer the bark and shrinks more along the grain. Consequently, boards containing both types of wood tend to warp 

during drying. 

Kink is observed in softwood dimension lumber where a knot 

or the grain distortion that surrounds a knot is located on one 

edge near the middle of the piece. The two portions of the 

piece are essentially flat and straight. Sometimes the sharp 

bend in the faces of a board caused by an offset sticker and 

considerable superimposed loading is also called a kink. 

Warp Reduction 

Warping is caused by the differences in shrinkage in the 

three grain directions and because of irregular and distorted 

grain. Warping can be minimized by following good 

stacking and piling practices. The tiers of stickers and other 

supporting members, such as foundation cross beams and 

bolsters, should be in good vertical alignment. Protecting the 

boards in the top course of a pile with a roof reduces exposure to the weather with its alternate wetting and drying 

cycles. Good sawing practices produce boards of uniform 

thickness, which is a desirable quality for stacking and drying 

uniformity. 

Figure 53—Various types of warp in lumber. 

Table 11—Warping tendency of various woods

Low Intermediate High 

Hardwoods 

Alder Ash Beech

Aspen Basswood Cottonwood 

Birch, paper, sweet Birch, yellow Elm, American 

Butternut Elm, rock Sweetgum 

Cherry Hackberry Sycamore 

Walnut Hickory Tanoak 

Yellow-poplar Locust Tupelo 

Magnolia, southern 

Maples 

Oaks 

Pecan

Willow 

Softwoods 

Cedars Baldcypress 

Pine, ponderosa Douglas-fir 

Pine, Sugar Firs, true 

Pine, white Hemlocks 

Redwood Larch, western 

Spruce Pine, jack 

Pine, lodgepole 

Pine, red 

Pine, Southern 
49

The more refractory (that is, difficult to dry) woods like oak 

and beech are very difficult to air dry during periods of good 

drying weather without surface checking. These failures 

result when the tension forces across the grain tend to localize at the small fractures created by the sawing process. 

Surfacing the green wood removes the sawing fractures, and 

the likelihood of surface checking in air drying is reduced. 

By presurfacing lumber to a uniform thickness, not only is 

the usable volume of a unit package increased, but the 

amount of warp present is reduced. 

Stacking, Package Piling,

and Pile Protection 

The volume and grade reduction as a result of poor stacking, 

improper piling practices, and inadequate pile protection can 

be appreciable. Warp is aggravated, and volume losses as a 

result of end checks, end splits, and surface checking are 

increased. 

Stacking Effects 

Lumber of uniform thickness and trimmed for length can be 

stacked in such a way that warp is restrained. As a result of 

poor sorting, stacking, and piling practices, however, good 

grade and volume recovery might not be attained. In stacking, even dry stickers may vary in thickness. When associated with poor sticker tier alignment, the introduced warp 

may be significantly increased. Perhaps length sorting cannot 

be justified, requiring that the packages be box piled. Insufficient sticker tiers and carelessly locating them may further 

fail to provide adequate board end support. Along with careless board placement, this further invites warp (Fig. 54). 

When long boards for the edges of the box piled courses are 

not readily available, shorter boards are often laid down, 

causing sticker overhang. If the space is not blocked, sag can 

be created in the courses of lumber above. This is a problem 

of greater concern in stacking the narrower unit packages. 

When stickered packages of lumber, particularly 25-mm 

(1-in.) softwoods of the less dense species, are transported by 

forklift trucks, two extra tiers of stickers are often included. 

Such tiers, located so they will be above the pickup points of 

the fork, are often installed six to eight layers from the bottom. The package is usually stiffened enough so that the end 

stickers will not fall out during transport. Carrier bunks 

should be spotted at these extra sticker tiers to minimize the 

sag in the bottom lumber layers of the package when the 

package is transported or temporarily stored (Fig. 55). 

Package Piling Effects 

Packages are usually piled one above the other two, three or 

four packages high, depending on the height of the packages. 

Thus, the lower packages are weighted by the upper ones. 

Any failure of the pile foundation, or the bolsters between the 

packages, to adequately support the load will warp the lumber. Green lumber can be bent and, if it dried while bent, the 

bends become fixed and are permanent. Where the number 

of cross beams in the foundation does not adequately support 

the pile at the sticker tiers, the lower courses are likely to 

sag. This introduced bow can result in appreciable grade 

reduction. 

Weathering Effects 

When piles are not covered, the upper layers are exposed and 

defects can develop that cause value losses as a result of 

grade reduction and volume losses in ripping and trimming to 

maintain grade. The economic effect is particularly notable in 

the upper grades of hardwoods and finish grades of softwoods. A rain tight roof with overhang on sides and ends will 

prevent excessive moisture regain and reduce the overall 

drying time in the yard. Where the output of air-dried lumber 

per square meter of yard area must be fairly high and production fairly constant, prevention of moisture regain during 

rainy seasons of the year is a necessity. In this case, good 

prefabricated pile covers are economical. The prevention of 

alternate wetting and drying also reduces grade losses caused 

by general weathering, mainly surface checking and warp. 

The cost of the pile roof construction, placement, tiedown, 

storage when not in use, and maintenance may justify designing and installing drying sheds, particularly for high value 

lumber. 

Figure 54—Example of poor sticker alignment. 
50

Figure 55—Forklift stickers in the lower layers of each 

package avoid damaging the lower layers when picked 

up by a forklift. 
51

Chapter 7

Protection of Air-Dried Lumber 

When lumber is as dry as can be expected in the air-drying 

yard, it should be stored where it is no longer exposed to 

wind, sun, and rain. The deteriorating effect of weathering 

continues as long as the lumber is left outdoors. When stickered lumber is taken down and solid stacked, it takes up less 

space and a shed can provide the needed protection. When 

yard space is needed for air drying lumber, less favorable 

drying areas are often set aside for bulk storage of lumber 

that has already been air dried. 

The seasonal changes that influence the air-drying potential 

of an area also affect the conditions of dry lumber storage. 

However, significant moisture changes in air-dried lumber 

result mainly from rains that rewet the lumber. If good, raintight pile roofs with adequate projections were used during 

drying or if the lumber were shed dried, the needed protection from adverse weather was provided. 

Lumber that has reached 20% moisture content or less is 

usually no longer threatened by the development of stains or 

fungus and insect attack. Consider storing and protecting this 

lumber without additional drying. 

Outdoors

The air-drying process is, in effect, outdoor lumber storage. 

The inventory is handled in such a way that drying takes 

place during the storage period. But the inventory period in 

the air-drying yard should be no longer than that required to 

reduce the moisture content to a level suitable for use, kiln 

drying, or shipment. Then, it becomes a matter of protecting 

the air-dried lumber against rewetting. This protection is 

especially important if the lumber is of high grade and the 

storage period is likely to be lengthy. Roofs should now be 

provided if they were not when the pile was first erected. The 

lumber may be left on sticks, or piles, and packages may be 

taken down and the lumber bulk piled for more efficient use 

of space. 

If the average moisture content of the lumber is greater than 

20%, it should be protected in such a way that drying can be 

continued, as under a roof or in a shed. It should not be bulk 

piled because of the hazards of stain and decay development. 

When the moisture content of the lumber is less than 20%, 

the piles or packages may be wrapped with plastic tarpaulins 

or enclosed in prefabricated waterproof wrappings for temporary outdoor storage. 

Foundations in the storage area should provide good support 

and ground clearance just as in the original air-drying pile. 

Support is still essential to prevent pile sagging. Ground 

clearance is not only needed to provide room for forklift 

operations but for ventilation to keep moisture regain in the 

bottom layers to a minimum. If the water table in the ground 

is high, a ground cover to provide a barrier to moisture vapor 

movement out of the soil may be desirable, particularly if 

the storage period is extended. Precautions should be taken 

to protect the lumber against infestation by termites and 

other insects. 

Indoors

Sheds can provide the indoor storage of air-dried lumber. 

The advantage of shed storage is the protection provided by 

the permanent roof. The air-dried lumber is stored in the shed 

either as stickered unit packages, as bulk-piled lumber in unit 

packages, or bulk piled in bins or bays. The sheds can be 

open, closed, or closed and heated; they may or may not be 

floored or paved. The design, size, and materials of shed 

construction will vary depending upon plant arrangement, 

storage volume requirements, and the cost of locally available building materials. 

Open-Shed Storage 

An open lumber storage shed is one that usually does not 

have sides or ends to block air movement through the building. Sheds with louvered walls are considered open sheds 

(Fig. 56). Wind-blown rain is baffled, a distinct feature of 

this type of shed. If the shed is not floored or paved, foundations for stickered or bulked packages of air-dried lumber 

will be necessary to provide ground clearance for ventilation. 

When hand-built piles in the yard are taken down, the lumber 

is often bulk piled in bins or bays in the “rough dry shed.” In 

a forklift yard operation, the stickered unit packages or packages of bulked lumber are stored in the open shed. The 

height of the package pile is governed by the height of the 

shed. Lumber storage volume can be increased by having a 

forklift available to place carrier packages two, three, or 

more units high, depending upon the height and construction 

of the shed. 
52

Closed, Unheated Storage 

A closed, unheated shed for the storage of air-dried lumber 

differs from an open shed in that the sides and ends of the 

buildings are walled, blocking air movement through the 

structure (Fig. 57). Doorways and other access openings for 

the transport of lumber in and out of the shed are usually kept 

closed when activity is not great or the sawmill or factory is 

not in operation. Ventilators are sometimes installed on the 

roofs of these buildings, but the air circulation within the 

shed remains virtually stagnant. Partially dry, stickered units 

of lumber can be stored in a closed shed but, if further drying 

at a reasonable rate is wanted, the air movement through the 

stickered package must be stimulated by introducing forced 

air circulation with fans. 

As air movement in and out of a closed shed is restricted by 

the solid walls, some heating of the air in the building results 

from solar radiation. If the roof and walls exposed to sunshine are dull black, these structural elements are heated 

when the sun shines and the air inside the building in contact 

with these heated elements is warmed. The air in the upper 

space of the building is warmer than the air at the ground 

level and, if warmed air is moved downward by fans, the 

EMC conditions of lumber storage are lowered. In a closed 

shed, air-dried lumber is more certain to remain dry and 

partially air-dried lumber in stickered unit packages will 

dry faster. 

If the closed shed is not floored or paved, foundations must 

be provided for the unit packages or the lumber in the bins in 

order to create a space between the ground and the lumber. 

The walls of the building are often open near the ground 

level in a closed shed to create air drift or ventilation under 

the lumber. These ground-level wall openings can be 

screened or louvered to discourage entry of birds and small 

animals into the shed. 

Closed, Heated Storage 

A closed, heated shed is most often utilized to store lumber 

that was kiln dried to moisture content levels lower than can 

usually be attained in conventional air drying. However, such 

a shed might sometimes be used to dry stickered unit packages of air-dried lumber because the systems used to heat the 

closed shed involve forced air circulation with fans. 

The closed, heated storage shed is usually floored or paved. 

The unit packages of lumber, or bulked lumber in bins, 

should not be in direct contact with a paved floor, however, 

because some moisture could migrate through the paving. 

The warmed air being circulated in the shed should be allowed to move under the stored lumber unit. In most instances, the bunks used in carrier or forklift operations provide adequate elevation of the lumber. The temperature 

maintained in a closed, heated lumber storage shed are not 

very high and insulated walls, roofs, or ceilings are seldom 

installed. 

The wood equilibrium moisture content (EMC) conditions in 

a heated storage shed depend upon the outdoor temperature 

and relative humidity and the temperature of the circulated 

air within the shed. When outdoor air is heated, the absolute 

humidity remains the same but the relative humidity is reduced. Thus, the wood EMC of outdoor air is reduced when 

the air is heated. Figure 58 shows the effect on EMC of 

increasing inside temperature above outside temperature. 

The values shown in Figure 58 apply to all outside temperatures from −1o

C to 32oC (30oF to 90oF). For example, if 

outside EMC is 14%, Figure 58 indicates that a temperature 

increase of 36o

C (20oF) reduces EMC in the heated shed 

from 14% to 7%. As another example, if during winter 

months the outdoor temperature averages 4o

C (40oF) and 

75% relative humidity, the wood EMC is about 15% 

Figure 56—Open shed with louvered side wall helps 

protect lumber from wind-blown rain. 

Figure 57—A closed, unheated lumber storage shed. 
53

(Table 2). To what temperature should the shed be heated 

to reduce the EMC to 12%? By interpolating between the 

14% and 16% outdoor EMC curves in Figure 58, we see a 

temperature increase of about 2.8o

C (5oF ), so the shed 

should be heated to 7o

C (45oF) to maintain an EMC of 12%. 

Air-dried lumber in storage does not require much heating of 

the ambient air to prevent moisture regain, and it is quite 

possible that in many areas of the country solar radiation will 

provide the needed heat energy. Circulation of the air within 

a closed, heated lumber storage shed by means of fans is 

necessary regardless of how heating is accomplished, 

particularly in a solar-heated system. The fans move the air 

across the warmed roof and walls and mix this warmed air 

with that being circulated around the dry, stored lumber. 

Closed sheds at millwork and furniture plants are often 

equipped with unit heaters complete with fans. Usually these 

unit heaters are supplied with steam from a central plant, but 

individual gas burning units are sometimes used. The heaters 

are located in such a way that the warmed air is directed 

toward the stored lumber. The conventional temperature 

control of a unit heater is a thermostatic control of the fan 

motor. When the thermostat calls for heat, the unit heater fan 

motor is started and air is forced through the finned coils of 

the heater. Forced air circulation in the shed is desirable at all 

times; therefore, the unit heater fan motor can be kept in fulltime operation and the temperature controlled by installing a 

motor valve on the steam or gas supply for “on and off” 

operation. This control mechanism is more expensive to 

install than a thermostatically controlled fan but it does ensure full-time fan operation to keep the warmed air in the 

heated shed from stagnating and stratifying. 

Temperature control in a closed, heated shed for dry lumber 

is mainly a problem of controlling wood EMC conditions at a 

predetermined value; therefore, the most simple control 

system is a humidistat that controls the shed temperature. By 

controlling the relative humidity in a heated shed, the wood 

EMC condition is controlled closely enough for most practical purposes. For example, from Table 2, the relative humidity that establishes a 12% EMC condition is about 65% over 

a fairly wide range of temperatures. By setting the humidistat 

at 65% relative humidity, the indoor air will be heated to 

decrease the indoor relative humidity to this level. The advantage of a humidistat control installation on the unit heater 

is that, with changing outdoor temperature and relative humidity, the wood EMC in the heated shed remains substantially constant. With humidistat control, the fans on the unit 

heaters should operate continuously and the humidistat 

should control the steam or gas input into the heater. 

The hazards of stain and decay developing in air-dried lumber stored in a heated shed are significantly reduced. However, partially air-dried lumber should be left on stickers so 

that drying can continue in storage. 

In Transit 

Air-dried lumber is shipped from the producing sawmills by 

railroad and truck to the factories where it may be kiln dried 

to a lower moisture content for final processing. However, 

the air-dried rough lumber may be kiln dried and processed 

to some extent at custom kiln-drying companies who carry 

out these services on an “in transit” basis. When the producer 

ships air-dried lumber, either by railroad or by truck, protection from the weather must be provided. 

20

18

16

14

12

10

8

6

4

EMC (%)

Degrees above ambient (°F)

0 5 10 15 20 25 30

10%

14%

16%

20

18

16

14

12

10

8

6

4

EMC (%)

Degrees above ambient (°C)

0 3 6 9 12 15 18

10%

12%

14%

16%

(b)

(a)

12%

18%

Outdoor EMC=20%

Outdoor EMC=20%

18%

Figure 58—EMC of wood when air in an enclosed 

space is heated greater than the temperature of 

the outside air. 
54

Railroad

High value, air-dried lumber is sometimes shipped to woodusing factories in boxcars. The lumber is thus protected from 

the weather and deterioration is not likely to occur. The labor 

costs of loading and unloading have affected the volume of 

shipments transported this way. To facilitate loading and 

unloading, boxcars are available with wide doors so that 

unitized packages of bulk-piled lumber can be moved in and 

out of the boxcar with lift trucks. Air-dried lumber is also 

shipped in gondola cars. The bulk-piled lumber may be 

loaded onto the gondola cars by hand, or unit packages can 

be loaded onto the car by crane or by a lift truck equipped 

with special slings. The unit packages of bulk-piled lumber 

are individually strapped, and the completed gondola load is 

strapped or chained in such a manner that the lumber cannot 

shift during rail transit. Protection from the weather is 

seldom provided. 

Flatcars are becoming widely used for the transport of airdried lumber because these cars can be quickly loaded and 

unloaded by a forklift (Fig. 59). When the carload is in place, 

it is anchored or bound to the car by numerous cables that are 

thrown over the load and tightened with ratchet load binders. 

Metal corner protectors may be used to prevent cable damage 

to the edges of the boards in the top layer of the top packages 

in the load. Flatcars with A-frames in the longitudinal center 

are also available, and the advantage is increased sidewise 

stability of the flatcar load. 

Truck

Considerable quantities of air-dried lumber are shipped by 

truck from the producing sawmills to the using factories or 

custom kiln-drying plants. Tractor trailer units are usually 

used for this purpose, and in most instances, the trailer is a 

flatbed unit that can be loaded and unloaded by a lift truck. 

The load of lumber is anchored to the trailer by chains tightened with load binders. Air-dried lumber should be protected 

by covering the load with canvas tarpaulins (Fig. 60). 

Figure 59—Flatcar loaded with lumber and held in 

place with cables. 

Figure 60—Tarpaulin protection on a trailer load

of air-dried lumber. 
55

Appendix

Summarized Guide

for Air-Drying Practices 

Air-Drying Yard 

Site 

High, well-drained ground with no obstructions to prevailing 

winds provides the greatest drying potential. Transportation is 

facilitated if the yard is located close to the sawmill or woodworking factory. 

Yard Layout 

Roadways of main and cross alleys divide the yard into 

rectangular blocks or areas for the lumber piles. The rowtype forklift yard has main alleys at least 7.3 m (24 ft) wide 

with cross alleys that may be 18 m (60 ft) wide. In a line-type 

forklift yard, an 18-m (60-ft) cross alley might be the principal roadway from the sawmill or woodworking factory. The 

lines of piles and the main alleys are placed at right angles to 

the cross alley. Yard boundaries or the need for cross alleys 

for insurance or fire fighting purposes determines the length 

of the lines. The hand-built pile yard is usually laid out in 

blocks of main alleys with a rear alley between each two 

rows of piles. The main alleys can be relatively narrow, but 

the blocks are often separated by wide cross alleys that could 

also be fire lanes. 

Yard Orientation 

Main alleys often run north and south to obtain faster roadway drying after rains or faster snow and ice melt. In forklift 

yards, the main alleys are sometimes laid out parallel to the 

prevailing wind to improve air movement in the unit 

packages. 

Row Spacing 

In a row-type yard, the row space needs to be wide enough to 

enable the forklift operator to maneuver and place the packages on the pile. A 1-m (3-ft) spacing between rows is recommended. A closer spacing might be practical in a paved 

yard. A “side shifter” on the forklift truck will make unit 

package placement much easier for the forklift driver. 

Line Spacing 

The space between the two lines in a line-type yard varies, 

but 0.6 m (2 ft) is considered a minimum. The two lines of 

permanent foundations need to be separated by this much 

space or more to ensure unobstructed air drift between 

the lines.

Pile Width 

Narrow packages of lumber tend to tip, often requiring long 

tie bolsters between piles. Wider unit packages minimize the 

possibility of tipping, and some hardwood operations are 

piling 2.4-m (8-ft) packages successfully. Hand-built piles of 

hardwoods are usually 2.4 m (8 ft) wide. Self-stickered softwood piles vary from 2.4 to 5 m (8 to 16 ft) wide, depending 

upon the sorted length of the lumber being stacked. 

Pile Height 

Although the pile height in a forklift yard is limited mainly 

by safety issues, a standard practice is to pile four unit packages on a foundation, making a pile height of about 6 m 

(20 ft). Higher piles of unit packages are not recommended. 

The hand-built pile, if built up from the ground, is usually 

about 5 m (15 ft) high, but those erected alongside an elevated dock can be as much as 9 m (30 ft) high. Compression 

of stickers and the indentation of the lumber in the lower 

courses because of superimposed loading may require using 

wider stickers in building these high piles. 

Pile Spacing 

The recommended minimum pile spacing in a row-type 

forklift yard is 0.6 m (2 ft). The pile spacing in the row can 

be varied to open up the yard when necessary. The space 

between ends of piles in a line-type forklift yard can be 

0.3 m (1 ft) or less, depending upon whether the forklift truck 

is equipped with a side shifter. The recommended space 

between piles in a hand-built yard is about 1 m (3 ft). 

Pile Placement 

In a row-type forklift yard, the rows are placed perpendicular 

to the main alley. The length of the row is determined by the 

inventory. In a line-type forklift yard, the lines are parallel to 

the main alleys. Hand-built piles are placed perpendicular to 

the main alley. 

Yard Transportation 

For easy and rapid handling, motorized transport equipment 

is used to move lumber from the sorter to the yard or from 

the stacker to the yard. 
56

Pile Foundation 

Wood foundation members are generally used for row-type 

forklift yards and hand-built pile yards. Preservative treatment of mudsills, posts, and piers is justified. When concrete 

piers are used to support stringers or cross beams, their 

footings should be below frostline. Concrete piers supporting 

railroad rails are frequently used in line-type forklift yards. 

The minimum foundation height for forklift yards is 200 mm 

(8 in.), and the preferred design allows free air movement in 

any direction under the pile. 

Pile Protection 

Sun and rain-tight pile roofs protect high value lumber from 

weathering. Sun shields or end coatings reduce footage losses 

caused by end checking and end splitting in thicker lumber 

items. Wind barrier tarpaulins can minimize surface checking 

when they are used to cover piles on the windward side of a 

forklift yard during periods of hot, dry winds. 

Stacking and Piling Lumber

Sorting 

Drying time varies with lumber thickness, and yard output is 

greater if thickness segregations are made for stacking and 

selected yard placement. Sorting by length is also recommended. Stacking of lumber sorted for length reduces grade 

losses caused by warp. Sorting by species is generally 

essential.

Sorting Equipment 

Smaller sawmills and woodworking plants will set up a 

conventional green chain or other mechanical or 

semimechanical means to sort lumber for stacking. At larger 

sawmills, mechanical sorting equipment such as edge, drop, 

or tray sorters can reduce lumber handling costs. 

Stacking of Lumber Into Unit Packages 

Sticker guides on stacking stalls or mechanical stackers 

ensure good sticker alignment. Mechanical stackers provided 

with an “even-ender” produce square ends on the packages. 

When random-length lumber is stacked, box piling is 

recommended. 

Board Spacing 

Edge-to-edge stacking of both softwoods and hardwoods is 

practical for unit packages 1.2 m (4 ft) or less in width. For 

wider packages of random-width hardwoods, board spacing 

may be necessary to minimize blue stain. Flues or chimneys 

are built into wide unit packages of stain-prone softwoods to 

promote better air circulation and minimize stain.. 

Package Height 

The load-carrying capacity of the lift truck often determines 

the height of a unit package. A package height of about 1.2 m 

(4 ft) is recommended as a general standard for most yarding 

operations. 

Bolster Size 

Small wood timbers 100 by 100 mm (4 by 4 in.) in cross 

section and as long as the unit package width support the 

entire package and allows for entry of the forks of the lift 

truck. 

Stacking Lumber in Hand-Built Pile Yards 

Random-width hardwood boards should be spaced. Very 

wide piles may require flues or a chimney in addition. Random-length hardwood boards will warp less if they are box 

piled. Chimneys or several flues are recommended in building sloped and pitched piles of softwood lumber. Board 

spacing between flues is usually not essential. Board spacing 

in addition to a large central chimney is recommended for 

self-stickered piles of softwood lumber. 

Stacking Stickers 

Stickers processed from high grade dry and surfaced heartwood lumber give longer and satisfactory service. Stickers 

should be uniform in thickness and stored under cover when 

not in use. 

Control of Drying Defects 

Chemical Brown Stain 

Treating green white pines with enzyme inhibitors is effective for preventing chemical brown stain. Sawn white pine 

lumber needs to be stacked promptly and subjected to good 

air-drying conditions as soon as possible after stacking. 

Sticker Marking 

Use dry stickers. The sawn lumber should be stacked and 

promptly subjected to good air-drying conditions. 

Blue Stain, Mold, and Decay 

Treatment of the green lumber with a suitable fungicide is 

recommended. Tight pile roofs prevent rain from wetting the 

lumber. Good yard sanitation practices are essential to keep 

sources of infection to a minimum. Good air movement and 

rapid drying helps reduce stain and mold. 

Insect Infestation 

Logs and lumber may require spray treatment with a suitable 

insecticide. Insect-breeding places need to be cleaned up 

or destroyed. 
57

End Checks and Splits 

Freshly trimmed ends of high value lumber thicker than 

32 mm (1-1/4in.) should be end coated to minimize end 

checks and splits. Sun shields may be more practical in 

some situations. 

Surface Checks 

Presurfacing check-prone species like oak and beech can be 

considered if grade losses as a result of surface checking are 

serious. Package and pile buildup can be modified to retard 

air movement. Pile spacing can be smaller during periods 

when the drying potential is severe. Wind baffles may be 

necessary at times. 

Honeycomb 

Precautions to prevent the development of end checks and 

surface checks are recommended. Honeycomb in air-dried 

hardwoods is generally caused by the extension of end 

checks and the penetration and extension of surface checks. 

Warp 

To prevent warp, good stickering and piling practices are 

recommended. Uniform-sized stickers, close sticker spacing, 

and good alignment are essential. Firm and straight foundations with supporting members at all sticker tiers are necessary. Improved thickness control in sawing or presurfacing 

may be required if miscut lumber significantly contributes to 

warp development. Clamping or superimposed loading may 

be a practical remedy for warp-prone species. 

Cost Reduction 

Degrade Studies 

Investigations to evaluate losses in grade and footage caused 

by drying defects can point out where improvements are 

needed. Changes in stacking and piling practices may be 

justified to reduce defects such as surface checking and warp. 

The treatment of green lumber with a fungicide to control 

blue stain may be profitable. 

Drying Time Reduction 

As overall air-drying costs increase with extended yard time, 

ways and means should be explored to decrease the time that 

a unit of lumber spends on the yard. A lower inventory being 

air dried may be warranted to reduce the average drying time. 

Labor Cost Reduction 

The switch from hand-built pile yarding methods to forklift 

yarding operations results from a need to reduce labor costs. 

Package handling methods allow increased use of mechanical 

equipment for stacking, transport, and piling. 

Protection of Air-Dried Lumber 

Outdoor Storage 

If air-dried lumber is at a moisture content of 20% or less, it 

can be bulk piled. However, protection from rain is essential 

and rain-tight roofs or tarpaulins can be used. Unit packages 

of bulk-piled lumber can be wrapped with waterproof paper 

for temporary storage. If lumber is at a moisture content of 

more than 20%, air drying in place can be continued. 

Indoor Storage 

If air-dried lumber is at a moisture content of 20% or less, 

bulk piling indoors in unit packages or bins is satisfactory. If 

the lumber is at a high moisture content, the stickered unit 

packages are stored without dismantling and lumber coming 

from hand-built piles is stickered. If the shed is not floored, 

unit package and bin foundations are required to provide 

ground clearance. Stickering lumber of high moisture content 

stored in bins would be a good precaution. Circulating air in 

closed sheds by fans is recommended. Humidistat control is 

recommended for closed, heated sheds. 

Truck Transit 

Tarpaulin protection from rain is often used for tractor-trailer 

transport of air-dried lumber. 

Railroad Transit 

Cars of bulk-piled strapped packages of air-dried lumber are 

not protected from the weather. If rains during shipment wet 

the lumber, prompt stickering for air or kiln drying is 

recommended. 
58

Glossary 

Active drying period. A time or season of the year when 

weather conditions are such that yarded lumber loses moisture rapidly. 

Air dried. The dried condition of lumber, usually 12% to 

20% moisture content, reached by exposing the lumber 

for a sufficient period to the prevailing outdoor weather 

conditions. 

Air drying. Syn. Air seasoning. The process of drying green 

lumber by exposure to prevailing atmospheric conditions 

outdoors. 

Alleys, cross. The passage ways that connect main alleys in 

an air-drying yard. 

Alleys, main. Syn. Runways. The roads in an air-drying yard 

for the transport of lumber. 

Alleys, rear. The space between the backs of rows of handbuilt piles in an air-drying yard. 

Annual growth ring. The growth layer put on a tree each 

year in temperate climates, or each growing season in other 

climates. Each ring includes earlywood and latewood. 

Anti-stain chemical. A chemical applied to lumber to prevent or retard chemical or fungus stain development. 

Blue stain. See Stain, blue. 

Board. (1) Yard lumber that is less than 51 mm (2 in.) thick 

and 25 mm (1 in.) or more wide. (2) A term usually applied 

to 25- mm- (l-in.-) thick lumber of all widths and lengths. 

Board foot. A unit of measurement equal to a board 305 mm 

(1 ft) long, 305 mm (1 ft) wide, and 25 mm (1 in.) thick. In 

finished or surfaced lumber, the board foot measure is based 

on the measurement before surfacing or other finishing. 

In the lumber industry, the working unit is 1,000 board 

feet = 2.4 m3

.

Bolster. Syn. Spacer. A square piece of wood usually 102 by 

102 mm (4 by 4 in.) in cross section, placed between stickered packages of lumber to provide space for the entry and 

exit of the forks of a lift truck. Bolsters need to be aligned 

with the stickers in the lumber packages. 

Box piling. Method of stacking random length lumber so that 

the length of the outside boards in each course is equal to the 

full length of the stack. 

Bound water. Syn. Adsorbed moisture, hygroscopic moisture. Moisture that is intimately associated, by weak chemical 

bonds, with the finer wood elements of the cell wall. 

Bow. Syn. Camber. A form of warp in which a board 

deviates from flatness lengthwise but not across the faces. 

Boxed heart. Syn. Boxed pith. When the pith falls entirely 

within the outer faces of a piece of lumber anywhere in its 

length, it is said to contain boxed heart. 

Bright. Syn. Unstained. The term is applied to lumber that is 

free from discolorations. The term bright sapwood is sometimes used to describe sapwood of natural color or in which 

the stain or discoloration can be removed by surfacing to 

standard thickness. 

Brown stain. See Stain, chemical brown. 

Bulk pile. Syn. Solid pile. The stacking of lumber into unit 

packages or into bins without vertical spaces or stickers 

between the layers. 

Bunk, carrier. Specially designed wood beams on which 

lumber is placed, enabling the straddle truck or carrier to 

pick up the unit for transport. 

Cambium. The one-cell-thick layer of tissue between the 

bark and wood that repeatedly subdivides to form new wood 

and bark cells. 

Canal, resin. Syn. Resin duct. Intercellular passages that 

contain and transmit materials, extending vertically or 

radially in a tree. 

Case hardening. A condition of stress and set in dry lumber 

in which the outer fibers are under compressive stress and the 

inner fibers under tensile stress. 

Cell. A general term for the minute units of wood structure 

having distinct cell walls and cell cavities including wood 

fibers, vessel segments, and other elements of diverse structure and function. 

Check. Syn. Drying check, checking. A separation of the 

wood fibers on any surface of a log, timber, or lumber resulting from tension stresses set up during drying, usually the 

early stages of drying. 

Check end. A failure usually in the plane of the wood rays 

on the end grain surface of a log, timber, or board resulting 

from stresses caused by too rapid or excessive end drying. 

Check internal. See Honeycombing. 

Check, surface. A check occurring on the tangential surface 

of a board and extending across the annual growth rings into 

the interior. 

Chemical brown stain. See Stain, chemical brown. 
59

Chimney. See Flue. A vertical space, more than 152 (6 in.) 

wide, in the center of a hand-built pile extending the length 

and depth of the pile, intended to facilitate air circulation in 

the pile. 

Collapse. Syn. Washboarding, crimping. A corrugated appearance of the surface of a piece of wood caused by an 

irregular drawing together of the cell walls as free water 

leaves the cavities; occurs only in the early stages of 

drying green heartwood of low permeability, especially 

quartersawn stock. 

Common lumber. A broad grade of lumber usually including several subgrades, applied to both hardwoods and 

softwoods. 

Compression failure. Deformation of the wood resulting 

from excessive pushing together of the fibers along the grain. 

It may develop in standing trees as a result of bending by 

wind or snow or as a result of internal longitudinal stresses 

imposed during felling. In surfaced lumber, compression 

failures appear as fine wrinkles across the face of the piece. 

Compression wood. See Reaction wood. Abnormal wood 

formed on the lower side of branches and inclined trunks of 

softwood trees. As seen on the cross-section surfaces of a 

branch or stem, it appears as relatively wide, eccentric 

growth rings with little or no demarcation between earlywood and latewood and more than normal amounts of latewood. Compression wood shrinks more longitudinally than 

does normal wood. 

Correction, temperature. An adjustment of the readings of 

the resistance-type electrical moisture meter to compensate 

for changes in the temperature of the wood. 

Course. Syn. Layer. A single layer of lumber of the same 

thickness in a stickered pile or package. 

Crib pile. Stacking lumber to usually form a hollow triangle. 

The boards lap at the corners. 

Crook. A form of warp in which a board deviates edgewise 

from a straight line from end to end. 

Cross break. A separation of the wood cells across the grain 

caused by internal strains resulting from the unequal longitudinal shrinkage commonly associated with reaction wood. 

Cross section. See Section, cross. 

Cup. A form of warp in which there is a deviation from 

flatness across the width of a board. 

Decay. Syn. Rot. The decomposition of wood substance 

by fungi. 

Defects. Any irregularity or imperfection in a tree, log, bolt, 

or lumber that reduces its volume or quality or lowers its 

durability, strength, or utility value. Defects may result from 

knots and other growth conditions and abnormalities; from 

insect or fungus attack; from milling, drying, machining, or 

other processing procedures. 

Diagonal grain. See Grain, diagonal. 

Dipping. Process of submerging lumber in a dipping vat 

containing fungicides or other chemicals to prevent stain or 

decay. 

Discoloration. Change in the color of lumber as a result of a 

fungus, chemical stain, or weathering. 

Drying. Syn. Seasoning. The process of removing moisture 

from lumber to improve its serviceability in use. 

Earlywood. Syn. Springwood. Wood formed during the 

early period of annual growth; usually less dense than wood

formed later. 

Edge piling. Stacking of wood products on edge so that the 

broad face of the item is vertical; usually done to restrain 

crook. 

Edge-to-edge stacking. Syn. Close piling. In stacking stickered unit packages for air drying, placing the edges of each 

item in the layer against the adjacent pieces. 

Electrodes. In testing wood for moisture content, devices 

made of electrically conducting material, usually steel pins, 

for connecting wood into the electric circuit of an electric 

moisture meter. 

Electrodes, insulated. In testing wood for moisture content, 

special electrodes for use with resistance-type electric moisture meters that are coated with an insulating material to limit 

or control the point of contact between the electrode and the 

wood. 

End coating. A coating of moisture-resistant material applied to the end grain surfaces of green lumber to retard end 

drying and consequently checking and splitting. 

End pile. Stacking of green lumber on end, and inclined in a 

long, fairly narrow row; the layers are separated by stickers. 

End racking. Placing of boards on end against a support to 

form a pile in the shape of an “X” or an inverted “V.” 

End splitting. See Split, end. 

Equilibrium moisture content. Abbr. EMC. The moisture 

content at which wood neither gains nor loses moisture when 

surrounded by air at a given relative humidity and temperature.

Extractives. Substances in wood, not an integral part of the 

cellular structure, that can be removed by hot or cold water, 

ether, benzene, or other solvents that do not react chemically 

with a wood substance. 

Fiber. A general term for a long narrow cell. 

Fiber saturation point. Abbr. fsp. The stage in the drying of 

wood at which the cell walls are saturated with water and the 
60

cell cavities are free of water. It is usually considered to be 

approximately 30% moisture content, based on the weight of 

the wood when ovendry. 

Flat pile. Stacking of lumber so that the broad face of the 

item is horizontal. 

Flat pile, sloped. A pile of stacked lumber on foundations 

that are not level, but deviate from the horizontal. The slope 

is usually lengthwise of the board and may be 25 mm or 

more per 0.30 m (1 in. or more per foot) of length. 

Flatsawn. See Grain, flat. 

Flue. See Chimney. Vertical spaces, 152 mm (6 in.) or less in 

width in the pile and extending the length and depth of the 

pile, intended to facilitate circulation of air within the pile. 

Foundation, open. Structural supports for the air-drying pile 

designed to facilitate air circulation under the pile. 

Free water. Syn. Capillary water. Water that is held in the 

cell cavities of wood. 

Fungi. Low forms of plants consisting mostly of microscopic 

threads that traverse wood in all directions, dissolving out of 

the cell wall materials they use for their own growth. 

Grade. A classification or designation of the quality of logs 

or manufactured lumber. 

Grain. The direction, size, arrangement, appearance, or 

quality of the fibers in lumber. 

Grain, cross. Lumber in which the fibers deviate from a line 

parallel to the sides of the piece. Cross grain may be either 

diagonal or spiral grain or a combination of the two. 

Grain, diagonal. Lumber in which the annual rings are at an 

angle with the axis of a piece as a result of sawing at an angle 

with the bark of the log. A form of cross grain. 

Grain, edge. Syn. Comb grain, edge sawn, quarter grain, 

quartersawn, rift grain, rift sawn, stripe grain, vertical grain. 

Lumber that has been sawn or split so that the wide surfaces 

extend approximately at right angles to the annual growth 

rings exposing the radial surface. Lumber is considered edge 

grained when the rings form an angle of 45o

 to 90o with the 

wide surface of the piece. 

Grain, end. The ends of logs or timbers, dimension, and 

boards that are cut perpendicular to the fiber direction. 

Grain, flat. Syn. Flatsawn, plain grain, plainsawn, slash 

grain, tangential cut. Lumber that is sawn or split in a plane 

approximately perpendicular to the radius of the log. Lumber 

is considered flat grained when the annual growth rings make 

an angle of less than 45o

 with the surface of the piece. 

Grain, spiral. A form of cross grain in lumber in which the 

fibers take a spiral course about the trunk of a tree instead of 

the normal vertical course. The spiral may extend in a righthanded or left-handed direction around the tree trunk. 

Grain, straight. Lumber in which the fibers and other longitudinal elements run parallel to the axis of a piece. 

Grained, close. Lumber with narrow, inconspicuous annual 

rings. The term is sometimes used to designate wood having 

small and closely spaced pores, but in this sense the term 

“fine textured” is more often used. 

Grained, coarse. Lumber with wide, conspicuous annual 

rings in which there is considerable difference between earlywood and latewood. The term is sometimes used to designate wood with large pores, such as oak, ash, chestnut, and 

walnut, but in this sense “coarse textured” is more often used. 

Hardwoods. Generally one botanical group of trees that has 

broad leaves, e.g., oak, elm, and basswood. Also, the wood 

produced from such trees. The term has no reference to the 

actual hardness of the wood. 

Heartwood. The inner layers of wood which, in the growing 

tree, have ceased to contain living cells and in which the 

reserve materials, e.g., starch, have been removed or converted into heartwood substances. It is generally darker 

in color than sapwood although not always clearly 

differentiated. 

High temperature drying. In kiln-drying wood, use of drybulb temperatures of 100o

C (212oF) or more. 

Honeycombing. Syn. Hollow horning, internal checking, 

interior checking, inner checking. In lumber, separation of 

the fibers in the interior of the piece, usually along the wood 

rays. The failures are often not visible on the surfaces, although they can be extensions of surface and end checks. 

Humidity, relative. The ratio of the vapor pressure of water 

in a given space compared with the vapor pressure at saturation for the same dry-bulb temperature. Under ordinary 

temperatures and pressures, it is the ratio of the weight of 

water vapor in a given space compared with the weight that 

the same space is capable of containing when fully saturated 

at the same temperature. 

Hygrometer. An instrument for measuring the dry- and wetbulb temperatures of the air, usually consisting of two 

thermometers. 

Infection. The invasion of lumber by fungi or other 

microorganisms. 

Infestation. The establishment of insects or other animals in 

lumber. 

Juvenile wood. Growth rings formed near the pith of the 

tree.

Kiln drying. The process of drying lumber in a closed 

chamber in which the temperature and relative humidity of 

the circulated air can be controlled. 

Kink. A form of warp in which there are sharp deviations 

from flatness or straightness as a result of exceptionally 
61

abrupt grain distortions, such as around knots, or the piece 

is sharply bent by misplaced stickers. 

Knot. That portion of a branch or limb that has been surrounded by subsequent growth of the tree. 

Latewood. Syn. Summerwood. The portion of the annual 

growth ring that is formed after the earlywood formation has 

ceased. Latewood is usually denser and stronger than 

earlywood. 

Lumber. The product of the sawmill and planing mill that is 

not further manufactured other than by sawing, resawing, 

passing lengthwise through a standard planing machine, cross 

cutting to length, and matching. 

Lumber, nominal size. As applied to lumber, the rough 

sawn commercial size by which it is known and sold in 

the market. 

Meter, electric moisture. An instrument used for rapid 

determination of the moisture content in wood by electrical 

means. 

Moisture content. The amount of water contained in the 

wood, usually expressed as a percentage of the weight of the 

ovendry wood. 

Moisture content section. A cross section approximately 

25 mm (1-in.) in length along the grain, cut from a board and 

used to determine moisture content by the oven-drying 

method. 

Mold. A fungus growth on lumber at or near the surface and, 

therefore, not typically resulting in deep discolorations. 

Ovendry. A term used to describe wood that has been dried 

in a ventilated oven at approximately 104o

C (220oF) until 

there is no additional loss in weight. 

Pile. Syn. Stack, rack. Stacking lumber layer by layer, separated by stickers or self-stickering, on a supporting foundation (hand stacking) or placing stickered unit packages by lift 

truck or crane, one above the other on a foundation and 

separated by bolsters. 

Pile pitch. The forward lean of the front face of a hand-built 

sloped pile of lumber. The pitch is usually approximately 

25 mm in 305 mm (1 in. in 12 in.) of pile height. 

Pile roof. Syn. Pile cover, cover boards, stack cover. A cover 

on top of the pile to protect the upper layers from exposure to 

the degrading influences of sun, rain, and snow. The sides 

and ends of the roof may project beyond the pile to provide 

added protection. 

Pile, roof. A method of flat stacking random-length lumber 

for air drying. Full-length boards are placed in the outer 

edges of each layer, and shorter boards are alternated 

lengthwise in between to produce square end piles or unit 

packages. 

Pitch pocket. Syn. Gum check. An opening extending parallel to the annual growth rings containing, or that has contained, resins, either solid or liquid. 

Pith. The small, soft core occurring in the structural center of 

a tree trunk, branch, twig, or log. 

Rays. A ribbon-like grouping of cells extending radially 

across the grain, so oriented that the face of the ribbon is 

exposed as a fleck on the quartersawn surface. 

Reaction wood. Wood with more or less distinctive anatomical characteristics, formed in parts of leaning or crooked 

stems and branches. In hardwoods, this consists of tension 

wood and in softwoods, compression wood. 

Refractory. Implies difficulty in processing or manufacturing by ordinary methods; resistance to the penetration of 

preservatives; difficulty in drying or difficulty in working. 

Resins. A class of amorphous vegetable substances secreted 

by certain plants or trees. 

Ring failure. Syn. Ring shake. A separation of wood along 

the grain and parallel to the annual rings, either within or 

between the rings. 

Rot. See Decay. 

Sap. The moisture in green wood and all that is held in 

solution. 

Sapstain. See Stain, blue. 

Sapwood. The outer layers of the stem that in the living tree 

contain living cells and reserve materials, e.g. starch. The 

sapwood is generally lighter in color than the heartwood. 

Section, cross. Syn. Transverse section. A section of a board 

or log taken at right angles to its longitudinal axis. 

Shake. See Ring failure. 

Shrinkage. The contraction of wood fibers caused by drying 

below the fiber saturation point. Shrinkage—radial, tangential, and longitudinal—is usually expressed as a percentage 

of the dimension of the wood when green. 

Softwoods. In general, one of the botanical groups of trees 

that, in most cases, has needlelike or scalelike leaves; the 

conifers; also, the wood produced by such trees. The term 

has no reference to the actual softness of the wood. 

Sorting. Segregation of rough sawn wood items into lots 

having similar drying characteristics, such as thickness, 

species, grades, sapwood, heartwood, grain patterns, and into 

classes for stacking, such as width and length. 

Specific gravity. The ratio of the ovendry weight of a piece 

of wood to the weight of an equal volume of water at 4o

C

(39o

F). Specific gravity of wood is usually based on the 

green volume. 
62

Split. A separation of the fibers along the grain forming a 

crack or fissure that often extends through the piece from one 

surface to another. 

Split, end. A split at the end of a log or board. Often an 

extension of end checks. 

Springwood. See Earlywood. 

Stack. To make a pile on a suitable foundation by placing 

layers of lumber on stickers or similar stock (self stickered). 

To construct by hand or with mechanical equipment, unit 

packages of the item to be dried, by separating layers of the 

stock with stickers. 

Stacking rack. Syn. Stacking jig, stacking stall. In hand 

building stickered unit packages of lumber, guides are provided to produce good sticker alignment and square sides. 

Stain. A discoloration in wood that may be caused by microorganisms, metal, or chemicals. 

Stain, blue. Syn. Sapstain. A bluish or grayish discoloration 

in the sapwood caused by the growth of certain dark-colored 

fungi. 

Stain, chemical brown. Syn. Brown oxidation stain, kiln 

brown stain, yard brown stain, kiln burn. A brownish discoloration of chemical origin in wood that sometimes occurs 

during the air drying of several softwood species, apparently 

caused by the concentration and oxidation of extractives. 

Sticker. Syn. Crosser, strip, piling strip, stick. A wood strip 

placed between courses of lumber in a pile or unit package 

and at right angles to the long axis of the stock to permit air 

to circulate between the layers. 

Sticker marking. Syn. Crosser stain. Indentation or compression of the lumber by the sticker when the superimposed 

load is too great for the sticker-bearing area. Sometimes 

identified as the discoloration caused by blue stain or chemical stain in the wood at the location of the sticker. 

Summerwood. See Latewood. 

Surface checking. See Check, surface. 

Swelling. Increase in the dimensions of wood caused by 

increased moisture content. Swelling occurs tangentially, 

radially, and, to a lesser extent, longitudinally. 

Temperature, dry-bulb. Temperature of air as indicated by 

a standard thermometer. 

Temperature, wet-bulb. Temperature indicated by any 

temperature-measuring device; the sensitive element of 

which is covered by a smooth, clean, soft, water saturated 

cloth (wet-bulb wick). 

Temperature, wet-bulb depression. The difference in the 

readings of wet- and dry-bulb thermometers. 

Texture. The size, distribution, and proportional volume of 

the cellular elements of which wood is composed; often used 

interchangeably with grain. Depending on the relative size 

and distribution of the cellular elements, texture may be 

coarse (open grain) or fine, even, or uneven. 

Tier. Stacking lumber or other wood products in vertical 

alignment; usually refers to sticker alignment. 

Tracheid. See Vessels. The elongated cells that constitute 

the greater part of the structure of the softwoods (fibers); also 

present in some hardwoods. 

Tramways, elevated. An elevated roadway, often constructed of wood, between rows of piles and well above the 

pile foundations to facilitate hand stacking. 

Twist. Syn. Winding, spiral distortion. A form of warp; a 

distortion caused by the turning or winding of the edges of a 

board, so that the four corners of any face are no longer in 

the same plane. 

Vessels. Syn. Tracheid. Tube-like structure of indeterminate 

length in porous woods; namely, hardwoods. 

Wane. Bark, or the lack of wood from any cause, on any 

edge of a piece of square-edged lumber. 

Warp. Distortion in lumber causing departure from its original plane, usually developed during drying. Warp includes 

cup, bow, crook, twist, and kinks, or any combination 

thereof. 

Weathering. The mechanical or chemical disintegration 

and discoloration of the surface of lumber that is caused 

by exposure to light, the action of dust and sand carried by 

winds, and the alternate shrinking and swelling of the 

surface fibers with continual variation in moisture content 

brought by changes in the weather. Weathering does not 

include decay. 

Weight of wood. The weight of wood depends on its specific gravity and moisture content. Weight equals the ovendry wood and the moisture it holds. It is expressed as pounds 

per cubic foot at a certain moisture content or weight per 

2.4 m3 (1,000 board feet) at a specified moisture content. 

Wetwood. Syn. Glassy, sinker, water core, water soak. Wood 

with abnormally high water content and a translucent or 

watersoaked appearance. This condition develops only in 

living trees and does not originate through soaking logs or 

lumber in water. 

Wood. Syn. Xylem. The tissues of the stem, branches, and 

roots of a woody plant lying between the pith and cambium, 

serving for water conduction, mechanical strength, and food 

storage, and characterized by the presence of tracheids or 

vessels.