United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

FPL–RP–580

Effect of Moisture

Content on Warp in

Hardwood 2 by 6’s

for Structural Use

William T. Simpson

John W. Forsman
Abstract 

Sugar maple (Acer saccharum), red maple (Acer rubrum), 

and yellow birch (Betula alleghaniensis) 2 by 6’s were dried 

and evaluated for warp as it affects ability to meet softwood 

dimension lumber grading rule requirements for warp. In the 

first part of the study, sugar maple was kiln-dried to three 

levels of final moisture content: 27%, 19%, and 12%. Warp 

during kiln drying increased as final moisture content decreased. Following kiln drying, the lumber was planed and 

then equilibrated in 12% equilibrium moisture content conditions. Warp during equilibration generally increased as the 

final moisture content after kiln drying increased. Crook, 

bow, and twist did not increase enough during equilibration 

to cause much structural lumber grade loss from warp. However, the percentage of boards still meeting structural lumber 

grade limits for cup fell to about 80%. In the second part of 

the study, sugar maple, red maple, and yellow birch 2 by 6’s 

were air- and predried to 27% moisture content and, in general, did not suffer much grade loss during equilibration. 

Keywords: drying, hardwoods, structural lumber, warp 

November 1999 

Simpson, William T.; Forsman, John W. 1999. Effect of moisture content 

on warp in hardwood 2 by 6’s for structural use. Res. Pap. FPL–RP–580. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 8 p. 

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Contents 

 Page 

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

Background ...........................................................................1 

Materials and Methods ..........................................................2 

Results and Discussion ..........................................................3 

Drying and Equilibration Times ........................................3 

Warp Measurements..........................................................3 

Grade Loss from Warp ......................................................6 

Conclusions and Recommendations ......................................8 

Acknowledgments .................................................................8 

Literature Cited......................................................................8 
Effect of Moisture Content on Warp in 

Hardwood 2 by 6’s for Structural Use 

William T. Simpson, Research Forest Products Technologist

Forest Products Laboratory, Madison, Wisconsin 

John W. Forsman, Assistant Research Scientist 

Michigan Technological University, Houghton, Michigan

Introduction 

Structural products such as trusses and I-joists are potential 

high value uses for lumber that is low grade by hardwood 

lumber grading rules. The use of hardwood lumber for structural uses requires that we rethink the drying process. Traditional hardwood kiln schedules were developed for appearance products such as furniture, cabinets, millwork, and 

flooring. They were necessarily mild, and thus slow, schedules to virtually eliminate any checking or discoloration. But 

checks and discoloration are not defects in structural lumber, 

so we have the opportunity to develop schedules that are 

accelerated and thus more efficient than the traditional hardwood schedules. A previous study (Simpson and others 

1998) showed that the schedule for maple 2 by 6’s could be 

accelerated to almost a 50% reduction in drying time without 

detracting from use as structural lumber. That previous study 

and this current study are part of a larger program to evaluate 

the feasibility of hardwoods for structural applications. 

As a continuation of developing an optimum schedule, we 

wanted to investigate the effect of final moisture content on 

warp and its influence on grade in sugar maple 2 by 6’s. 

Softwood structural lumber is generally dried to a moisture 

content of 15% to 19%, even though it is expected to eventually dry to 10% to 12% (and even less in the arid southwest) 

in use. Reasons for this practice are to shorten drying time 

and to minimize downgrade from warp that is thought to 

increase as final moisture content decreases. A potential 

problem associated with drying to a higher final moisture 

content than what the lumber will later attain is warp during 

post-kiln moisture loss that may cause grade loss or adversely affect truss or joist construction and performance. 

This study was in two parts. One part involved kiln drying 

sugar maple 2 by 6’s at the USDA Forest Service, Forest 

Products Laboratory in Madison, Wisconsin, and the other 

part involved air drying and predrying sugar maple, red 

maple, and yellow birch 2 by 6’s in cooperation with Michigan Technological University and a lumber mill in South 

Range, Michigan. The objectives of the kiln drying part of 

the study were to determine if warp in sugar maple 2 by 6’s 

increases as final moisture content immediately after kiln 

drying decreases and if warp develops during post-kiln 

moisture loss as the lumber equilibrates to ambient 

equilibrium moisture content (EMC) conditions. In the 

second part of the study, we explored the warp characteristics 

of other northern hardwoods and other drying methods, but 

to keep the study within manageable size, we examined only 

one level of final moisture content after air and low 

temperature, forced-air predrying and before equilibration 

to ambient EMC. 

Background 

Although, apparently, there are no data in the literature on 

the effect of final moisture content or post-kiln drying on 

warp of hardwood lumber, there are some available for softwood lumber. Bassett (1973) studied the effect of final 

moisture content on degrade in kiln drying Douglas-fir 

dimension lumber. He found that degrade increased with 

decreasing final moisture contents between 20% and 10%. 

He did not distinguish what types of degrade increased, but 

presumably some of the degrade was warp, in addition to 

increased degrade caused by splitting at the planer. Koch 

(1971) kiln dried southern pine 2 by 4’s at high temperature 

to a final moisture content of 9% and then observed their 

warp (crook, bow, and twist) 48 hours out of the kiln, after 

planing, after exposure to 81o

F (27oC) and 87% relative 

humidity (18.7% EMC), and then after exposure to 130o

F 

(54o

C) and 10% relative humidity (1.6% EMC). He found an 

increase in all forms of warp in the 48 hours after removal 

from the kiln, then a decrease after planing, a further decrease after exposure to the high humidity, and then a large 

increase after exposure to the low humidity. Markstrom and 

others (1984) kiln-dried young growth (70–80 years old) 

ponderosa pine studs to two of the common softwood standards (below 19% or an average of 15%) and measured both 

grade and the three forms of warp after drying. With the arid 

southwest in mind as the likely market for these studs, they 

then further dried the studs to 9% and then 6% moisture 

content, measuring grade and warp at each level, to simulate 

the conditions the studs would probably go through in transit 

or service. As expected, all forms of warp increased and 

grade recovery decreased as the studs were further dried. 
2 

Materials and Methods 

Sugar maple, red maple, and yellow birch 2- by 6-in. (51- by 

152-mm) boards, 8 ft (2.4 m) long, were taken from the 

production line of a mill in the Upper Peninsula of Michigan. 

The lumber was sawn from cants remaining after the high 

grade boards were sawn from the outer parts of the logs. The 

hardwood lumber grades of the 2 by 6’s (NHLA 1994) were 

3A, 3B, and Below Grade. 

For the kiln-drying part of the study (sugar maple only), the 

boards were divided into three groups of 119 boards 

each—one group to be dried to 27% moisture content, another to 19%, and the third to 12%. The kiln schedule used 

(Table 1) started at a dry-bulb temperature of 160o

F (71oC) 

and a wet-bulb temperature of 153o

F (67oC). Each of the 

three groups was kiln-dried separately, with eight kiln sample 

boards (Forest Products Laboratory 1991), four on each side 

of the stacks, to monitor moisture content for schedule 

changes and to estimate when the kiln charges reached their 

desired final moisture content. 

For the air and predrying part of the study, 105 boards of 

each of the three species were taken from the line for air 

drying and the same number for predrying (except 128 

boards were taken for air drying sugar maple). Four sample 

boards were selected to monitor moisture content. Final 

moisture content, as determined by kiln samples, after air 

and predrying was 27%. 

Crook, bow, and twist were measured on each of the boards 

before drying, after drying but before planing, and after 

planing (Fig. 1). Boards were planed to 1.5 by 5.5 in. (38 by 

140 mm). The boards that were kiln-dried were then stacked 

on stickers and equilibrated in conditions that resulted in 

12% EMC. The boards that were air- or predried were left to 

equilibrate with outdoor ambient conditions, which resulted 

in boards reaching 12% to 14% EMC. After equilibration, 

crook, bow, twist, and cup were measured again. 

Table 1—Kiln schedule for drying hard maple 2 by 6’s 

Moisture 

content 

Dry-bulb 

temperature 

Wet-bulb 

temperature EMC 

(%) (°F (°C)) (°F (°C)) (%) 

50+

 160 (71) 153 (67) 13.4 

50 to 40 160 (71) 150 (66) 11.5 

40 to 35 160 (71) 145 (63) 9.4 

35 to 30 160 (71) 135 (57) 6.8 

30 to 25 170 (77) 130 (54) 4.4 

25 to final 180 (82) 130 (54) 3.3 

Figure 1—Warp measuring table and wedges used for measuring twist. 
3 

Results and Discussion 

Drying and Equilibration Times 

Table 2 shows the drying times to the nominal final moisture 

contents. Kiln-drying times for sugar maple ranged from 

5.6 days required to dry to 12% moisture content to 3.3 days 

to dry to 27%. Predrying times to 27% moisture content 

ranged from 9 days for sugar maple to 17 days for yellow 

birch. Air-drying times to 27% moisture content were about 

25 days for all three species. Equilibration curves are shown 

in Figure 2. 

Warp Measurements 

We were concerned with two aspects of warp during drying 

and equilibration. First, we were concerned with the net warp 

that occurred during drying; that is, the difference between 

warp in the lumber before drying, which was presumably 

caused by growth stresses unbalanced by sawing, and warp 

after drying but before planing. Similarly, the net warp after 

equilibration is the difference between warp measured immediately after planing (not all warp is necessarily eliminated 

by planing) and after equilibration. The second aspect we 

were concerned with was gross warp, from the standpoint of 

utility and meeting grade warp requirements; that is, the 

actual state of warp at the time of interest, not the net amount 

of change. The gross warp values are summarized in Table 3. 

Table 4 shows the net warp that occurred during drying and 

equilibration. The values are the averages of the absolute 

value of the changes in warp during either drying or equilibration. Use of absolute values was necessary because sometimes warp was more and sometimes it was less after drying 

or equilibration than it was before, and the absolute value 

best describes the change that occurred. Therefore, the differences between before and after warp measurements in 

Table 3 (gross values) are not always the same as those in 

Table 4. Bow in particular was affected in this way because 

of the common presence of bow before the lumber was dried. 

The amount of net warp that occurred during kiln drying 

sugar maple is shown in Table 5. These warp values are the 

net warp; that is, the difference between warp measurements 

before drying and after drying but before planing. The data 

clearly show that all forms of warp increase as final moisture 

content decreases. The amount of warp that occurred in sugar 

maple during equilibration after kiln drying and planing is 

shown in Table 6. The values shown are the differences in 

warp measured after drying and after planing and then after 

equilibration. The reason warp developed in the boards dried 

to 12% is the distribution of final moisture contents. Even 

though the average final moisture content was estimated (by 

kiln samples) to be 12%, the distribution of final moisture 

contents results in some boards being at a higher moisture 

content (Simpson and others 1998) and thus having the 

potential to warp during equilibration to 12%. 

Table 6 shows that the amount of crook that developed during equilibration was greater when the final moisture content 

was 19% than when it was 12%, which is what we expected. 

However, Table 6 also shows that the amount of bow and 

twist that developed in the boards dried to 19% moisture 

content was less than the amount that developed in the 

boards that were dried to 12%. This was an unexpected result 

because the boards that were kiln-dried to 19% would be 

expected to warp more during equilibration than those kilndried to 12%. This result might be explained by the stacking 

arrangement during equilibration. Because of limitations in 

the 12% EMC storage space, it was necessary to stack one 

Table 2—Kiln-, pre-, and air-drying times for 

hardwood 2 by 6’s 

Species and drying method Drying time (days) 

Sugar maple 

Kiln-dried to 27% 3.3 

Kiln-dried to 19% 4.3 

Kiln-dried to 12% 5.6 

Predried to 27% 9 

Air-dried to 27% 25 

Red maple 

Predried to 27% 12 

Air-dried to 27% 25 

Yellow birch 

Predried to 27% 17 

Air-dried to 27% 25 

*

*

* ** * *

*

Time (days)

Moisture content (%)

29

27

25

23

21

19

17

15

13

0 20 40 60 80 100 120 

27%

19%

12%

Final MC

***

Figure 2—Curves showing moisture content loss 

during equilibration of sugar maple 2 by 6’s. 
4 

Table 3—Gross warp values for sugar maple, red maple, and yellow birch 

2 by 6’s during drying and equilibration to ambient EMC 

Experimental variablesa Crook 

(in. (mm)) 

Bow 

(in. (mm)) 

Twist 

(in. (mm)) 

Cup 

(in. (mm)) 

SM KD to 27% 

BD 0.089 (2.28) 0.115 (2.39) 0.022 (0.56) 0.000 (0.00) 

AD-BP 0.171 (4.34) 0.167 (4.24) 0.236 (5.99) 0.108 (2.74) 

AD-AP 0.043 (1.09) 0.134 (3.40) 0.159 (4.04) 0.000 (0.00) 

AE to 12% 0.128 (3.25) 0.218 (5.54) 0.230 (5.84) 0.052 (1.32) 

SM KD to 19% 

BD 0.089 (2.26) 0.144 (3.66) 0.019 (0.48) 0.000 (0.00) 

AD-BP 0.210 (5.33) 0.213 (5.87) 0.257 (6.53) 0.140 (3.56) 

AD-AP 0.049 (1.24) 0.162 (4.11) 0.161 (4.09) 0.000 (0.00) 

AE to 12% 0.095 (2.41) 0.181 (4.60) 0.173 (4.39) 0.056 (1.42) 

SM KD to 12% 

BD 0.091 (2.31) 0.144 (3.66) 0.024 (0.61) 0.000 (0.00) 

AD-BP 0.269 (6.83) 0.245 (6.22) 0.387 (9.83) 0.155 (3.94) 

AD-AP 0.048 (1.22) 0.210 (5.33) 0.268 (6.81) 0.000 (0.00) 

AE to 12% 0.078 (1.98) 0.191 (4.85) 0.184 (4.67) 0.051 (1.30) 

SM PD to 27% 

BD 0.066 (1.68) 0.070 (1.78) 0.027 (0.69) 0.004 (0.10) 

AD-BP 0.165 (4.19) 0.131 (3.33) 0.322 (8.18) 0.086 (2.18) 

AD-AP 0.023 (0.58) 0.088 (2.24) 0.263 (6.68) 0.001 (0.03) 

AE to 12% 0.133 (3.38) 0.173 (4.39) 0.484 (12.29) 0.028 (0.71) 

SM AD to 27% 

BD 0.107 (2.72) 0.087 (2.21) 0.005 (0.13) 0.004 (0.10) 

AD-BP 0.143 (3.63) 0.106 (2.69) 0.125 (3.18) 0.036 (0.91) 

AD-AP 0.039 (0.99) 0.084 (2.13) 0.091 (2.31) 0.000 (0.00) 

AE to 19% 0.098 (2.49) 0.111 (2.82) 0.140 (3.56) 0.012 (0.30) 

AE to 12% 0.190 (4.83) 0.152 (3.86) 0.305 (7.75) 0.047 (1.19) 

RM PD to 27% 

BD 0.060 (1.52) 0.076 (1.93) 0.019 (0.48) 0.002 (0.05) 

AD-BP 0.089 (2.26) 0.093 (2.36) 0.174 (4.42) 0.032 (0.81) 

AD-AP 0.005 (0.13) 0.086 (2.18) 0.126 (3.20) 0.000 (0.00) 

AE to 12% 0.495 (12.57) 0.164 (4.17) 0.467 (11.86) 0.052 (1.32) 

RM AD to 27% 

BD 0.069 (1.75) 0.087 (2.21) 0.023 (0.58) 0.001 (0.03) 

AD-BP 0.128 (3.25) 0.123 (3.12) 0.302 (7.67) 0.047 (1.19) 

AD-AP 0.047 (1.19) 0.115 (2.92) 0.206 (5.23) 0.001 (0.03) 

AE to 19% 0.110 (2.79) 0.123 (3.12) 0.288 (7.32) 0.013 (0.33) 

AE to 12% 0.122 (3.10) 0.115 (2.92) 0.314 (7.98) 0.027 (0.69) 

YB PD to 27% 

BD 0.091 (2.31) 0.064 (1.63) 0.026 (0.66) 0.000 (0.00) 

AD-BP 0.122 (3.10) 0.069 (1.75) 0.163 (4.14) 0.035 (0.89) 

AD-AP 0.013 (0.33) 0.065 (1.65) 0.090 (2.29) 0.000 (0.00) 

AE to 12% 0.167 (4.24) 0.144 (3.66) 0.302 (7.67) 0.033 (0.84) 

YB AD to 27% 

BD 0.113 (2.87) 0.094 (2.39) 0.030 (0.76) 0.003 (0.08) 

AD-BP 0.183 (4.65) 0.142 (3.61) 0.307 (7.80) 0.054 (1.37) 

AD-AP 0.028 (0.71) 0.107 (2.72) 0.214 (5.44) 0.000 (0.00) 

AE to 12% 0.123 (3.12) 0.145 (3.68) 0.298 (7.57) 0.009 (0.23) 

a

SM, sugar maple; RM, red maple; YB, yellow birch; KD, kiln-dried; PD, predried; 

 AD, air-dried; BD, before drying; AD-BP, after drying, before planing; AD-AP, after 

 drying, after planing; AE, after equilibration. 
5 

Table 4—Absolute values of net warp for sugar maple, red maple, and yellow birch 

2 by 6’s during drying and equilibration to ambient EMC 

Experimental variablesa Crook 

(in. (mm)) 

Bow 

(in. (mm)) 

Twist 

(in. (mm)) 

Cup 

(in. (mm)) 

SM KD to 27% 

AD-BP 0.088(2.24) 0.106(2.69) 0.219(5.56) 0.108(2.74) 

AE to 12% 0.088(2.24) 0.095(2.41) 0.095(2.41) 0.052(1.32) 

SM KD to 19% 

AD-BP 0.124(3.15) 0.135(3.43) 0.241(6.12) 0.140(3.56) 

AE to 12% 0.050(1.27) 0.045(1.14) 0.066(1.68) 0.056(1.42) 

SM KD to 12% 

AD-BP 0.183(4.65) 0.158(4.01) 0.365(9.27) 0.155(3.94) 

AE to 12% 0.043(1.09) 0.066(1.68) 0.100(2.54) 0.051(1.30) 

SM PD to 27% 

AD-BP 0.108(2.74) 0.078(1.98) 0.291(7.39) 0.082(2.08) 

AE to 12% 0.109(2.77) 0.093(2.36) 0.224(5.69) 0.028(.071) 

SM AD to 27% 

AD-BP 0.054(1.37) 0.060(1.52) 0.122(3.01) 0.034(0.86) 

AE to 19% 0.060(1.52) 0.055(1.40) 0.068(1.73) 0.013(2.06) 

AE to 12% 0.151(3.84) 0.093(2.36) 0.218(5.54) 0.046(1.17) 

RM PD to 27% 

AD-BP 0.038(0.97) 0.044(1.12) 0.156(3.96) 0.032(0.81) 

AE to 12% 0.450(11.43) 0.110(2.79) 0.341(8.66) 0.052(1.32) 

RM AD to 27% 

AD-BP 0.066(1.68) 0.052(1.32) 0.283(7.18) 0.047(1.19) 

AE to 19% 0.064(1.63) 0.070(1.78) 0.119(3.02) 0.013(0.33) 

AE to 12% 0.076(1.93) 0.067(1.70) 0.140(3.56) 0.026(0.66) 

YB PD to 27% 

AD-BP 0.042(1.07) 0.034(0.89) 0.140(3.56) 0.035(0.89) 

AE to 12% 0.167(4.24) 0.097(2.46) 0.217(5.51) 0.033(0.84) 

YB AD to 27% 

AD-BP 0.085(2.16) 0.076(1.93) 0.272(6.91) 0.051(1.30) 

AE to 12% 0.095(2.41) 0.072(1.83) 0.090(2.29) 0.009(0.23) 

a

SM, sugar maple; RM, red maple; YB, yellow birch; KD, kiln-dried; PD, predried; 

 AD, air-dried; AD-BP, after drying, before planing; AE, after equilibration. 
6 

group on top of the other. The boards kiln-dried to 27% were 

stacked on top of those kiln-dried to 19%, which resulted in 

more weight restraint on the 19% boards during equilibration. This restraint apparently reduced the amount of bow 

and twist that developed during equilibration. The boards 

kiln-dried to 12% did not have anything stacked on top during equilibration. The difference in response between the 

crook that developed in the 19% boards (which was more 

than that of the 12% boards) and the bow and twist that 

developed in the same boards is in accord with the observation that Koch (1974) made on southern pine 2 by 4’s. His 

research showed that top restraint was effective in reducing 

bow and twist but was not effective in reducing crook. 

Table 7 shows the effect of drying method on warp in sugar 

maple. There are few clear patterns that suggest that drying 

method affects the amount of warp that develops during 

drying or during equilibration. Bow and cup may be greater 

in kiln drying than in pre- or air drying, and twist may be 

greater in predrying than in air or kiln drying. However, it 

is difficult to rationalize reasons for these observations. 

Table 8 shows the effect of species on the amount of warp 

occurring during drying and during equilibration. As was the 

case with the drying method comparisons, there does not 

appear to be any general pattern to suggest any species 

included in this study warps more than any other. 

Grade Loss from Warp 

To get an idea of how serious the observed warp was, we 

evaluated it by southern pine grading rules (SPIB 1994). 

Whether or not grade warp limits for hardwoods would be 

the same as for softwoods is unknown at this point. The 

limits for warp in various structural grades of 2 by 6’s are 

shown in Table 9. Table 10 shows the percentage of boards 

that meet Number 2 grade warp requirements immediately 

after drying and surfacing and also after equilibration. The 

percentages are shown according to type of warp, drying 

method, species, and final target moisture content (in the case 

Table 7—Effect of drying method on warp in sugar 

maple 2 by 6’s dried to 27% moisture content 

Experimental 

variablesa 

Crook 

(in. 

(mm)) 

Bow 

(in. 

(mm)) 

Twist 

(in. 

(mm)) 

Cup 

(in. 

(mm)) 

 Gross warp 

AD-BP 

Kiln-dried 0.171 

(4.34) 

0.167

(4.24) 

0.236 

(5.99) 

0.108

(2.74) 

Predried 0.165 

(4.19) 

0.131

(3.33) 

0.322 

(8.18) 

0.086

(2.18) 

Air-dried 0.143 

(3.63) 

0.106

(2.69) 

0.125 

(3.18) 

0.036

(0.91) 

Equilibration to 12% (AE) 

Kiln-dried 0.127

(3.23) 

0.218

(5.54) 

0.230 

(5.84) 

0.052

(1.32) 

Predried 0.132

(3.35) 

0.173

(4.39) 

0.484 

(12.29) 

0.028

(0.71) 

Air-dried 0.190

(4.83) 

0.152

(3.86) 

0.305 

(7.75) 

0.047

(1.19) 

Net warp 

AD-BP 

Kiln-dried 0.088

(2.24) 

0.106

(2.69) 

0.219 

(5.56) 

0.108 

(2.74) 

Predried 0.108

(2.74) 

0.078

(1.98) 

0.291 

(7.39) 

0.082 

(2.08) 

Air-dried 0.054

(1.37) 

0.060

(1.52) 

0.122 

(3.10) 

0.034 

(0.86) 

Equilibration to 12% (AE) 

Kiln-dried 0.088

(2.24) 

0.095

(2.41) 

0.09 

(2.41) 

0.052 

(1.32) 

Pre-dried 0.109

(2.77) 

0.093

(2.36) 

0.224 

(5.69) 

0.028 

(0.71) 

Air-dried 0.151

(3.84) 

0.093

(2.36) 

0.218 

(5.54) 

0.047 

(1.19) 

a

AD-BP, after drying, before planing; AE, after 

 equilibration. 

Table 5—Net warp occurring during kiln drying of 

sugar maple 2 by 6’s 

Final moisture 

content 

(%) 

Crook 

(in. 

(mm)) 

Bow 

(in. 

(mm)) 

Twist 

(in. 

(mm)) 

Cup 

(in. 

(mm)) 

12 0.183 

(4.65) 

0.158 

(4.01) 

0.365 

(9.27) 

0.155 

(3.94) 

19 0.124 

(3.15) 

0.135 

(3.43) 

0.241 

(6.12) 

0.140 

(3.56) 

27 0.088 

(2.24) 

0.106 

(2.69) 

0.218 

(5.54) 

0.108 

(2.74) 

Table 6—Net warp occurring during post-kiln 

equilibration of sugar maple 2 by 6’s 

Final moisture 

content 

(%) 

Crook 

( in. 

(mm)) 

Bow 

(in. 

(mm)) 

Twist 

(in. 

(mm)) 

Cup 

(in. 

(mm)) 

12 0.0421

(1.07) 

0.0664 

(1.69) 

0.0999 

(2.54) 

0.0507 

(1.29) 

19 0.0496

(1.26) 

0.0453 

(1.15) 

0.0652 

(1.66) 

0.0557 

(1.41) 

27 0.0878

(2.23) 

0.0952 

(2.42) 

0.0952 

(2.42) 

0.0523 

(1.33) 
7 

of kiln drying). Although commercial planing equipment and 

methods probably differ from our laboratory capabilities, the 

data show that the planing operation is capable of greatly 

reducing downgrade from warp that occurs during drying. 

Surfacing out warp to meet grade requirements requires 

larger target dimensions for sawn lumber, which reduces 

yield and wastes resource. However, considering the poor 

use potential of this type of material, reduced yield may be 

justifiable. The results in Table 10 are somewhat varied. In 

all cases, the percentage of boards meeting grade warp requirements after drying and planing was high, from 97% to 

100%. In many cases, the percentage was still high after 

equilibrating to 12% moisture content. The worst case was 

red maple equilibrated to 12% after predrying to 27% moisture content. For this group, the percentage of boards meeting grade warp requirements was only 38.6%, and there is no 

apparent reason for this outlying result. There is concern that 

cup might be a problem in equilibration. For kiln-dried sugar 

maple, the percentage of boards that met grade warp requirements based on cup dropped into the 80% range during 

equilibration to 12% moisture content. The percentage of red 

maple boards predried to 27% moisture content that met 

grade warp requirements after equilibration also dropped into 

the 80% range. 

Currently, most structural lumber is kiln-dried to 15% to 

19% moisture content. Because of post-kiln drying to ambient equilibrium conditions of 12% moisture content or below 

and the potential danger of warp during that drying, some are 

suggesting that structural lumber be dried closer to these 

ambient EMCs (Koch 1971, Markstrom and others 1984). 

The concern with this approach is that warp and thus grade 

loss during kiln drying may be increased, and kiln residence 

time and thus required kiln capacity will certainly be increased. At the other extreme, it would be desirable to dry to 

final moisture contents higher than 19% to reduce required 

kiln capacity and residence time, even though the danger of 

post-kiln drying warp exists. This study provides data to help 

make a decision on optimum final moisture content of 

Table 8—Effect of species on warp in 2 by 6’s 

Experimental 

variablesa

Crook 

(in. 

(mm)) 

Bow 

(in. 

(mm)) 

Twist 

(in. 

(mm)) 

Cup 

(in. 

(mm)) 

Gross warp 

AD-BP 

SM-PD 0.165 

(4.19) 

0.131 

(3.33) 

0.322 

(8.18) 

0.086 

(2.18) 

RM-PD 0.089 

(2.26) 

0.093 

(2.36) 

0.174 

(4.42) 

0.032 

(0.81) 

YB-PD 0.121 

3.07) 

0.069 

(1.75) 

0.163 

(4.14) 

0.035 

(0.89) 

SM-AD 0.143 

(3.63) 

0.106 

(2.69) 

0.125 

(3.18) 

0.036 

(0.91) 

RM-AD 0.128 

(3.25) 

0.123 

(3.12) 

0.302 

(7.67) 

0.047 

(1.19) 

YB-AD 0.183 

(4.65) 

0.142 

(3.61) 

0.307 

(7.80) 

0.054 

(1.37) 

Equilibration to 12% (AE) 

SM-PD 0.133 

(3.38) 

0.173 

(4.39) 

0.484 

(12.29) 

0.028 

(0.71) 

RM-PD 0.495 

(12.57) 

0.164 

(4.17) 

0.467 

(11.86) 

0.052 

(1.32) 

YB-PD 0.167 

(4.24) 

0.144 

(3.66) 

0.302 

(7.67) 

0.033 

(0.84) 

SM-AD 0.190 

(4.83) 

0.152 

(3.86) 

0.305 

(7.75) 

0.047 

(1.19) 

RM-AD 0.122 

(3.10) 

0.115 

(2.92) 

0.314 

(7.98) 

0.027 

(0.69) 

YB-AD 0.123 

(3.12) 

0.145 

(3.68) 

0.298 

(7.57) 

0.009 

(0.23) 

Net warp 

AD-BP 

SM-PD 0.108 

(2.74) 

0.078 

(1.98) 

0.291 

(7.39) 

0.082 

(2.08) 

RM-PD 0.039 

(0.99) 

0.044 

(1.12) 

0.156 

(3.96) 

0.032 

(0.81) 

YB-PD 0.042 

(1.07) 

0.034 

(0.86) 

0.140 

(3.56) 

0.035 

(0.89) 

SM-AD 0.054 

(1.37) 

0.060 

(1.52) 

0.122 

(3.10) 

0.034 

(0.86) 

RM-AD 0.066 

(1.68) 

0.053 

(1.35) 

0.283 

(7.19) 

0.047 

(1.19) 

YB-AD 0.085 

(2.16) 

0.076 

(1.93) 

0.272 

(6.91) 

0.051 

(1.30) 

Equilibration to 12% (AE) 

SM-PD 0.109 

(2.77) 

0.093 

(2.36) 

0.224 

(5.69) 

0.028 

(0.71) 

RM-PD 0.450 

(11.43) 

0.110 

(2.79) 

0.341 

(8.66) 

0.052 

(1.32) 

YB-PD 0.167 

(4.24) 

0.097(

2.46) 

0.217 

(5.51) 

0.033 

(0.84) 

SM-AD 0.151 

(3.84) 

0.093(

2.36) 

0.218 

(5.54) 

0.046 

(1.17) 

RM-AD .076 

(1.93) 

0.067 

(1.70) 

0.140 

(3.56) 

0.026 

(0.66) 

YB-AD 0.095 

(2.41) 

0.072 

(1.83) 

0.090 

(2.29) 

0.009 

(0.23) 

a

AD-BP, after drying, before planing; AE, after 

 equilibration; SM, sugar maple; RM, red maple; 

 YB, yellow birch; PD, predried; AD, air-dried. 

Table 9—Warp limits for 8-ft- (2.4-m-) long 

southern pine structural 2 by 6’s (SPIB 1994) 

Grade 

Crook 

(in. 

(mm)) 

Bow 

(in. 

(mm)) 

Twist 

(in. 

(mm)) 

Cup 

(in. 

(mm)) 

Select Structural 0.250

(6.4) 

0.500

(12.7) 

0.563

(14.3) 

0.0625

(1.6) 

1 0.250

(6.4) 

0.500

(12.7) 

0.563

(14.3) 

0.0625

(1.6) 

2 0.313

(2.5) 

0.750

(19.1) 

0.750

(19.1) 

0.0625

(1.6) 

3 0.500

(12.7) 

1.000

(25.4) 

1.125

(28.6) 

0.1250

(3.2) 
8 

hardwoods for structural purposes. However, the results 

suggest that even though increased warp develops when final 

moisture content immediately after drying is reduced to 12%, 

it probably can be surfaced out in most planing operations. 

But because the additional warp that develops during equilibration of sugar maple kiln-dried to only 27% may not reduce grade appreciably, it may also be appropriate to rethink 

and further investigate the idea of increasing final moisture 

content to some level above the common 15% to 19%. However, before a higher final moisture content could be recommended, the effects of post-kiln drying shrinkage in terms of 

variable size and fastener integrity would have to be 

investigated. 

Conclusions and 

Recommendations 

This study confirms the expected result that the amount of 

warp that develops in kiln drying hard maple 2 by 6’s increases as the final average moisture content decreases. The 

results also show that if this lumber is kiln-dried to a final 

moisture content higher than 15% to 19%, warp will increase 

but grade loss (assuming softwood grade warp limits would 

also apply to hardwoods) may not seriously increase from the 

warp that occurs as moisture content decreases to typical 

ambient levels of about 12%. Similarly, in most cases, sugar 

maple, red maple, and yellow birch 2 by 6’s air- or predried 

to 27% moisture content did not suffer much grade loss from 

warp during equilibration. The results of one laboratory 

study are not sufficient to recommend higher final moisture 

contents in commercial applications, but they do suggest that 

for sugar maple, red maple, and yellow birch, it may be 

possible to increase kiln efficiency for structural applications 

by raising final moisture content above the 15% to 19% 

range if shrinkage and fastener integrity are not problems. 

We recommend that further research or field tests be conducted to confirm or disprove the results of this study. 

Acknowledgments 

We gratefully acknowledge the assistance of Northern Hardwoods, South Range, Michigan, for supplying the lumber 

for this study and John Erickson, former FPL Director, for 

guidance. 

Literature Cited 

Bassett, K.H. 1973. A look at redry. In: Proceedings of Western 

Dry Kiln Clubs, 24th annual meeting; 1973 May 10–11; Vancouver, BC, Canada. Corvallis, OR: Western Dry Kiln Clubs. 

Forest Products Laboratory. 1991. Dry kiln operator’s manual. 

Agric. Handb. 188. Madison, WI: USDA Forest Service, Forest 

Products Laboratory. 274 p. 

Koch, P. 1971. Process for straightening and drying southern pine 

2x4s in 24 hours. Forest Products Journal. 21(5): 17–24. 

Koch, P. 1974. Serrated kiln sticks and top load substantially 

reduce warp in southern pine studs dried at 240o

F. Forest Products 

Journal. 24(11): 30–34. 

Markstrom, D.C.; Shuler, C.E.; King, R.M. 1984. Warpage of 

studs from young growth ponderosa pine from northern New 

Mexico. Res. Pap. RM–RP–257. Fort Collins, CO: Rocky Mountain Forest and Range Experiment Station. 

NHLA. 1994. Rules for the measurement and inspection of hardwood and cypress. Memphis, TN: National Hardwood Lumber 

Association. 124 p. 

Simpson, W.T.; Forsman, J.W.; Ross, R.J. 1998. Kiln drying 

maple structural lumber from log heart cants. Forest Products 

Journal. 48(6): 70–76. 

SPIB. 1994. Grading rules. Pensacola, FL: Southern Pine 

Inspection Bureau. 

Table 10—Percentage of boards that meet southern 

pine grade requirements for warp in Number 2 grade 

(Forest Products Laboratory 1991) 

Percentage of boards 

Experimental variablesa Crook Bow Twist Cup 

SM Kiln-dried to 27% 

AD-AP 100 100 100 100 

AE to 12% 97.3 100 100 82.9 

SM Kiln-dried to 19% 

AD-AP 100 100 100 100 

AE to 12% 99.1 100 100 84.7 

SM Kiln-dried to 12% 

AD-AP 100 99.1 97.3 100 

AE to 12% 98.2 100 100 85.6 

SM Predried to 27% 

AD-AP 100 100 100 100 

AE to 12% 95.1 99.0 87.3 100 

SM Air-dried to 27% 

AD-AP 100 100 100 100 

AE to 19% 100 100 100 100 

AE to 12% 86.1 100 96.0 92.1 

RM Predried to 27% 

AD-AP 100 100 100 100 

AE to 12% 38.6 100 89.1 85.1 

RM Air-dried to 27% 

AD-AP 100 99.0 99.0 100 

AE to 19% 98.0 100 93.1 100 

AE to 12% 96.0 100. 94.1 100 

YB Predried to 27% 

AD-AP 100 100 100 100 

AE to 12% 88.1 100 100 100 

YB Air-dried to 27% 

AD-AP 100 99.0 100 100 

AE to 12% 99.0 100 97.0 100 

a

SM, sugar maple; RM, red maple; YB, yellow birch; 

 AD-AP, after drying, after planing; AE, after equilibration.