Effect of Drying 

Temperature on Warp 

and Downgrade of 2 by 4ís 

From Small-Diameter 

Ponderosa Pine 

William T. Simpson 

United States

Department of 

Agriculture 

Forest Service 

Forest 

Products 

Laboratory 

Research 

Paper 

FPL−RP−624 
Abstract 

Kiln drying at high temperature may reduce warp in dimension lumber sawn from small-diameter trees. In this study, 

we examined the effect on warp of high drying temperatures 

in conjunction with top loading immediately after drying and 

after storage in typical conditions that result in further moisture loss. Eight-foot-long 2- by 4-in. (2 by 4) boards sawn 

from open-grown, 30- to 35-year-old, 9-in.-diameter at 

breast height ponderosa pine trees were kiln dried at three 

temperatures, 160°F, 230°F, and 280°F. Crook, bow, and 

twist were measured and compared. Total grade loss from 

exceeding Structural Light Framing grade warp limits was 

also measured. Crook, bow, and twist measured after boards 

had equilibrated to 14% moisture content decreased by 12%, 

29%, and 20%, respectively, with increase in kiln drying 

temperature from 160°F to 280°F. Total grade loss caused 

by all forms of warp was 24% lower in boards dried at 

230°F than at 160°F and 39% lower in boards dried at 280°F 

than at 160°F. When boards were equilibrated to 4% to 5% 

moisture content, warp and grade loss from warp increased 

significantly. 

Keywords: kiln drying, ponderosa pine, small-diameter 

timber, drying degrade 

September 2004 

Simpson, William T. 2004. Effect of drying temperature on warp and 

downgrade of 2 by 4ís from small-diameter ponderosa pine. Res. Pap. 

FPL-RP-624. Madison, WI: U.S. Department of Agriculture, Forest Service, 

Forest Products Laboratory. 8 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 53726ñ2398. This publication is also available online at 

www.fpl.fs.fed.us. 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. 

The United States Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national 

origin, sex, religion, age, disability, political beliefs, sexual orientation, or 

marital or familial status. (Not all prohibited bases apply to all programs.) 

Persons with disabilities who require alternative means for communication 

of program information (Braille, large print, audiotape, etc.) should contact 

the USDAís TARGET Center at (202) 720ñ2600 (voice and TDD). To file a 

complaint of discrimination, write USDA, Director, Office of Civil Rights, 

Room 326-W, Whitten Building, 1400 Independence Avenue, SW, Washington, DC 20250ñ9410, or call (202) 720ñ5964 (voice and TDD). USDA is 

an equal opportunity provider and employer. 

Contents 

 Page 

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

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

Procedure .............................................................................. 2

Analysis ................................................................................ 2

Results................................................................................... 3

Drying Time...................................................................... 3

Warp Soon After Drying................................................... 3

Warp After Equilibration .................................................. 4

Conclusions and Recommendations ..................................... 7

Literature Cited ..................................................................... 8

SI conversion factors 

English unit 

Conversion 

factor SI unit 

inch (in.) 25.4 millimeter (mm) 

foot (ft) 0.3048 meter (m) 

pound (lb) mass 0.454 kilogram (kg) 

pound per square foot 

(lb/ft2

)

 4.88 kilogram per square 

meter (kg/m2) 

temperature (°F) [TF−32]/1.8 temperature (°C) 
Effect of Drying Temperature on Warp 

and Downgrade of 2 by 4ís From 

Small-Diameter Ponderosa Pine 

William T. Simpson, Research Forest Products Technologist

Forest Products Laboratory, Madison, Wisconsin 

Introduction 

In some instances, ponderosa pine (Pinus ponderosa) nominal 2- by 4-in. (2 by 4) lumber has been shown to experience 

less grade loss from warp when dried at 230°F compared to 

160°F (Simpson and Green 2001). When boards were measured immediately after kiln drying, drying temperature did 

not show any consistent effect on crook, bow, and twist. 

However, after equilibrating to approximately 12% moisture 

content, boards that had been dried at 230°F showed significantly less grade loss from warp than did boards dried at 

160°F. The average percentage of boards meeting No. 3 

warp limits in the Structural Light Framing grading system 

(WWPA 1998) was 59.7% for boards kiln dried at 160°F 

and 79.0% for boards dried at 230°F. Another way of analyzing the results after planing is to consider the total amount 

of grade loss; that is, the difference between the actual afterplaning grade and the grade if no warp were present. This 

method of analysis recognizes when a board drops more than 

one grade because of warp. The average total grade loss 

from warp in boards kiln dried at 230°F was half the grade 

loss in boards dried at 160°F. The results also showed that 

heavy top loading (200 lb/ft2

) was effective in reducing 

warp. These results present encouraging evidence that high 

drying temperatures might significantly reduce grade loss 

from warp. The purpose of the study reported here was to 

explore this issue in more depth. 

Background 

The results of a previous study (Simpson and Green 2001) 

showed that high temperature kiln drying may have potential 

in reducing warp in ponderosa pine lumber sawn from smalldiameter treesóespecially warp that occurs during moisture 

equalizationópossibly by providing stress relief during 

moisture equalization after kiln drying. Koch (1971) found 

that 2 by 4ís sawn from small-diameter Southern Pine logs 

and veneer cores experienced less warp when dried at 240°F 

than when dried at temperatures of no more than 180°F, both 

48 h after kiln drying and after equilibrating for 20 days. 

Price and Koch (1980) found that drying temperature (180°F 

as opposed to 270°F) had no appreciable effect on warp in 

Southern Pine nominal 2- by 6-in. (2 by 6) lumber 

immediately after kiln drying. After equalization, however, 

lumber dried at the higher drying temperature had an average 30.6% reduction in warp. 

Other studies have explored the effect of drying temperature 

on warp but do not specify the time between removal of 

lumber from the kiln and measurement of warp. In a study 

by Arganbright and others (1978), studs sawn from smalldiameter ponderosa pine trees (diameter unspecified) and 

dried by a high temperature schedule that varied from 210°F 

to 230°F had more warp than did studs dried by a schedule 

that varied from 135°F to 170°F. However, the studs were 

dried to lower moisture content at the higher temperatures, 

which may have been why they developed more warp. 

Maeglin and Boone (1983) found that 2 by 4ís from smalldiameter (8- to 12-in. diameter at breast height (dbh)) ponderosa pine trees dried at high temperature (240°F) developed 12.9% less crook, 9.6% less bow, and 25.0% less twist 

than did lumber dried at temperatures not exceeding 180°F, 

although only the reduction in twist was statistically significant. Taylor (1993) found no difference in the amount of 

warp in Southern Pine 2 by 6 boards dried at 245°F and 

boards dried at 305°F. Wu and Smith (1998) compared warp 

in loblolly pine (Pinus taeda, L.) 2 by 6ís kiln dried by a 

schedule that ramped from 170°F to 200°F in 52 h to boards 

dried by a schedule that ramped from 180°F to 230°F for 

12 h and was then held at 230°F for 18 h. At the higher 

drying temperatures, crook, bow, and twist were reduced by 

18.6%, 6.0%, and 4.7%, respectively. Only the crook reduction of 18.6% was statistically significant. Milota (2000) 

compared warp in HemñFir (predominately western hemlock, Tsuga heterophylla, but with some true fir, Abies spp.) 

2 by 4ís dried at four temperatures (180°F, 240°F, 255°F, 

and 270°F). Although the differences were not statistically 

significant, the boards dried at 180°F had more crook, 

bow, and twist than did boards dried at any of the higher 

temperatures. 

Several studies have explored the effect of equilibration to 

low equilibrium moisture content (EMC) after drying, which 

simulates end-use conditions in the arid interior western 

states. Markstrom and others (1984) showed the effect of 
2 

post-kiln drying (at 165°F to 200°F) and moisture equalization on grade in studs sawn from 9- to 14-in.-diameter ponderosa pine trees. The studs were kiln dried to between 15% 

and 19% moisture content and then stored in conditions that 

reduced moisture content initially to 9% and then to 6% 

(typical in the interior western United States). The authors 

noted a marked reduction in the percentage of boards that 

met STUD grade as moisture content decreased to 9% and 

6% in storage. Fahey and others (1986) also noted this effect 

in 2 by 4ís and 2 by 6ís sawn from ≤12-in.-diameter ponderosa pine trees. 

This background literature provides some level of optimism 

that high temperature drying may reduce warp in dimension 

lumber sawn from small-diameter ponderosa pine trees. The 

previous study on ponderosa pine warp (Simpson and Green 

2001) was designed to investigate the effect of several variables on warp and was not specifically designed to study the 

effect of equalization during storage. The purpose of the 

study reported here was to focus more closely on the effect 

of high drying temperatures, in conjunction with top loading, 

on warp both immediately after drying and after storage in 

typical conditions that result in further moisture loss. 

Procedure 

The 8-ft-long 2 by 4ís were sawn from small-diameter 

(about 9 in. dbh) open-grown ponderosa pine trees from a 

thinning treatment on a 30- to 35-year-old stand in Idaho. 

The logs were sawn near the harvesting area, and the lumber 

shipped to the Forest Products Laboratory in Madison, Wisconsin. The lumber was wrapped in plastic to prevent drying 

during shipment, and then held, still wrapped, in cold storage 

until it was kiln dried. 

When the lumber was received, it was graded green as Structural Light Framing (WWPA 1998). Measurements were 

made of dynamic modulus of elasticity (MOE) by transverse 

vibration for each board in the rough, green condition. These 

data were used to assign each board to one of eight experimental groups on the basis of MOE within each grade. Then 

within a grade, the boards were ordered by MOE. The first 

eight boards in the ranking were randomly assigned to one 

of the eight groups. Then, the next eight boards were randomly assigned. This process was repeated until all boards 

were assigned to a group. 

The study design was six experimental groups of 152 boards 

each (kiln loads of 12 boards wide by 13 courses high). The 

plan was for three kiln schedules with two replicate kiln runs 

per schedule. The two extra experimental groups were 

planned for test runs to ensure the kiln was operating correctly and to establish some background data for determining 

some drying times for the six experimental kiln runs. The 

kiln schedules were at three temperature levels (160°F, 

230°F, and 280°F) and are shown in Table 1. Top loading of 

200 lb/ft2 was included. Target final moisture content was 

12% to 15%, which was estimated with a pin-type conductance moisture meter at intervals near the end of drying. 

After kiln drying, top loads were left in place during a 24-h 

cooling period. 

Soon after drying and cooling, the 2 by 4 boards were measured for crook, bow, and twist using a wedge gauge and flat 

reference table. These warp measurements were made within 

1 to 2 days after the weights were removed from the stacks. 

Each board was metered in two places along its length for an 

estimate of moisture content. The boards were then planed. 

After planing, the boards were stickered and equilibrated in 

an environment of 75°F and 70% relative humidity, which is 

about 13% EMC. After equilibration, crook, bow, and twist 

were remeasured and the boards metered for moisture content. The boards were then equilibrated in an environment of 

approximately 60°F to 70°F and 20% to 30% relative humidity, which is about 4% to 5% EMC and simulates typical 

conditions in the interior west. Crook, bow, and twist were 

again measured after equilibration. 

Analysis 

The data for the crook, bow, and twist measurements were 

analyzed using two-way analysis of variance with kiln run 

and drying temperature as the two factors tested for their 

effect. The analysis was conducted soon after kiln drying 

and also after the equalization period (Glanz 1992). When 

results of the analysis of variance showed a significant effect, the Tukey test for individual group comparisons was 

used (Glanz 1992). 

The results were also analyzed in terms of the number of 

boards that failed to meet the maximum warp allowed in 

8-ft-long 2 by 4 lumber in the Structural Light Framing 

grades (WWPA 1998). The general approach was to determine if each 2 by 4 maintained the green grade after drying 

(on the basis of meeting the warp limit of the grade). These 

Table 1óKiln drying schedules for ponderosa pine 

2 by 4ís 

Kiln schedule 

Time 

(h) 

Dry-bulb 

temperature 

(°F) 

Wet-bulb 

temperature 

(°F) 

Conventional 0 to 24 160 140 

 24 to 36 165 140 

 36 to end 170 140 

High temperature #1 0 to end 230 180 

High temperature #2 0 to end 280 180 
3

warp limits are shown in Table 2. The green grade distribution in all groups was 2% Select Structural, 40% No. 1, 48% 

No. 2, and 10% No. 3. It is possible for a Select Structural 

2 by 4 to drop up to four grade levels: from Select Structural 

to No. 1, 2, or 3, or below No. 3. Similarly, a No. 1 board 

can drop three grade levels and a No. 2 board one or two 

grade levels; a No. 3 board can drop only one grade level. 

Results 

One test run from the two extra experimental groups was 

conducted at 280°F. We had no experience with the required 

drying time at 280°F, and there are no guidelines in the 

literature. We conducted three kiln runs at 280°F and obtained a range of final moisture contents. One run was overdried, another underdried, and the third within the target 

final range of 12% to 15% moisture content. Because the 

results offered some possible insight into the effectiveness of 

high temperature drying in controlling grade loss from warp, 

we decided to include all three 280°F runs in the statistical 

analysis even though this unbalanced the experimental design. Inclusion of the third group for only one drying temperature precluded analysis of interactions, but the interactions between drying temperature and replicate were of little 

useful interest. 

Drying Time 

Kiln drying time was reduced from about 84 h by the conventional temperature (160°F) schedule to 36 h at 230°F and 

15 h at 280°F (Fig. 1). Although surface checking and honeycomb are not degrading factors in softwood dimension 

lumber, it is noteworthy that no honeycomb was observed on 

the ends of boards, and only a few boards showed any surface checking. 

Warp Soon After Drying 

Warp measured within 1 to 2 days after drying is shown in 

Table 3. Results of the analysis of variance and Tukey test 

are shown in Tables 4 and 5. Both kiln run and drying temperature significantly affected all three types of warp. However, there does not appear to be any pattern in the effects. 

One possible reason for differences observed as a result of 

both factors (kiln run and drying temperature) is the effect of 

variation in final moisture content. It was not possible to end 

each kiln run at exactly the same final moisture content, and 

a distribution of final moisture contents occurred regardless 

of the average final moisture content. We would expect 

overdried 2 by 4ís to show more warp than would underdried 2 by 4ís. For example, in Table 3, replicate B at 280°F, 

which was overdried to 6.8% moisture content, had an average bow of 0.785 in. Replicate C, which was underdried to 

23.4% moisture content, had only 0.262 in. of bow. This 

underscores the necessity of equilibrating all boards to a 

common moisture content for evaluating the effects of drying temperature on warp. 

Table 2óWarp limits for 8-ft-long 2 by 4ís under 

Structural Light Framing grading rules (WWPA 1998) 

 Warp (in.) 

Grade Crook Bow • Twist 

Select Structural • 0.250 • 0.50 • 0.375 

No. 1 • 0.250 • 0.50 • 0.375 

No. 2 • 0.375 • 0.75 • 0.500 

No. 3 • 0.500 • 1.00 • 0.750 

Figure 1. Amount of time required to dry ponderosa pine 

2 by 4ís to 12%ñ15% moisture content as a function of 

drying temperature. 

Table 3óWarp in 8-ft-long ponderosa pine 2 by 4ís soon 

after kiln drying

Warp (in.) 

Temp Replicate Crook Bow Twist 

MCa

(%) 

160°F A 0.211 0.407 0.201 14.7 

 B 0.199 0.525 0.269 11.6 

 Average 0.205 0.466 0.235 13.2 

230°F A 0.213 0.509 0.190 14.2 

 B 0.179 0.391 0.144 17.2 

 Average 0.196 0.450 0.167 15.7 

280°F A 0.201 0.505 0.235 12.4 

 B 0.267 0.785 0.228 6.8 

 C 0.155 0.262 0.120 23.4 

 Average 0.208 0.517 0.194 14.2 

a

MC is moisture content. 

100

160 180 200 220 240 260 280

0

20

40

60

80

Drying time (h) 

Drying temperature (°F)

Drying time = 719 ñ 125 In (temp) 

R2

 = 0.998 
4 

Table 5óTukeyís pairwise multiple comparison for 

effects of drying temperature on warp measured 

1 to 2 days after kiln drying 

• Warp • Comparison • p 

• Crook • 230°F vs 280°F • 0.0176*

• • 160°F vs 280°F • 0.0936 

• • 160°F vs 230°F • 0.8419 

• Bow • 230°F vs 280°F • <0.0001*

• • 160°F vs 280°F • <0.0001*

• • 160°F vs 230°F • 0.8718 

• Twist • 230°F vs 280°F • <0.0001*

• • 160°F vs 280°F • 0.9615 

• • 160°F vs 230°F • 0.0002*

Warp After Equilibration 

In the three kiln runs at 280°F, one run was overdried to 

6.8% moisture content, one underdried to only 23.4%, and 

one dried to the target 12% to 15% range. These results are 

illustrated in Figure 2, where the amounts of crook, bow, and 

twist both before and after equilibration to 14% EMC are 

plotted against moisture content measured soon after drying. 

Figure 2 shows that the higher the out-of-kiln moisture 

content, the lower the amount of warp, as measured before 

equilibration. Although unplanned this observation was fully 

expected because when warp was measured, the 2 by 4ís 

with higher out-of-kiln moisture content had not dried as 

much as the boards with lower out-of-kiln moisture content 

and thus would be expected to have less warp. The unexpected observation was that after equilibration to 14% moisture content, a decrease in out-of-kiln moisture content was 

associated with a decrease in warp. For example, crook 

(measured at 14% moisture content) averaged 0.272 in. 

when out-of-kiln moisture content was 23.4% (replicate C), 

but only 0.115 in. when out-of-kiln moisture content was 

6.8% (replicate B) (Table 6). The 2 by 4ís that were dried to 

6.8% moisture content were exposed to the high temperature 

at low moisture contents but the boards dried to only 23.4% 

were not exposed to high temperature at low moisture contents, and this difference could be the cause for the different 

response. This observation suggests the possibility that 

overdrying at 280°F is more effective at reducing warp after 

equilibration than is underdrying at 280°F. 

Warp after the 2 by 4ís had equalized at 75°F and 70% 

relative humidity is shown in Table 6 and Figure 3. Crook, 

bow, and twist all decreased with an increase in drying 

temperature. As drying temperature was increased from 

160°F to 280°F, crook, bow, and twist were reduced 12%, 

29%, and 20%, respectively. The statistical summary is 

shown in Tables 7 and 8. The overall effect of drying temperature on all three forms of warp is statistically significant, 

and in most individual comparisons, the higher drying temperature resulted in significantly less warp. Kiln run replicate was also a significant factor in the amount of crook, 

bow, and twist. This observation was unexpected and its 

cause is unknown. Warp after equilibration to 4%ñ5% moisture content is shown in Table 9. The amount of warp increased from the amount measured at 14% EMC. 

Table 4óTwo-way analysis of variance for effects of drying temperature on warp measured 

1 to 2 days after kiln drying 

Warp 

Source of 

variation 

Degrees of 

freedom 

Sum of 

squares 

Mean 

square F p a

Crook Kiln run 2 0.6411 0.3205 10.73 <0.0001*

 Temp 2 0.2427 0.1213 4.06 0.0175*

 Residual 1059 31.63 0.0299 

 Total 1063 32.30 0.0304 

Bow Kiln run 2 16.467 8.233 71.80 <0.0001*

 Temp 2 6.843 3.422 29.84 <0.0001*

 Residual 1059 121.431 0.1147 

 Total 1063 138.777 0.1306 

Twist Kiln run 2 1.248 0.624 18.69 <0.0001*

 Temp 2 0.893 0.446 13.67 <0.0001*

 Residual 1059 35.357 0.0334 

 Total 1063 37.325 0.351 

a Asterisk indicates statistical significance. 
5

Figure 2óRelationship of moisture content measured 

within 1 to 2 days after kiln drying to amount of warp: 

(a) crook, (b) bow, (c) twist. 

Figure 4 shows the effect of drying temperature on the number of grades lost (on a per 100 boards basis for ease of 

comparison) from the green grade because of exceeding the 

warp limits of Structural Light Framing because of crook, 

bow, twist, or any combination of these. The results of the 

two-way analysis of variance and Tukeyís multiple comparison are shown in Tables 10 and 11. Not all the results are 

statistically significant, but they suggest that higher drying 

temperatures reduce grade loss from warp in 2 by 4ís that 

have been allowed to equilibrate to about 14% and 4% moisture content. After boards were equilibrated to 14% moisture 

content, the loss in grade resulting from warp was 24% 

lower for 2 by 4ís dried at 230°F than at 160°F and 39% 

lower for lumber dried at 280°F than at 160°F. Higher drying temperature also reduced grade loss after equilibration to 

4% EMC, but the overall grade loss was very large compared to that after equilibration to 14% EMC. Very few 2 by 

4ís escaped downgrading after equilibrating to 4% EMC. 

Table 12 offers some insight into the question of whether or 

not any particular form or warpócrook, bow, or twistówas 

responsible for a larger share of the total grade loss. Twist 

was an insignificant source of grade lossóless than two 

grades were lost per 100 boards, regardless of drying temperature. Crook was responsible for more total grade loss 

than was bowógrade loss per 100 boards ranged from 27.6 

grades at 160°F drying temperature to 22.1 grades at 280°F. 

Total grade loss per 100 boards for bow ranged from 22.0 

grades at 160°F to 9.2 grades at 280°F. The data in Table 12 

suggest that reduction in bow at the higher drying temperatures is the most important factor in reducing total grade 

loss, with grade loss per 100 boards dropping from 22.0 

grades at 160°F to 10.5 and 9.2 grades at 230°F and 280°F, 

respectively, which represent reductions in total grade loss 

of 52.3% and 58.2%, respectively. The comparable reductions in total grade loss from crook were 14.1% and 19.9%. 

Table 6óWarp in planed 8-ft-long ponderosa pine 

2 by 4ís after kiln drying and equilibrating at 70°F 

and 70% relative humidity 

 Warp (in.) 

Temp Replicate Crook Bow Twist 

MC 

(%) 

160°F A 0.255 0.379 0.162 15.0 

 B 0.170 0.390 0.156 12.6 

 Average 0.213 0.385 0.159 13.9 

230°F A 0.191 0.337 0.140 13.5 

 B 0.199 0.358 0.127 15.0 

 Average 0.195 0.348 0.134 14.3 

280°F A 0.175 0.304 0.150 13.4 

 B 0.115 0.162 0.055 12.6 

 C 0.272 0.349 0.177 14.5 

 Average 0.187 0.272 0.127 13.5 

6 8 10 12 14 16 18 20 22 24

0.04 

0.08 

0.12 

0.16 

0.20 

0.24

After equilibration to 14%: Twist = 0.0341 + 0.00655 MC

R2 = 0.773 

Before equilibration to 14%: Twist = 0.291 - 0.00681 MC 

R2 = 0.848 

Moisture content soon after kiln drying (%) 

(c) 

Twist (in.)

6 8 10 12 14 16 18 20 22 24

0 

0.20 

0.40 

0.60 

0.80 

(b) 

After equilibration to 14%: Bow = 0.128 + 0.0101 MC 

R2

 = 0.789 

Before equilibration to 14%: Bow = 0.934 - 0.0292 MC 

R2 = 0.946 

Bow (in.)

6 8 10 12 14 16 18 20 22 24

0.08

0.12 

0.16 

0.20 

0.24 

0.28 

Crook (in.)

After equilibration to 14%: Crook = 0.0564 + 0.00920 MC

R2 = 0.998 

Before equilibration to 14%: Crook = 0.298 - 0.00633 MC 

R2 = 0.917 

(a) 
6 

Figure 3. Relationship of warp to drying temperature: 

(a) crook, (b) bow, (c) twist. 

Table 7óTwo-way analysis of variance for effects of 

drying temperature on warp measured after equilibration 

to approximately 14% moisture content 

Warp 

Source 

of 

variation 

Degrees 

of 

freedom 

Sum 

of 

squares 

Mean 

square F pa

Crook Kiln run 2 2.141 1.070 25.30 <0.0001*

 Temp 2 0.756 0.378 8.93 0.0001*

 Residual 1059 44.802 0.0423 

 Total 1063 47.059 0.0443 

Bow Kiln run 2 1.656 0.828 20.37 <0.0001*

 Temp 2 3.789 1.894 46.61 <0.0001*

 Residual 1059 43.040 0.0406 

 Total 1063 47.225 0.0444 

Twist Kiln run 2 0.903 0.451 33.56 <0.0001*

 Temp 2 0.486 0.243 18.05 <0.0001*

 Residual 1059 14.242 0.0135 

 Total 1063 15.336 0.0144 

a

Asterisk indicates statistical significance. 

Table 8óTukeyís pairwise multiple comparison for 

effects of drying temperature on warp measured after 

equilibration to approximately 14% moisture content 

Warp Comparison p 

Crook 230°F vs 280°F 0.0069*

160°F vs 280°F 0.0002*

160°F vs 230°F 0.6426 

Bow 230°F vs 280°F <0.0001*

160°F vs 280°F <0.0001*

160°F vs 230°F 0.1223 

Twist 230°F vs 280°F 0.0031*

160°F vs 280°F <0.0001*

160°F vs 230°F 0.0472*

Table 9óWarp in planed 8-ft-long ponderosa pine 

2 by 4ís after kiln drying and equilibrating at 70°F 

and 20%ñ30% relative humidity

Warp (in.) 

Temperature Replicate Crook Bow Twist 

160°F A 0.692 1.332 0.452 

 B 0.469 1.261 0.397 

 Average 0.581 1.297 0.425 

230°F A 0.473 1.173 0.316 

 B 0.439 0.933 0.261 

 Average 0.456 1.053 0.289 

280°F A 0.390 0.983 0.298 

 B 0.259 0.650 0.164 

 C 0.729 1.143 0.519 

 Average 0.459 0.925 0.327 

160 180 200 220 240 260 280

0.18 

0.19 

0.20 

0.21 

0.22 Crook = 0.247 ñ 0.000219 TMP 

R2

 = 0.985 

Crook (in.)

160 180 200 220 240 260 280

0.24 

0.28 

0.32 

0.36 

0.40 Bow = 0.541 ñ 0.000923 TMP 

R2

 = 0.915 

Bow (in.)

160 180 200 220 240 260 280 

0.12 

0.13 

0.14 

0.15 

0.16

Twist (in.)

Twist = 0.199 ñ 0.000266 TMP 

R2

 = 0.925 

Drying temperature (°F)

(c) 

(b) 

(a) 
7

Figure 4óEffect of drying temperature on total number 

of grades lost per 100 boards for boards equilibrated to 

(a) 14% moisture content and (b) 4% moisture content. 

Table 10óTwo-way analysis of variance for effects of 

drying temperature on grade lossa

Source of 

variation 

Degrees 

of 

freedom 

Sum of 

squares 

Mean 

square F p 

Kiln run 2 12.675 6.338 11.68 <0.0001*

Temp 2 4.147 2.074 3.82 0.0222*

Residual 1059 574.772 0.543 

Total 1063 593.420 0.558 

a

Warp measured after equilibrating lumber to approximately 

 14% moisture content. 

Table 12óTotal grades lost in ponderosa pine 

2 by 4ís by form of warp and drying temperature 

• Grades lost/100 boards a

Warp • 160°F • 230°F • 280°F 

Crook • 27.6 • 23.7 (14.1) • 22.1 (19.9) 

Bow • 22.0 • 10.5 (52.3) • 9.2 (58.2) 

Twist • 1.6 • 6 (0) • 0.9 (43.8) 

a Numbers in parentheses are percentage of reduction in 

 total grades lost compared to total grades lost at 160° 

 drying temperature. 

Conclusions and 

Recommendations

The results show that kiln drying time for ponderosa pine 2 

by 4ís can be reduced from about 84 h at 160°F to 36 h at 

230°F and 15 h at 280°F. The results also offer evidence that 

the amount of crook, bow, and twist that develop in 2 by 4ís 

sawn from open-grown 30- to 35-year-old ponderosa pine 

trees of about 9 inches in diameter progressively decreases 

with an increase in drying temperature from 160°F to 230°F 

to 280°F. The percentage of decrease from 160°F to 280°F 

drying temperature was 12%, 29%, and 20% for crook, bow, 

and twist, respectively. This observation applies to 2 by 4ís 

that were allowed to equilibrate to about 14% moisture 

content. Warp measured immediately after drying, that is, 

before boards were equilibrated to 14%, did not show any 

consistent relationship to drying temperature. 

The total loss in grade because of exceeding warp limits 

under the Structural Light Framing rules decreased with 

increase in drying temperature. The total number of grades 

lost because of any form of warp was 24% lower for 2 by 4ís 

dried at 230°F than at 160°F and 39% lower for 2 by 4ís 

dried at 280°F than at 160°F. Warp and grade loss from 

warp were excessively large after boards were equilibrated 

to 4% equilibrium moisture content. 

• Table 11óTukeyís pairwise multiple 

comparison for effects of drying 

temperature on grade lossa

Comparison p 

230°F vs 280°F 0.6870 

160°F vs 280°F 0.0192*

160°F vs 230°F 0.2368 

• a

Warp measured after equilibrating lumber 

 to approximately 14% moisture content. 

160 180 200 220 240 260 280

160 180 200 220 240 260 280

150 

160 

170 

180 

190 

200 

210 Equilibrated to 4% MC 

Number grades lost/100 boards = 264 ñ 0.387 TMP 

R2 = 0.995

(b) 

Drying temperature (°F)

25 

30 

35 

40 

45 

50 Equilibrated to 14% MC 

Number grades lost/100 boards = 70.4 ñ 0.151 TMP 

R2 = 0.998 (a) 

Number of grades lost per 100 boards
8 

The results of this study and our previous work indicate that 

high temperature kiln drying may be effective in reducing 

warp and grade loss from warp in dimension lumber sawn 

from small-diameter ponderosa pine. Suggested next steps in 

the verification of this approach could be pilot-scale studies 

in full-size commercial kilns. Another question to be addressed is the length of time after kiln drying that is necessary for warp reduction to develop and how this might affect 

the timing of grading and sales. 

Literature Cited 

Arganbright, D.G.; Venturino, J.A.; Gorvad, M. 1978. 

Warp reduction in young-growth ponderosa pine studs dried 

by different methods with top-load restraint. Forest Products 

Journal. 28(8): 47ñ52. 

Fahey, T.D.; Max, T.A.; Sachet, J.K.A. 1986. Factors 

affecting grade change of ponderosa pine lumber in the 

desert Southwest. Forest Products Journal 36(6): 36ñ40. 

Glanz, S.A. 1992. Primer of Bio-Statistics. New York, NY: 

McGrawñHill, Inc. 

Koch, P. 1971. Process for straightening and drying 

Southern Pine 2 by 4ís in 24 hours. Forest Products Journal 

21(5):17ñ24. 

Maeglin, R.R.; Boone, R.S. 1983. An evaluation of Sawñ

DryñRip (SDR) for the manufacture of studs from small 

ponderosa pine logs. Res. Pap. FPLñRPñ435. Madison, WI: 

U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 8 p. 

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

Warpage of studs from young growth ponderosa pine from 

northern New Mexico. Res. Pap. RMñRPñ257. Fort Collins, 

CO: U.S. Department of Agriculture, Forest Service, Rocky 

Mountain Forest and Range Experiment Station. 

Milota, M.R. 2000. Warp and shrinkage of hem-fir stud 

lumber dried at conventional and high temperature. Forest 

Products Journal. 50(11/12): 79ñ84. 

Price, E.W.; Koch, P. 1980. Kiln time and temperature 

affect shrinkage, warp, and mechanical properties of 

Southern Pine lumber. Forest Products Journal. 

30(8): 41ñ47. 

Simpson, W.T.; Green, D.W. 2001. Effect of drying methods on warp and grade of 2 by 4ís from small diameter 

ponderosa pine. Res. Pap. FPLñRPñ601. Madison, WI: 

U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory. 17 p. 

Taylor, F.W. 1993. Warp threat unaffected by higher drying 

temps. Wood Technology. May/June: 36ñ37. 

Wu, Q.; Smith, W.R. 1998. Effects of elevated and high 

temperature schedules on warp in southern yellow pine 

lumber. Forest Products Journal. 48(2): 52ñ56. 

WWPA. 1998. Western Lumber Grading Rules. Portland, 

OR: Western Wood Products Association.