Time-Based Kiln 

Drying Schedule for 

Sugar Maple for 

Structural Uses 

William T. Simpson 

Xiping Wang 

United States

Department of 

Agriculture 

Forest Service 

Forest 

Products 

Laboratory 

Research 

Note 

FPL−RN−0279
Abstract 

This report describes the conversion of a moisture-contentbased kiln schedule to a time-based schedule for drying 

nominal 2- by 6-in. (standard 38- by 140-mm) sugar maple 

lumber for structural use. The recommended schedule requires 115 h of drying time followed by a 15-h equalizing 

period. 

Keywords: kiln schedule, sugar maple 

February 2001 

Simpson, William T.; Wang, Xiping. 2001. Timed-based schedule for kiln 

drying sugar maple for structural uses. Res. Note. FPL-RN-0279. Madison, 

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

Laboratory. 4 p. 

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Time-Based Schedule for Kiln Drying 

Sugar Maple for Structural Uses 

William T. Simpson, Research Forest Products Technologist 

Xiping Wang, Research Scientist

Forest Products Laboratory, Madison, Wisconsin 

Introduction 

A previous study (Simpson and others 1998) showed that 

maple could be kiln dried successfully for structural uses by a 

schedule that was accelerated in comparison with the traditional maple schedule intended for products where appearance is important, such as furniture. In addition to being 

unnecessarily conservative and slow for lumber intended for 

structural uses, the traditional maple schedule is controlled 

on the basis of moisture content. Changes in kiln conditions 

are made when the lumber reaches certain levels of moisture 

content; this requires some method of estimating moisture 

content during the kiln run. Because of the less exacting 

quality requirements of structural lumber compared to 

appearance-grade lumber, traditional kiln schedules for 

softwood structural lumber are usually based on time; that 

is, changes in kiln conditions are made at predetermined time 

intervals rather than at predetermined moisture content 

levels. Thus, periodic estimates of moisture content during 

the kiln run are not necessary, and time-based schedules 

are therefore more efficient than are schedules based on 

moisture content. 

The primary objective of the study reported here was to 

convert the moisture-content-based kiln schedule developed 

in the previous study (Simpson and others 1998) to a schedule based on time. A secondary objective was to examine the 

effectiveness of several equalizing periods on moisture 

content variability. 

Material and Methods 

Nominal 2- by 6-in. (standard 38- by 140-mm), 8-ft- (2.4-m-) 

long sugar maple lumber was obtained from the Upper Peninsula of Michigan. Enough boards were obtained for three 

kiln runs of 84 boards each. The moisture-content-based kiln 

schedule developed in the previous study (Simpson and 

others 1998) and the time-based schedule used in the study 

reported here are shown in Table 1. The time-based schedule 

was developed from the relationship between moisture content and time found in the previous study; this relationship is 

shown in Figure 1, along with notations for the time-based 

schedule. Air velocity ranged from 500 to 600 ft/min 

(2.54 to 3.03 m/s), with fan reversal every 6 h. 

The kiln schedule was followed by three different equalizing 

periods (12, 24, and 48 h) at 180°F (82°C) dry-bulb and 

170°F (77°C) wet-bulb temperatures. Moisture content variability was determined gravimetrically. Each board was 

weighed three times: in the green condition, after drying, and 

after oven drying at 220°F (104°C) for long enough to reach 

a constant weight. 

Results 

Lumber green moisture content levels and their frequency 

distributions for the three kiln runs are shown in Figure 2. 

Moisture content after drying is summarized in Figure 3, 

histograms of final moisture content distributions for the 

three kiln runs, which had different equalizing periods. Runs 

1 and 2 each had one outlying data point; that is, a final 

moisture content value that was very different from other 

values. These outliers (2% and 41% moisture content) were 

eliminated from the analysis. 

Table 1—Moisture-content- and time-based kiln 

schedules for nominal 2- by 6-in. (standard 38- by 

140-mm) sugar maple lumber intended for 

structural use 

Dry-bulb 

temperature 

Wet-bulb 

temperature Moisture 

content 

(%) 

Time 

(h) (°F) (°C) (°F) (°C) 

50+

 0–24 160 71 153 67 

50–40 24–48 160 71 150 66 

40–35 48–60 160 71 145 63 

35–30 60–72 160 71 135 57 

30–25 72–84 170 77 130 54 

25–Final 84–120a

 180 82 130 54 

Equalizing — 180 82 170 77 

a

In the recommended time-based schedule developed 

in the study reported here, final drying time is shortened 

to 115 h and followed by 15 h of equalizing. 
2 

One way to analyze the influence of equalizing time on final 

moisture content is to compare standard deviation values. If 

longer equalizing time reduces the standard deviation in final 

moisture content, then a longer equalizing time can be judged 

effective in reducing the variation in final moisture content. 

If standard deviation does not decrease as equalizing time is 

increased, then there is no justification for prolonging equalizing time. If the outliers had been included in the analysis, 

they would have had such a great effect on the standard 

deviations that the remaining results would have been 

distorted and would likely have led to false conclusions. 

Average final moisture content values for the three kiln runs, 

in order of progressively increasing equalizing time, were 

13.4%, 14.1%, and 13.2%, with corresponding standard 

deviations of 1.91%, 2.75%, and 1.64%. Because there was 

no progressive decrease in standard deviation with increasing 

equalizing time, we cannot conclude that longer equalizing 

time is necessary to reduce variation in final moisture 

content. 

The final moisture content values observed may be a little 

lower than would be necessary for structural lumber. Structural lumber is commonly kiln dried to a target moisture 

content of 15%. The data for this study were only for the 

final moisture content at the end of drying plus equalizing 

Figure 1—Drying curve for nominal 2- by 6-in. 

(standard 38- by 140-mm) sugar maple lumber 

derived from a kiln schedule based on moisture 

content (Simpson and others 1998) and converted 

to a schedule based on time. 

Figure 2—Distribution of green moisture content of 

lumber for three experimental kiln runs. 
3 

time. To estimate the time required for the lumber to reach 

15% moisture content instead of the actual lower final moisture content of 13.4%, 14.1%, and 13.2% for the three kiln 

runs, we pooled the initial and final moisture content data of 

the runs. We then used a computer drying simulation (Hart 

1982, Simpson and others 1998) to generate a drying curve 

that is a composite of the three experimental kiln runs 

(Fig. 4). As Figure 4 indicates, the drying time to 15% moisture content was about 115 h instead of the 120 h estimated 

in the schedule (Table 1, Fig. 1). 

The drying curve shown in Figure 5 was obtained by 

shortening the last step of the schedule from 36 to 31 h, so 

that lumber moisture content reached 15% in a total drying 

time of 115 h. Figure 5 also shows that an equalizing time 

of about 15 h (bringing total drying plus equalizing time to 

about 130 h) will decrease average final moisture content to 

15% after the moisture content is increased by adding 

moisture to the overdried boards and then decreased as the 

underdried boards continue drying and the overdried boards 

no longer gain moisture. 

Recommendations 

The time-based kiln schedule developed in this study, in 

which the final step is shortened from 36 to 31 h and is 

followed by a 15-h equalizing period, will result in a final 

moisture content of about 15%. This schedule is recommended as a starting point for individual kiln operators, with 

adjustments to be made according to observations over a 

number of production kiln runs. 

Figure 3—Distribution of final moisture content of 

lumber for three kiln runs. 

Figure 4—Composite drying time curve representing 

three kiln runs, with drying time to actual moisture 

content below 15%. 
4 

Acknowledgments 

We gratefully acknowledge the cooperation of Northern 

Hardwoods, South Range, Michigan, and John Forsman 

of Michigan Technological University, Institute of Wood 

Research, Houghton, Michigan, for the lumber and 

procurement arrangements. 

Literature Cited 

Hart, C.A. 1982. SIMSOR: A computer simulation of water 

sorption in wood. Wood and Fiber. 13(1):46–71. 

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

maple structural lumber from log heart cants. Forest Products 

Journal. 48(6):70–76. 

Figure 5—Drying curve representing three kiln runs 

adjusted to 15% final moisture content.