Internal Moisture Content and 

Temperature of Standardized 

Aboveground Wood Durability 

Test Specimens

Stan T. Lebow

Patricia K. Lebow

United States Department of Agriculture

Forest

Service

Forest Products

Laboratory

Research Paper

FPL–RP–694

July

2018
Abstract

Durability of wood products intended for use in outdoor 

aboveground applications are evaluated using one or more 

standardized test methods. These methods use relatively 

small specimens in an effort to accelerate decay growth and 

allow detection of decay. In this study, we evaluated internal 

moisture content and temperature of standardized specimens 

and compared them with specimens of a simulated post and 

beam connection that is recommended for residential deck 

construction. Specimens of each type were cut from endmatched Southern Pine timbers and exposed above ground 

at a test site near Madison, Wisconsin. Pairs of stainless 

steel screws were inserted at multiple locations within each 

specimen to allow moisture content determination with a 

resistance-type moisture meter. Internal temperature was 

monitored in one specimen of each type using a thermistor 

probe and data logger. The specimens of the simulated post 

and beam connection sustained higher moisture contents and 

experienced less moisture content fluctuation than any of 

the standardized test specimens. Internal temperature also 

varied by specimen type but not to the extent observed for 

moisture content. The findings of this study suggest that 

frequent drying of standardized specimens has the potential 

July 2018

Lebow, Stan T.; Lebow, Patricia K. 2018. Internal moisture content and 

temperature of standardized aboveground wood durability test specimens.

Research Paper FPL–RP–694. Madison, WI: U.S. Department of 

Agriculture, Forest Service, Forest Products Laboratory. 14 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 53726-2398. 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 use of trade or firm names in this publication is for reader information 

and does not imply endorsement by the United States Department of 

Agriculture (USDA) of any product or service.

to inhibit decay and may underestimate the decay hazard 

present within larger members of aboveground structures. 

Of the current standardized methods, the E18 ground 

proximity test appears to offer the best combination of 

relatively high moisture contents and ease of inspection. 

Other approaches, such as those that stack or layer groups of 

smaller specimens to entrap moisture and enhance decay, 

may warrant further consideration.

Keywords: durability testing, aboveground, standardized 

specimens, moisture content, temperature

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Introduction

In the past decade, there has been increasing interest in 

using less toxic preservative systems or lower retentions of 

preservative for wood used above ground. Preservative 

formulations intended for aboveground applications are not 

necessarily evaluated with ground-contact stake tests and 

instead are evaluated using standardized aboveground test 

methods (AWPA 2017a). These methods typically use 

relatively small specimens to facilitate inspection, and it is 

unclear how consistently these test specimens sustain 

moisture contents necessary to allow rapid growth of decay 

fungi. Moisture content is a critical factor in the risk of 

biologic and nonbiologic degradation of wood used above

ground (Lebow and Carll 2011, Lebow and Highley 2008,

Meyer and Brischke 2015, Zabel and Morrell 1992), and in 

many climates, it is the primary determinant of the 

aboveground decay hazard. Decay fungi require a moisture 

content of at least 20% to sustain any growth, and higher 

moisture contents (over 29%) are required for initial spore 

germination (Clausen 2010, Morris 1998, Zabel and Morrell 

1992). Most brown and white rot decay fungi prefer wood in 

the moisture content range of 40% to 80% (Zabel and 

Morrell 1992), although Schmidt (2007) concluded that 

moisture contents in excess of 100% are within the 

favorable range of decay fungi. Growth at lower moisture 

contents is much slower, and typically, wood with a 

moisture content of less than 25% cannot be attacked unless 

the fungus has another source of moisture nearby. In 

addition, wetting and drying cycles can negatively affect

fungal growth, with the return to decay activity significantly 

delayed after a drying period (Wadsö and others 2013).

Moisture conditions in structures are dependent on 

construction details that affect the uptake of free water and 

the dissipation of water (primarily by evaporation) from the 

wood. Although the importance of moisture content is 

widely acknowledged, there has been relatively little effort 

to evaluate moisture content within in-service wood 

products or within test specimens used to evaluate 

durability. Of the current American Wood Protection 

Association (AWPA) methods for evaluating the durability 

of wood products used above ground, only method E28 

(Standard Field Test for Serviceability of Decking) suggests 

monitoring of wood moisture content (AWPA 2014).

Interestingly, E28 is intended as a test of decking 

appearance and dimensional stability rather than a test for 

decay development. In addition, these standard test 

specimens have relatively small dimensions compared with

some of the members used in construction. As a result, 

specimens in this test may dry more rapidly than some areas 

within actual structures. For example, the American Wood 

Council (AWC) Prescriptive Residential Deck Construction 

Guide specifies that deck beams be supported on a notched 

140- by 140-mm (nominal 6- by 6-in.) or larger post (AWC 

2015). This configuration would be expected to trap 

moisture in the area where the beam members sit on the 

end-grain of the post. The lack of data on specimen moisture 

contents limits our ability to relate the results from test 

specimens to in-service structures.

Materials and Methods

Test specimens were prepared according to five different 

AWPA evaluation methods. A simulated post and beam 

connection as prescribed for deck construction by the AWC

(AWC 2015) was also prepared as a “real world” 

comparison. Moisture content was monitored as these 

specimens were exposed to natural weathering.

Moisture Content Determination

Moisture contents were determined using a resistance-type 

(also referred to as pin-type or conductance-type) moisture 

meter. This approach allows for estimating moisture content 

at various depths within the wood, depending on the 

placement of the metal pins. Although these meters are 

widely used in wood drying and for monitoring indoor wood 

moisture content, their use for measuring the moisture 

content of wood exposed outdoors has been more limited.

This is largely because the meters were thought to have little 

value for measuring moisture contents above the wood fiber 

saturation point (approximately 25% to 30% moisture 

Internal Moisture Content and 

Temperature of Standardized 

Aboveground Wood Durability 

Test Specimens

Stan T. Lebow, Research Forest Products Technologist

Patricia K. Lebow, Research Mathematical Statistician

USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin
Research Paper FPL–RP–694

2

content) (ASTM 2014, James 1988), and some commercial 

meters do not display moisture contents above 40%.

However, some change in resistivity does occur at higher 

moisture contents, and researchers have presented data 

indicating that measurements above 30% can be at least 

semiquantitative (Smith and others 2007, Brischke and 

Lampen 2014). Recent research noted that some of the 

variability at higher moisture contents was attributable to 

poor contact between the wood and the electrodes and that 

this variability could be lessened by using screws rather than 

pins as electrodes (Lebow and Lebow 2016). A segregated 

regression model was developed to relate moisture content 

readings above the fiber saturation point to oven-dry 

moisture content, and temperature correction was also 

developed to allow monitoring moisture content in 

specimens exposed outdoors (Lebow and Lebow 2016).

This approach was used successfully to monitor moisture 

content in leaching specimens exposed to natural 

precipitation (Lebow and others 2017).

In this study, pairs of trim-head stainless steel screws spaced 

25 mm apart were used as electrodes. The depth of the 

screws and their location depended on the type of specimen, 

but in each case, the screws were driven to at least the center 

of the specimen. The moisture meter used was a General 

Electric Protimeter Timbermaster (General Electric Sensing, 

Danbury, Connecticut, USA), which displays moisture 

content readings between 7% and 100%. A hole was drilled 

in one replicate of each specimen type to allow insertion of 

a temperature probe at the time readings were taken.

Internal Temperature Monitoring

One additional replicate specimen of each type was added to 

the exposure for use in monitoring internal temperature. A 

6-mm-diameter hole was drilled into each specimen, 

allowing a thermistor probe (5 mm diameter, 25 mm length) 

to be inserted into the center of the specimen near an area of 

moisture content determination (Figs. 1–6). The hole was 

then sealed around the probe wire using silicone caulk. The 

thermistor probe was connected to a thumb drive data logger 

(EL-USB-TP-LCD, Lascar Electronics Incorporated, Erie, 

Pennsylvania, USA), which was programmed to record 

temperature at 30-min intervals. An additional thermistor 

probe and data logger were installed to monitor ambient air 

temperature at the exposure site.

Test Specimen Configurations

Specimens were cut in accordance with AWPA standards 

E9, E16, E18, E25, and E27 (AWPA 2017b-f). In addition, a 

simulated post–beam connection was devised to simulate 

the AWC deck construction recommendations (AWC 2015).

Five replicates of each specimen type were prepared. To 

minimize moisture content differences resulting from wood 

properties, matched specimens of each type were cut from 

five replicate 140- by 140-mm (nominal 6- by 6-in.) 

timbers. Inclusion of heartwood was avoided except in the 

case of the “post” member in the simulated post and beam 

specimens, which used the entire cross section of the timber.

The various specimen configurations are described in the 

following paragraphs:

• E9, L-joint method (AWPA 2017b): In this method, two 

38- by 38-mm (1.5- by 1.5-in.) pieces are joined at right 

angles with a mortise and tenon joint (Fig. 1). A 

protruding tenon cut at the end of one piece fits into a 

matching recess (mortise) in the other piece to create a 

moisture-trapping joint. In this study, moisture probe 

screws (30 mm long) were drilled into three locations 

within each specimen as shown in Figure 1. In 

accordance with the standard, specimens were exposed 

on a rack tilted at a 10° angle, with spacers to prevent the 

specimens from lying flat against the test rack.

• E16, lap-joint method (AWPA 2017c): This method 

prescribes that specimens 89 mm wide and 38 mm thick 

be cut to create an overlapping joint as shown in 

Figure 2. In this study, sets of moisture probe screws 

76 mm long were drilled into a narrow face of each 

specimen in such a way that they crossed its width and 

extended to within 13 mm of the opposite narrow face.

An additional set of 30-mm-long moisture probe screws 

was drilled directly into the lap joint from the bottom 

wide face of the specimen in such a way that they 

extended to within 8 mm of the upper wide surface

(Fig. 2B).

• E18, ground proximity method (AWPA 2017d): This 

method does not use a moisture trapping joint but instead 

places 19-mm-thick by 50-mm-wide specimens on 

concrete blocks that are resting on the ground. The test 

array is covered with shade cloth to further slow drying.

In this study, four 41-mm-long moisture probe screws 

were drilled into a narrow face of each specimen in such 

a way that they extended to within 9 mm of the opposite 

narrow face. The distance between the screws was 

25 mm, allowing three moisture readings to be taken 

from each specimen (Fig. 3).

• E25, decking test (AWPA 2017e): This method 

simulates a deck constructed near the ground and uses

commodity-sized 38-mm-thick by 140-mm-wide 

specimens. One end of the specimens is butted against 

what the standard refers to as an “inert material” while 

the other is open to the air. In this study, the inert 

material was wood covered with rubber roofing 

membrane. A space of 3 to 5 mm was left between the 

specimen and the rubber membrane. Sets of moisture 

probe screws 102 mm long were drilled into the narrow 

faces of the specimens near each end at the midpoint. 

The screws extended to within 38 mm of the opposite 

narrow face (Fig. 4).

• E27, accelerated lap joint (AWPA 2017f): This method 

uses three overlapping parts held together with PVC pipe 
Internal Moisture Content and Temperature of Standardized Aboveground Wood Durability Test Specimens

3

sections that are cut and slipped over both long edges of 

Figure 1. E9 L-joint specimen showing location of screw electrodes (A, B, and C) for five replicate specimens and 

location of thermistor probe (one additional specimen).

Figure 2. E16 lap-joint specimen showing location of screw electrodes (A, B, and C) for five replicate specimens and 

location of thermistor probe (one additional specimen).

Figure 3. E18 ground proximity specimen showing location of screw electrodes (A, B, and C) for five replicate 

specimens and location of thermistor probe (one additional specimen).
Research Paper FPL–RP–694

4

sections that are cut and slipped over both long edges of 

Figure 4. E25 decking specimen showing location of screw electrodes (A, B, and C) for five replicate specimens and 

location of thermistor probe (one additional specimen).

Figure 5. E27 accelerated lap-joint specimen showing location of screw electrodes (A, B, C, and D) for five replicate 

specimens and location of thermistor probe (one additional specimen).

Figure 6. Simulated post and beam specimen showing location of screw electrodes (A, B, and C) for five replicate 

specimens and location of thermistor probe (one additional specimen).
Internal Moisture Content and Temperature of Standardized Aboveground Wood Durability Test Specimens

5

sections that are cut and slipped over both long edges of 

the specimens (Fig. 5). The three parts of the specimens 

have relatively small dimensions. The two upper parts of 

the specimens are each 12 mm thick by 27 mm wide, 

whereas the bottom piece is 12 mm thick and 55 mm 

wide. The PVC clamps along the long edges of the 

specimens to slow drying. To further slow drying, the 

standard prescribes that the specimens be placed onto a 

test rack in such a way that each end is pressed against 

plastic foam pads. In this study, moisture probe screws 

were drilled upward from the wide bottom face of each 

specimen in such a way that they contacted both the 

bottom piece and one of the two top pieces of the 

assembly. Four sets of probe screws were installed, two

on each side of the specimen (Fig. 5). The length of the 

screws was 25 mm, but they were left protruding 6 mm 

from the bottom of the specimen, leaving only 19 mm 

inserted into the specimen.

• Simulated post–beam: A simulated post–beam 

configuration was prepared by notching a 280-mm-long 

section of 140- by 140-mm (nominal 6- by 6-in.) timber

to a depth of 140 mm and a width of 76 mm. Two 38- by 

140- by 280-mm members (nominal 2 by 6 in.) were 

then bolted into the notch to simulate a beam (Fig. 6).

The cut ends of these beam members were sealed with 

an elastomeric neoprene maintenance coating (N-700, 

Republic Powdered Metals Coatings and Lining 

Systems, Beachwood, Ohio, USA) to simulate the 

wetting–drying characteristics of a longer beam.

Specimen Exposure and Evaluation

Specimens used for determination of moisture content were

exposed to natural weathering in an open field test site west 

of Madison, Wisconsin, in March 2016 and were monitored 

through October of 2017. Test racks were constructed to 

support and expose each type of specimen in accordance 

with its respective standard. On approximately a weekly 

basis, moisture content readings were taken from the probe 

locations shown in Figures 1 through 6. Evaluations were 

conducted at least 12 h after rainfall events to prevent 

surface moisture from affecting resistance moisture content 

readings. Additional specimens for monitoring internal 

temperature were added to the exposure in August of 2016. 

Daily precipitation data during the exposure were obtained 

from a United States Geologic Survey monitoring station 

7.3 km northwest of the exposure site (USGS 2017).

Madison’s climate is characterized as humid-continental 

(Köppen classification: Dfa) (Schatz and Kucharik 2014) 

with an average of 876 mm of precipitation annually 

(NOAA 2017). June, July, and August are the months with 

the greatest precipitation, and average temperatures in 

December, January, and February are below freezing.

Results and Discussion

Precipitation During Exposure

The pattern of rainfall that occurred during specimen 

exposure is shown in Figure 7. Greater than average rainfall 

occurred during both the 2016 and 2017 exposure periods. A 

total of 1,902 mm of precipitation was received, which was 

higher than the 1,566 mm that would have been expected for 

this period based on historical averages (SCO 2017).

Average monthly temperatures were often above and were 

rarely below historic averages (Fig. 8). Every month, except 

April 2016 and January, February, September, and October 

2017, temperatures exceeded the historical average.

Figure 7. Daily precipitation for the duration of the exposure.

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Research Paper FPL–RP–694

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Observed Specimen Moisture Contents

It is important to note that the moisture probes detect the 

path of least electrical resistance present anywhere between 

the probes. Thus, the moisture content reported is the 

highest in that area and is not necessarily the average or 

typical moisture within the specimen. For the larger 

specimens, the highest moisture content is likely to occur 

near the center of the specimen with lower moisture 

contents near the surface. The probe moisture contents may 

be somewhat more representative for the small specimens 

such as the E18 ground proximity and E27 accelerated lap 

joint, but even with these specimens, it is likely that the 

outer portion of the specimen was dryer.

The types of test specimens differed substantially in their 

pattern of moisture absorption and retention. For larger 

specimens, consistent moisture differences were also 

observed by location within the specimen. The E9 L-joint 

specimens tended to have the lowest moisture contents, with 

rapid redrying after rainfall events (Fig. 9). This is probably

attributable to the relatively small dimensions of the 

specimens and the fact that they were exposed in a manner 

that allowed air circulation between the specimen and the 

exposure rack. As expected, the joint tended to have the 

highest moisture content, whereas the vertical mortise part 

of the specimen typically had the lowest moisture content.

The even smaller dimensions of the accelerated lap-joint 

specimens (E27) also appeared to allow rapid drying, 

despite the use of PVC pipe sections on the sides of the 

specimens and foam pressed against the end-grain (Fig. 10).

Moisture contents were relatively consistent among probe 

locations for these specimens, perhaps because of their 

small size. The larger E16 lap-joint specimens often had

moisture contents equivalent to or greater than the E27 

accelerated lap-joint specimens even without the use of PVC 

pipe or foam to hold moisture. There was not a consistent 

trend of the joint area of the E16 lap specimens having the 

highest moisture content, although this was the case initially 

(Fig. 11). In contrast, the E25 decking specimens exhibited a 

consistent trend of the highest moisture content occurring 

near the end of the specimen that was abutted against the 

inert surface (Fig. 12). The moisture content at this location 

was often higher than those observed in all specimen types 

except the post–beam. However, moisture contents at the 

other two locations on the decking specimen were much 

lower, with the middle section of the specimen tending to be 

the driest. Not surprisingly considering their small 

dimensions and simple design, the E18 ground proximity 

specimens had much more uniform within-specimen 

moisture contents (Fig. 13). They also generally had higher

moisture contents than the other types of standardized 

specimens. However, it is notable that these specimen did 

dry below 20% moisture content at various times despite 

their direct contact with concrete and shade cloth covering.

This tended to occur in the summer months when heat and 

sunlight were most intense. After initially wetted, the post 

and inner beam locations of the post–beam specimens more 

consistently maintained high moisture contents compared 

with any of the standardized test specimens (Fig. 14). The 

post part of the specimen was slow to gain moisture but then 

maintained high moisture contents even during periods 

when the standardized specimens dried substantially.

These results clearly show that, as expected, specimens with 

larger dimensions are more resistant to drying and thus have 

sufficient moisture to promote decay for a greater proportion 

of time. To better visualize this effect, the percentage of 

observations for which the geometric mean of each 

specimen type exceeded various moisture thresholds is 

Figure 8. Average monthly temperatures during exposure compared with historical average temperatures.

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Average monthly temperature (°C)

Month of exposure

Historical Average

2016

2017
Internal Moisture Content and Temperature of Standardized Aboveground Wood Durability Test Specimens

7

displayed in Figure 15a. For each specimen type, only the 

Figure 9. Geometric mean of moisture contents observed for three locations within the E9 L-joint specimens.

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Figure 10. Geometric mean of moisture contents observed for four locations within the E27 accelerated 

lap-joint specimens.

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Figure 11. Geometric mean (five replicates) of moisture contents observed for three locations within the 

E16 lap-joint specimens.

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Lap 1 Joint Lap 2
Research Paper FPL–RP–694

8

displayed in Figure 15a. For each specimen type, only the 

Figure 12. Geometric mean of moisture contents observed for three locations within the E25 decking specimens. Readings 

capped at 100%.

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Figure 13. Geometric mean of moisture contents observed for three locations within the E18 ground proximity specimens. 

Readings capped at 100%.

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Figure 14. Geometric mean of moisture contents observed for three locations within the simulated post and beam 

specimens. Readings capped at 100%.

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Outer Beam

Inner Beam

Post
Internal Moisture Content and Temperature of Standardized Aboveground Wood Durability Test Specimens

9

displayed in Figure 15a. For each specimen type, only the 

probe location that most frequently had the highest moisture 

content is shown. For example, in the E25 decking 

specimens, only the moisture contents observed near the end 

of the specimen that was abutted against the inert surface is 

shown. Although all specimen types had a probe location

with moisture content above 20% for most observations, the 

proportion of observations above 40% was much lower for 

the AWPA standard specimens than for the post–beam

specimens. This indicates that although standard specimens 

typically had enough moisture to sustain decay, they were

much less likely to reach the 40% to 80% moisture content 

range thought to be optimal for brown rot decay fungi 

(Zabel and Morrell 1992). The relatively low moisture 

contents of the standardized specimens become even more 

of a concern when viewing only the summer months (June–

August), when temperatures are most conducive to decay 

(Fig. 15b). During these months, even the E18 ground 

proximity specimens exceeded 40% moisture content for 

only 42% of observations, and the percentage of moisture 

contents favorable for decay was even lower for the other 

types of standardized specimens. This occurred despite 

Figure 15. Percentage of moisture content geometric means above various moisture content thresholds for (a) entire 

exposure period or (b) months of June, July, and August. Data are for the electrode location that most frequently had the 

highest moisture content within each specimen type. Readings capped at 100%.
Research Paper FPL–RP–694

10

relatively frequent rainfall during the summer months. An 

average of only 3 days passed between rainfall events, and 

the longest dry spell during the two summers was 9 days.

Although the highest moisture content noted for the post–

beam specimens was typically above the 80% to 90%

reported to be the upper limit for optimal growth (Brischke 

and Rapp 2008, Zabel and Morrell 1992), the moisture 

probe detects the area of highest moisture content, and it is 

likely that a gradient of gradually decreasing moisture 

content extends toward the surface of the specimen. In 

addition, Schmidt (2007) concluded that the literature 

supported preferred moisture content ranges well in excess 

of 100%.

The effect of drying observed in the smaller specimens may 

be greater than that solely attributable to time at a lower 

moisture content. Wadsö and others (2013) noted that there 

can be a significant delay in the return to fungal activity 

after a drying period. The differences in moisture content 

between the post–beam and standard specimens might affect 

not only the rate of decay but also the types of decay fungi 

present because moisture tolerances have been shown to 

vary by type of fungus (Meyer and Brischke 2015).

Observed Specimen Temperatures

The internal temperature of the specimen types differed as a 

function of air temperature (recorded in the shade adjacent 

to the specimens). During summer heating (Fig. 16a), the 

internal temperature of all specimens exceeded the air 

temperature, with the highest temperatures observed in the 

E25 decking and E16 lap-joint specimens. Even the E18 

ground proximity specimens, which were partially protected 

by shade cloth, were affected by heating from solar 

radiation. The post–beam specimens had a more muted and 

somewhat delayed response to summer daytime temperature 

increases because of their larger size but still reached 

temperatures a few degrees above the air temperature. In 

contrast, the temperature of the specimens was often at or 

below the air temperature during winter days and nights

(Fig. 16b) although the E25 decking and E16 lap-joint 

specimens still experienced the highest daytime 

temperatures. The temperature swings within the post–beam 

specimens were again relatively subdued but not to the 

extent exhibited by the E18 ground proximity specimens.

The E18 specimens stayed notably warmer during the cold 

nights, perhaps because they rested on concrete that was

directly in contact with the ground. The shade cloth may 

also have served to slow heat loss from the concrete block. 

However, this same effect apparently limited warming of 

the E18 ground proximity specimens during warm spring 

days that followed the freezing temperatures (Fig. 16c).

Under those conditions, the E18 specimens remained cooler 

than the post–beam specimens, which exhibited their 

characteristic delayed response to changing air temperature.

It is apparent that the effect of specimen dimensions on 

internal temperature is complex in a climate such as 

Wisconsin, which has wide seasonal temperature variation.

Other researchers have reported that growth of most decay 

fungi is negligible at temperatures below 2 °C and is 

relatively slow at temperatures below 10 °C. The growth 

rate then increases rapidly, with most fungi having optimum 

growth rates between 24 and 35 °C (Boddy 1983, Clausen

2010, Zabel and Morrell 1992). There are a few wood 

destroying fungi that prefer temperatures in the 34 to 36 °C 

range, and these are most commonly found in wood exposed 

to sunlight (Morris 1998). The rate of growth declines 

Figure 16. Examples of internal specimen temperatures for 

48-h periods in (a) June, (b) January, and (c) April. Dashed 

horizontal lines (a and c only) indicate the 24 to 35 °C 

temperature range (c) that is reported to be preferred by 

decay fungi.
Internal Moisture Content and Temperature of Standardized Aboveground Wood Durability Test Specimens

11

steeply at higher temperatures, with little growth above 

40 °C and no growth above 46 °C. The proportions of 

recorded specimen temperatures from September 2016 to 

September 2017 falling within three temperature ranges 

reported to be favorable for growth of decay fungi are 

shown in Figure 17. Specimen temperatures were within the 

range where fungal growth is possible (2–46 °C) more than

65% of the time, with the E27 accelerated lap-joint 

specimens having a slightly higher proportion of 

temperatures in this range than other specimen types. 

Between 46% and 50% of readings were within the more 

favorable 10 to 40 °C temperature range, and in this case,

the post–beam specimen had a slightly higher proportion of 

temperatures within this range. Only 8% to 12% of readings 

were in the most favored 24 to 35 °C temperature range, 

with the E18 ground proximity specimens having the lowest 

proportion of favorable temperatures. It is likely that contact 

with the concrete block on soil slows initial warming during 

a heating period and then causes more rapid cooling as the 

temperature drops. Examples of this trend can be seen in 

Figure 16a and c. For all specimen types, the highest 

proportion of favorable temperatures occurred in the 

summer months when wood moisture contents were often 

below the range favorable for decay in the standardized test 

specimens (Fig. 15b).

Both the E16 lap-joint and E25 decking specimens recorded 

temperatures well above 46 °C on several occasions, at 

times exceeding 50 °C. This is likely to occur more 

frequently for specimens exposed in warmer climates, but it 

is unclear if daily brief periods of high temperatures might 

have a significant negative effect overall on fungal growth.

Previous researchers have reported somewhat conflicting 

values on the effect of high temperatures on fungal growth, 

with one report concluding that a temperature of 50 °C is 

lethal for some types of decay fungi in less than 8 h and 

lethal for most others evaluated after 24 h (Hulme and 

Stranks 1976). However, Chidester (1939) found that many 

decay fungi could tolerate temperatures up to 65 °C for 10 h

or more, although one fungus (Poria incrassata) was killed 

at 45 °C. In his summary of the literature, Schmidt (2007) 

reported that most decay fungi can tolerate temperatures up 

to 65 °C for at least 4 h. Given that decay of untreated 

decking proceeds rapidly in southern climates, it seems 

evident that some types of decay fungi have adapted to 

intermittent exposure to high temperatures. However, it is 

possible that the high temperatures experienced by the E16 

lap-joint and E25 decking specimens could influence the 

types of decay fungi that colonize the wood. Temperature 

affects not only the rate of fungal growth but possibly also 

the type of fungi that are successful in colonization (Hiscox 

and others 2016). Fluctuations in temperature have also 

been shown to increase the diversity of fungi present by 

limiting the ability of dominant fungi to exclude competitors 

(Toljander and others 2006). Previous temperature 

exposures also appear to have a lingering effect, because

fungal communities established at warmer temperatures 

decay wood more rapidly than those established during 

cooler temperatures (Rubenstein and others 2017).

Not surprisingly, given the climate at the test site, specimen 

temperatures were often below those thought to be favorable 

for fungal growth. Although internal temperatures did vary 

somewhat by specimen type, these differences did not have 

a large effect on the percentage of readings within ranges 

favorable for fungal growth (Fig. 17). At this location,

specimen configuration apparently had a much greater effect 

on moisture content than temperature.

Figure 17. Percentage of temperature readings falling within temperature ranges reported to be optimal (24 to 35 °C), 

preferred (10 to 40 °C), or tolerated (2 to 46 °C) by wood decay fungi. Based on temperature observations for one year, from 

September 2016 to September 2017.
Research Paper FPL–RP–694

12

Results of moisture content monitoring clearly show that 

AWPA standardized aboveground test specimens may 

frequently have lower moisture contents than those that 

occur in larger members of in-service structures, at least in 

this location. This was especially apparent in the summer 

months when temperatures are most conducive to growth of 

decay fungi. The moisture content of the standardized 

specimens also fluctuated much more widely than moisture 

contents observed in the larger post–beam specimens. This 

finding has implications for the use of current standard test 

specimen dimensions in evaluating and predicting future 

long-term performance of preservative treatments. Although 

these tests are often portrayed as a means of accelerated 

testing, it is possible that frequent drying in standardized 

specimens is actually decelerating decay or at least 

underestimating the extent of the hazard. It is also possible 

that the lower moisture contents and wider moisture 

fluctuations observed in the standardized specimens favor a 

different composition of fungi and other microorganisms 

than would exist within in-service structures. The types of 

decay fungi present could influence both rate of decay and, 

possibly, extent of tolerance to a specific preservative 

formulation. To evaluate this possibility, decay development 

in these specimens will be monitored and compared with

that observed in the post–beam specimens.

One possible solution to frequent drying could be to use 

larger specimens. However, it may difficult to detect decay 

during inspections because decay is likely to develop 

internally. This is an existing disadvantage of the current 

E25 decking specimens. Other methods that use stacks or 

layers of somewhat smaller specimens that serve to trap 

moisture while still allowing disassembly for internal 

inspection may warrant further consideration (De Groot 

1992, Jenkins 2012, Meyer and others 2016). The E27 

accelerated lap-joint method does use two layers, but it 

appears that larger pieces and/or additional layers are 

needed to enhance moisture trapping. Of the current 

standardized methods, the E18 ground proximity specimens 

appeared to most closely simulate moisture contents 

observed in the post–beam specimens while still using small 

dimensions that ease inspection. The E18 specimens did 

tend to have slightly lower temperatures because of their 

contact with concrete, but this should be less of a concern in 

test sites with warmer climates. Tropical or subtropical test 

sites can provide acceleration of test results for wood 

products that are subsequently used in cooler or drier 

climates with lower decay hazard. However, providing 

conservative durability prediction for wood products that are 

used above ground in tropical or subtropical areas remains 

challenging.

Conclusions

The findings of this study indicate that the current AWPA 

standardized methods for evaluating aboveground durability 

do not adequately simulate the moisture conditions that can 

exist within larger wood members used in aboveground

construction. The nonstandard post–beam specimens, which 

simulated a recommended deck construction configuration, 

sustained higher moisture contents and experienced less 

moisture content fluctuation than did any of the standardized 

specimens. This finding suggests that frequent drying of the 

standardized specimens may underestimate the decay hazard 

potential for aboveground structures and/or create moisture 

conditions that favor a different combination of wood decay 

organisms. Internal temperatures also varied by specimen 

type, but the temperature differences between specimen 

types were proportionally much less than the moisture 

content differences. Further consideration of specimen 

configurations that enhance moisture-trapping conditions, 

such as stacks or layers of specimens, appears warranted. Of 

the current standardized methods, the E18 ground proximity 

test appeared to offer the best combination of relatively high 

moisture contents and ease of inspection. The E18 ground 

proximity method did not appear to present an overly severe 

test of aboveground efficacy, at least on the basis of 

moisture content and temperature. Additional data on 

internal moisture contents of outdoor structures would be 

beneficial in developing test methods that more closely 

simulate actual exposure conditions.

Acknowledgments

The authors gratefully acknowledge the assistance of Joseph 

Balczewski and Steven Halverson of the USDA Forest 

Service, Forest Products Laboratory, for their assistance in 

preparing the test specimens.

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