Durability of Hardboard 

Lap Siding 

Determination of Performance Criteria 

Charles Carll 

Anton TenWolde 

United States

Department of 

Agriculture 

Forest Service 

Forest 

Products 

Laboratory 

Research 

Paper 

FPL−RP−622 
Abstract 

This report describes a study in which a current U.S. industry quality assurance test for hardboard siding was evaluated 

as a predictor of siding performance in accelerated exterior 

exposure. Additional laboratory test procedures were also 

performed. The study included all commercially available 

hardboard sidings manufactured in the United States at time 

of study inception, as well as siding that was specifically 

produced for this study at one manufacturing plant. The 

siding was installed on outdoor fences that were sprayed 

with water for 1 hour every day during the summer. We 

periodically monitored siding performance over 38 months 

of exposure. Performance of commercially available boards 

varied appreciably, both in laboratory testing and on the test 

fences. However, there was some consistency in the types of 

performance problems that were exhibited. The ìweatherability of substrateî test procedure contained in the American National Standard proved to be an imperfect, but nevertheless useful, predictor of some performance characteristics 

on the test fences. The data suggest that the current criterion 

in the standard (20% maximum residual thickness swell) 

does exclude boards prone to serious performance problems 

but that the incidence of such problems may be significantly 

reduced by lowering the maximum allowable level to 17% or 

lower.

A painted drip edge significantly retarded development of 

performance problems. Water stains on the back surfaces of 

siding provided no convincing evidence of significant vertical capillary rise of water at siding laps but nevertheless 

suggested that back surface wetting and liquid water retention in the laps played a role in siding degradation. Sorption 

and desorption isotherms, representing equilibrium moisture 

content at various ambient relative humidities, are also presented in this report. 

Keywords: moisture, durability, hardboard siding, thickness 

swell, edge welt, mildew, vapor sorption, paint cracks, 

moisture content, staining 

Acknowledgments 

Gerry Laughinghouse, of Georgia-Pacific Corporation, 

assisted in design of this study. We also thank Gerry and 

Georgia-Pacific Corporation for producing specially fabricated hardboards for this study. Harry Warren (retired from 

the Masonite Corporation) and Louis Wagner (formerly with 

the American Hardboard Association) provided advice and 

assisted us in obtaining commercial hardboards and OSB 

siding. We appreciate the assistance of Vyto Malinauskas, 

formerly of the Forest Products Laboratory (FPL), in laboratory and field testing. Lastly, we acknowledge the following 

FPL personnel: Jessie Micales for mold and mildew identification, Dan Foster for chemical analysis of OSB siding for 

borate inclusion, James Beecher and Fred Matt for chemical 

extraction and analysis of hardboard extracts, and Steve 

Verrill for design and execution of statistical analyses. 

December 2004 

Carll, Charles; TenWolde, Anton. 2004. Durability of hardboard lap 

sidingóDetermination of performance criteria. Res. Pap. FPL-RP-622. 

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

Products Laboratory. 29 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. 

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Persons with disabilities who require alternative means for communication 

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an equal opportunity provider and employer. 
3

Summary 

This report describes results of a study on durability of 

hardboard siding. The objectives of this study were (1) to 

examine the extent to which performance in the current 

industry standard test procedures for ìweatherability of 

substrateî correlates with siding performance in use, (2) to 

determine if changes in the acceptance criterion for ìsubstrate weatherabilityî are justified, and (3) to examine to 

what degree results from other laboratory tests correlate with 

siding performance in use. The results cover outdoor exposure of hardboard siding on water-sprayed test fences over 

the period July 1997 to October 2000, as well as the results 

of several types of laboratory tests. 

We observed the following in laboratory testing: 

• Residual thickness swell (RTS) and 24-h edge water absorption test results varied considerably among commercial hardboards. 

• Most of the commercial hardboard siding tested met the 

industry criterion of 20% maximum RTS. However, 14% 

of tested specimens from one of the classes of commercial 

hardboard exceeded 20% RTS. 

We observed the following on water-sprayed test fences: 

• The most noticeable failure mode was surface welting, 

which occurred earlier and to a greater extent near drip 

edges (in lap areas) than it did on board end cuts. The next 

most noticeable failure modes were (1) paint film breakage on drip edges and (2) board swelling that resulted either in cracks in the paint film at the nail heads or in ìdimplingî around nail heads. 

• There was no noticeable buckling, but test specimen size 

and attachment to fences was such that noticeable buckling would have been unlikely. 

• No visible evidence of decay appeared in any of the siding. Open end joints at board ends evidently permitted water entry but also provided drainage and drying. 

• Minor amounts of mildew appeared on the painted surfaces of some boards. The exposure regimen (appreciable 

sun exposure, daily spraying with quantities of water sufficient to flush the board surfaces), however, may not have 

been conducive to mildew growth. 

• Paint performance was generally good, even on the test 

fence where drip edges had been removed. 

• There were substantial differences in performance of 

commercial boards, especially with regard to edge welting. 

• A painted drip edge substantially retarded development of 

surface welting at the drip edge, reduced the magnitude of 

measured in-service swelling, and also reduced development of paint cracks at nail heads. 

Correlations between laboratory RTS data and performance 

on the fences showed the following: 

• Direct correlations of RTS and any given measurement of 

field performance were rarely strong. There was usually 

substantial scatter in the data, but RTS nevertheless often 

was a useful indicator of field performance. 

• RTS proved to be a good indicator of the likelihood of 

edge welting. The data suggest that lowering the test criterion in AHA/ANSI Standard A135.6 to 16% or 17% 

would noticeably reduce the incidence of edge welting in 

service and that lowering it still further would have additional benefits. 

• RTS proved to be a reasonably good indicator of the likelihood of mildew growth on painted surfaces. The data 

suggest that lowering the RTS criterion in the standard to 

16% or 17% is likely to reduce the incidence of mildew 

growth in service but do not suggest that further reductions would be beneficial. 

• RTS proved to be a good indicator of the likelihood of 

paint cracking on drip edges. The data suggest that lowering the RTS criterion in the standard to 18% would noticeably lower the incidence of drip edge paint cracking 

and that further reductions would yield additional benefits. 

Our review of the ANSI/AHA Standard A135.6 for hardboard siding led us to two conclusions: 

• The standard in its current form lacks an appropriate 

statistical basis for sample selection. 

• The standard does not account for the effects of variability 

in the test results. 

Other conclusions and observations included the following: 

• The degree of back-surface water staining varied appreciably between commercial hardboards. Despite significant water staining on the back surface of some boards, 

there is no convincing evidence that vertical capillary suction in the laps between boards played a significant role in 

back-surface wetting. With the short specimen length and 

open end joints, migration of water from board ends appeared to play a substantial role in back-surface wetting. 

However, retention of water in lap areas may be an important factor in board degradation. 

• Considerable mold growth occurred on specimens exposed 

to 97% relative humidity (RH) during the sorption tests, 

but there was no visible mold on any of the specimens 

maintained at 79% RH. 

Based on the conclusions and observations, we recommend 

that ANSI/AHA Standard A135.6 be revised to include 

statistically based criteria for sample selection and that statistically based criteria be included to account for variability in 

the measurements. The report contains a specific suggestion 

for determination of sample size. We also recommend that 

the RTS passñfail criterion in the standard be lowered to 

17% or lower, which likely will decrease in-service edge 

welt and drip-edge paint cracking and potentially reduce the 

chance of mildew growth. Although there likely would be 

additional performance benefits from lowering the criterion 

further, the authors believe that such a decision should also 

be based on factors such as the effect on mill operation and 

economics, which are outside the scope of this report. 
4 

Contents 

 Page 

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

Objectives ............................................................................. 2

Approach and Methodology ................................................. 2

Materials and Specimen Selection .................................... 2

Outdoor Exposure Tests.................................................... 3

Laboratory Tests ............................................................... 7

Data Analysis.................................................................... 8

Results................................................................................... 8

Laboratory Tests ............................................................... 8

Outdoor Exposure Tests.................................................. 11

Correlations With Residual Thickness 

Swell Test Results........................................................... 20

Issues Related to Standard ANSI/AHA 135.6 .................... 27

Conclusions......................................................................... 27

Recommendations............................................................... 28

Literature Cited ................................................................... 28
Durability of Hardboard Lap Siding 

Determination of Performance Criteria 

Charles Carll, Forest Products Technologist 

Anton TenWolde, Supervisory Research Physicist

Forest Products Laboratory, Madison, Wisconsin 

Background 

Although failure of hardboard siding often occurs due to 

improper installation or detailing (Keplinger and Waldman 

1988, HUD 1992), more stringent test requirements than 

stated in industry standard ANSI/AHA A135.6-1998 (AHA 

1998) might reasonably lead to improved performance and 

durability of hardboard siding in service. The literature to 

date does not include evidence that the current minimum test 

requirements correlate with satisfactory performance in 

service. 

Accelerated aging tests have long been used as a method to 

screen products for durability, but it is difficult to correlate 

results of accelerated exposure with exposure to natural 

weathering (Ruffin 1960). The hardboard industry currently 

uses a number of tests (ANSI/AHA A135.6-1998) to evaluate hardboard siding and has formulated minimum test criteria. Keplinger and Waldman (1988) and Baldwin (1988) 

suggest that when there are durability problems with hardboard siding, irreversible (or ìresidualî) thickness swell is 

frequently a contributing factor to or a cause of problems. 

We therefore postulated that results from the ANSI/AHA 

A135.6 test procedure for ìweatherability of substrateî 

would be a reasonably good predictor of durability in service. This procedure prescribes six consecutive cycles of 

wetting, drying, and freezing of the hardboard and requires 

the measurement of the permanent increase in the thickness 

of the drip edge (residual thickness swell, or RTS). 

ANSI/AHA A135.6-1998 allows a maximum RTS of 20%. 

Published measured values for commercial hardboard siding 

vary between 2% and 17% (Biblis 1989, 1991). The degree 

of residual swell of individual products primarily depends on 

chemical additives or heat treatments, raw materials, prevention of buildup or relief of residual stresses during manufacturing, amount of adhesives used, or sizing additives 

(Carll 1996). 

Permanent thickness swelling in response to consecutive 

wetting and drying cycles is one important way in which 

wood composition materials distinguish themselves from 

solid wood. Wood swells when wetted and returns to virtually its original thickness when dried. Wood composition 

materials, on the other hand, because of inter-leaving of the 

wood elements (particles, flakes, or fibers) in the mat prior 

to pressing and because of compaction under high pressure 

during pressing, contain internal residual stresses that may 

relax during wettingñdrying cycles. Hardboard siding generally shows less RTS than other commercial wood composition materials bonded with similar adhesive systems, in 

particular those made from wood particles or flakes (River 

1994). However, hardboard siding can exhibit significant 

levels of RTS (Biblis 1989, 1991, River 1994). If swelling is 

extreme, paint may crack, caulking may fail, and nail heads 

may be pulled through the paint surface, providing pathways 

for water entry into the board. Although no published evidence directly links residual swell and siding degradation, 

Kelly and others (1984) showed that hardboard siding with 

appreciable residual swelling can absorb water much more 

easily than hardboard with small amounts of residual 

swelling. 

Table 2 of the Canadian standard CAN/CGSB-11.3-M87 for 

exterior hardboard specifies a limit of 15% allowable RTS 

after exposure to cyclic accelerated aging (Canadian General 

Standards Board 1987). The Canadian standard prescribes 

full immersion of the sample in hot water, with subsequent 

exposure to 200°F steam, and follows procedures described 

in ASTM Standard D 1037 (ASTM 1995a). The ANSI/AHA 

weatherability of substrate procedure, in contrast, calls for 

soaking of one sample edge only and does not include a 

steam cycle. The literature suggests that edge soaking is 

better able to identify boards with appreciable levels of 

water absorptivity than is a full immersion test (Kelly and 

others 1984, Biblis 1989). Kelly and others (1984) saw 

greater water absorption in edge soak exposures than in full 

immersion tests. Biblis (1989) found that RTS as determined 

with the ANSI/AHA substrate weatherability test usually 

exceeded RTS after full immersion testing. Differences 

between boards that were not detectable by full immersion 

testing could be identified by the ANSI/AHA substrate 

weatherability test. Although the literature suggests that it 

would be a better predictor of in-service siding performance 

than other test procedures, the literature to date does not 

include evidence that the ANSI/AHA procedure correlates 

with natural or accelerated outdoor exposure or with 

satisfactory performance in service. 
2 

Various researchers have identified correlations between 

mechanical property values of wood composites after laboratory test procedures and after natural weathering of small 

unpainted specimens placed in outdoor exposure with all 

surfaces of the boards exposed to the weather (River 1994, 

Beech and others 1974, Dinwoodie 1981). River (1994) 

found reasonably good correlations for mechanical properties between laboratory test procedures and outdoor exposure for various outdoor exposure periods. When thickness 

swelling was the measurement criterion, River (1994) found 

that the correlation between laboratory test results and field 

performance was generally weaker than that for mechanical 

properties and that the correlation depended on exposure 

period. His findings were complicated by the fact that some 

boards underwent appreciable in-field swelling during the 

initial years of exposure but then decreased in thickness due 

to the loss of surface flakes or fibers from their unpainted 

faces. Our study employed natural outdoor exposure and 

accelerated outdoor exposure of hardboard siding, with test 

specimens primed and painted and in contact with sheathing, 

in an attempt to realistically simulate installation on a building. Furthermore, we evaluated physical behaviors that are 

indicative of siding performance. 

Objectives 

The objectives of this study were to (1) examine to what 

extent performance in the current industry standard test 

procedures for ìweatherability of substrateî correlates with 

siding performance in use, (2) determine if changes in the 

acceptance criterion for ìsubstrate weatherabilityî are justified, and (3) examine to what degree results from other 

laboratory tests correlate with siding performance in use. 

Approach and Methodology 

The basic premise of this study was that siding durability is 

to some extent related to moisture absorption properties. 

Although siding can degrade for a variety of reasons, it is 

logical that siding with low hygroscopicity and liquid absorptivity would provide better performance when paint 

application procedures are imperfect, caulk seals fail, or 

maintenance is less than ideal. In addition, a siding that 

exhibits little thickness swell in response to water uptake 

would reasonably be expected to develop less cracking of 

paint coating or problems with nail heads breaking the paint 

surface. We hypothesized, therefore, that laboratory tests 

that assess the moisture absorption and thickness swell of 

hardboard siding could be useful tools in predicting the 

durability of hardboard siding in the field. 

We conducted a series of laboratory and field exposure tests 

on a variety of hardboard sidings and correlated the observations from field exposure tests with the results of laboratory 

tests. The materials were obtained from various sources and 

showed wide ranges of residual thickness swell (RTS) and 

liquid water absorptivity. Siding specimens were installed on 

a test building (exterior exposure without additional water 

spray) and on two test fences that were sprayed with water 

for 1 h once a day during the summer (accelerated exterior 

exposure). We conducted regular extensive inspections of 

the exposed siding. 

Materials and Specimen Selection 

The materials in this study included seven commercial hardboard sidings, one OSB siding, and six non-commercial 

hardboard sidings, which we selected from different batches 

of boards that were specially produced for this study. All 

U.S. hardboard manufacturing plants producing hardboard 

siding in 1996ñ1997 were represented in this study. The 

Forest Products Laboratory (FPL) did not retain any information on the origin of the commercial boards. 

Non-Commercial Siding 

Georgia Pacific Corporation produced dry-process hardboard siding specifically for this study at lower than usual 

steam pressures and shorter press times, to provide FPL with 

test materials exhibiting a wide range in properties. This 

material was produced in three separate lots. Some board 

was also produced at normal temperatures and press times. 

These boards were shipped to FPL unprimed and unpainted 

in 4- by 8-ft (1.2- by 2.4-m) sheets. At FPL the sheets were 

sawn into 8-ft (2.4-m) by 8-in.- (0.2-m-) wide strips of lap 

siding; these siding strips therefore did not have shaped drip 

edges. Strips from the separate lots were kept segregated and 

were marked as cut to retain a record of the sheet and position within the sheet from which they were cut. 

After preliminary tests of RTS, we found that the strips of 

non-commercial board could provide six separate classes of 

lap siding. The six classes were determined by lot and by 

position within the sheet (see Table 1 and Fig. 1). 

Table 1óClass designations for non-commercial siding

Class 

Average 

RTSa

(%) Notesb 

1 5.9 Produced with normal temperatures and 

press time 

2 6.7 Lot 1, strips 2 through 5 

3 8.9 Lot 2, strips 2 through 5; Lot 3, strips 3 

and 4 

4 12.0 Lot 3, strips 2 and 5 

5 22.7 Lot 2, strips 1 and 6 (edge strips) 

6 30.1 Lot 3, strips 1 and 6 (edge strips) 

a

Residual thickness swell, average of 80 samples, 

 ANSI/AHA A135.6. 

b

Strip designation numbers refer to their location in the 

 sheet corresponding to Figure 1. 
3

Of the non-commercial siding, class 1 had the lowest RTS. 

The RTS of the other material varied widely with location in 

the sheet, with strips from sheet edges showing the highest 

RTS. Classes 2, 3, and 4 were therefore selected from the 

center of the sheets, producing low to medium values for 

RTS. Classes 5 and 6 were cut from the edge of the sheet, 

and the average RTS of this material did not meet the industry minimum standard of 20%. 

Commercial Siding 

The seven commercial hardboards were cut from 4- by 16-ft 

(1.2- by 4.9-m) master sheets at the producing mills using 

the identification scheme shown in Figure 2. The lap siding 

delivered to FPL was 8 ft (2.4 m) long and nominal 8 in. 

(0.2 m) wide. All commercial hardboard was factory primed 

and had beveled or slightly rounded drip edge, which had 

been ìpreformedî prior to priming. Because OSB siding was 

available only in 4- by 8-ft (1.2- by 2.4-m) panels, the siding 

was cut from these panels at FPL. All board had flat, smooth 

(not textured) front surfaces except the OSB, which had a 

textured face. OSB siding contained zinc borate, which was 

confirmed by atomic emission spectroscopy. 

Commercial boards from each mill were randomly assigned 

a class number (classes 7 to 14). Unlike class numbers 1 to 

6, which reflect relative performance in the substrate 

weatherability test procedure, class numbers for commercial 

boards are not indicative of any performance ranking. 

Boards in class 12 contained press blows, but these defects 

were so inconspicuous that we did not notice them until we 

observed unusual on-fence behavior of these boards. 

Specimen Selection 

Twenty boards from each board class (a total of 280 boards) 

were selected to provide specimens. Each board yielded 

three specimens for outdoor exposure tests, four specimens 

for weatherability of substrate (residual thickness swell) 

tests, and one specimen for the water absorption test (Fig. 3). 

Specimens for vapor sorption tests were cut from the interior 

edge of the end pieces. 

Figure 2óDiagram for cutting and identifying 

commercial hardboard lap siding from master sheets. 

Specimens for exterior exposure were brush-painted with 

two thin coats of satin exterior latex paint. On the commercial hardboards, the latex paint was applied over the factory 

primer. On the non-commercial hardboards, which were not 

factory-primed, the latex paint was applied over brushapplied alkyd primer.1

 The OSB siding had a resinimpregnated paper face, which is intended to serve as a 

topcoat base, and was painted with two coats of satin latex 

paint without priming. Average spread rate for the combined 

two coats of latex paint was 387 ft2

/gal (9.5 m2/L) on the 

(smooth) hardboard and 267 ft2

/gal (6.6 m2/L) on the OSB. 

All brush painting was performed in a conditioned laboratory. Specimen end-cuts were left bare. 

Outdoor Exposure Tests 

Test Fence Exposure 

The test fences were located at the FPL Valley View test site 

just west of Madison, Wisconsin. Boards on the two test 

 

1

The alkyd primer was from the same manufacturer as the latex 

topcoat. It was compatible with the topcoat and recommended by 

the paint manufacturer for use on hardboard that had not been 

factory-primed.

Strip 6 

Strip 5 

Strip 4 

Strip 3 

Strip 2 

Strip 1 

Figure 1óStrip designation numbers and 

their location in the sheet. 

X 1 C

X6A

X5A

X4A

X3A

X2A

X1A

X6B 

X5B 

X4B 

X3B 

X2B 

X1B 

8 ft 8 ft 

X6C

X5C

X4C

X3C

X2C

X1C

X6D 

X5D 

X4D 

X3D 

X2D 

X1D 

X6E

X5E

X4E

X3E

X2E

X6F 

X5F 

X4F 

X3F 

X2F 

8-in. 

nominal 

width 

8-ft segment

Location across sheet 

Mill number 

X1E X1F 
4 

Figure 3óDiagram for cutting specimens from boards. 

Figure 4óDiagram of test fence construction (top view). 

fences faced south and were exposed to the weather and 

artificial wetting for 38 months. Boards were mounted on the 

test fences on a 0.75-in. (19-mm) thick foam sheathing to 

simulate exposure on an insulated wall. On each fence, 

boards were installed as lap-siding courses in 28 columns of 

10 boards, each course with 6.625 in. (0.17 m) of exposure. 

Column width (or specimen length) was 22.5 in. (0.57 m). 

Plasticñwood composite lumber, which was fastened to the 

(19 mm) plywood sheathing, separated adjacent columns. 

There was a gap of roughly 0.25 in. (6 mm) between each 

board end and the column separator (Fig. 4). These gaps 

simulated open (uncaulked) joints between siding and corner 

trim.2

 Boards were installed on the fences in July and August 

1997 and removed on October 12 and 13, 2000. Figure 5 

shows both test fences immediately after installation of the 

boards. The two fences were identical, with one exception: 

on fence 1, all drip edges were cut and left bare, whereas on 

fence 2, the painted drip edges were left intact. End cuts 

were left bare on both fences. 

 

2

Open joints are contrary to AHA application instructions; they 

allow water entry at board ends. The open joints remained open 

down past the bottom siding course, unlike an open joint or a failed 

caulk joint between siding and window jamb casing. The open 

joints past the bottom siding provided appreciable drainage potential. The appreciable width of open end gaps, coupled with the fact 

that the siding was true lap siding (as opposed to shiplap siding) 

also provided for back-face venting. 

Each column of siding on the two test fences was individually sprayed for 1 h each day from May through late November. The spray was turned off during the winter. Spraying started mid-August 1997. The water spray hit above the 

top board and water ran down each column of siding. Column separators prevented water movement between columns. The water was drawn from a well on the test site. To 

verify that each column received comparable amounts of 

spray water, we measured the water spray rate of each spray 

nozzle in July 1997 and again in June 1998. We found that 

although the rate varied with water pressure, the average 

flow rate was about 0.23 gal/min (15 mL/s), with maximum 

variations between nozzles of about 10%. Each nozzle delivered a fan-shaped spray of water, with the long dimension of 

the fan oriented horizontally (Fig. 6). The spray was directed 

at a siding strip above the top board in each column. The 

spray fan could be deflected by wind but did not reach board 

ends, even under moderately windy conditions. 

The spray nozzles therefore did not spray water directly into 

the open end joints, except perhaps under very windy conditions. Water ran down the face of the specimens in rivulets, 

which sometimes remained continuous between adjacent 

courses, and sometimes ended at drip edges, with water 

dripping off the drip edge. It was common to find rivulets 

across the entire width of a board specimen. Water was also 

commonly present on vertical end surfaces of a test specimen during spraying. 

Front Side of Board

Drip edge 

Outdoor exposure

Water absorption test 

Vapor sorption test 

Weatherability of substrate test 

(Specimens A, B, C, and D) 

6 in. 

2 in. 

4 in.

Left Middle Right 

A B C D

23-1/2 in. 

Outdoor exposure Outdoor exposure

23 in.

OSB

2 in. 

Plywood 

Separator 

Siding

Nailing strip 3/4-in. extruded polystyrene

4 by 4 support post 
5

Figure 5óTest fences at the FPL Valley View test site. 

Figure 6óSpraying of the fences. 

Test Building Exposure 

Boards also were installed on the south-facing wall of a test 

building near the test fences. These boards are still on the 

building at the time of this report; we intend to monitor this 

siding until September 2007, for a total of 10 years. The wall 

is exposed to the weather but is not sprayed with water. The 

boards were treated the same as for fence 2 (that is, drip 

edges were left intact and painted, but cross cuts were left 

bare). The boards were cut to a length of 17.75 in. (0.45 m). 

Each column was separated from adjacent columns by 
6 

vertical column separators made of treated lumber. The open 

(uncaulked) joints between siding ends and column separators were similar to those on the test fences. 

Specimen Placement 

Specimen placement was identical on the two test fences and 

the building. The 20 boards of each of the 14 classes were 

placed randomly over the 28 columns. However, because the 

water was sprayed at the top of the wall, we felt it appropriate that all 14 classes be represented equally in the top four 

positions. Thus, separate randomization was applied to the 

top four courses and the rest of the courses. Eight boards of 

each class were placed in the top four positions on each of 

the fences and the wall, and the remaining 12 on the middle 

and lower sections. 

Inspections 

During the period of May to November, two or three full 

inspections were made each year, always by the same person. The inspection protocol evolved over time, but it was 

consistent between inspection dates by November 1998. 

Each board was inspected for surface discoloration of the 

painted face by mildew, evidence of decay, edge welt, and 

paint condition. We did not attempt to immediately identify 

mildew species as mildew was found, although an FPL 

mycologist identified mildew species on a few board specimens after the final inspection. We also checked for breaks 

in the paint surface at each nail head, which could provide a 

path for water entry. 

The first inspection took place in late September 1997. The 

last inspection of the siding on the test fences occurred 

between September 26 and October 4, 2000, with siding 

removal from the fences following shortly thereafter on 

October 12 and 13, 2000. Siding on the building continues 

to be inspected at yearly intervals. 

In-Place Thickness Measurements 

In-place thickness was measured at approximately the same 

time as inspections were made. Thickness measurements 

were assumed to be less subject to individual technique or 

perception than were other inspection observations and thus 

were not always performed by the same person. In-place 

thickness was measured in two locations on the drip edge, 

using the technique described by Carll and others (2000). 

The measurement device consisted of a dial gauge with a 

square stationary foot, which was placed against the bottom 

of the drip edge to be measured, on the siding course below 

(Fig. 7). The round foot on the dial gauge rested on top of 

the drip edge. We placed the side of the stationary square 

base against the column separator. This provided for consistent location of the measurements, at about 3 in. (74 mm) 

from each corner of the drip edge. On the building, we used 

a smaller square base and measured thickness at about 2 in. 

(50 mm) from each specimen corner, to avoid measuring on 

top of the nail heads. 

Measurement During and After Siding Removal 

Staining on the backóWe made note of staining on the 

back of specimens when we removed them from the test 

fences in October 2000. We quantified the extent of staining 

during a subsequent reexamination. 

Moisture content at time of removalóWe determined the 

moisture content (MC) at time of removal by weighing each 

board immediately after removal. We subsequently transported boards to FPL and placed them in a room maintained 

at about 70oF (18oC), 50% relative humidity (RH) for over 

6 months. We then weighed the reconditioned boards. Finally, we determined the ovendry weight, allowing us to 

determine MC at time of removal and after re-conditioning 

to 50% RH, 70oF. 

Final thickness swellóWe measured board thickness after 

reconditioning at 70oF (18oC), 50% RH. We determined the 

thickness of the drip edge near the side edges and in the 

middle. These results were compared with measurements 

made at the same locations before installation on the fence. 

Figure 7óIn-place thickness measurement device. 
7

Laboratory Tests 

Weatherability of Substrate 

We performed weatherability tests on 80 samples of each 

siding class according to section 4.1, Weatherability of 

Substrate, of ANSI/AHA A135.6-1998. Each sample measured 6 by 2 in. (150 by 51 mm) with unprimed edges, and 

four samples were cut from each of the 20 siding boards as 

shown in Figure 3. The drip edge was cut off and left bare. 

Specimens were first conditioned at 70°F (18°C), 50% RH, 

after which thickness was measured at the center of the edge 

to be submerged. Specimens were then exposed to six 24-h 

cycles, each cycle consisting of the following steps: 

1. Suspension in 100°F (38°C) deionized water for 18.5 h. 

Specimens were suspended vertically with the bottom 

edge immersed to a depth of 1 in. (25 mm). 

2. Placement in a 220°F (104°C) oven for 30 min. 

3. Placement in a freezer at 0°F (ñ18°C) for 2 h. This was 

slightly colder than the temperature of 5°F (ñ14°C) 

specified in ANSI/AHA A135.6-1998. 

4. Placement back in the oven for 30 min. 

Steps 3 and 4 were repeated to complete a 24-h cycle. 

Fresh deionized water was used for each cycle. After the 

sixth cycle, the specimens were reconditioned at 70°F 

(18°C), 50% RH, after which the thickness of the edge was 

again measured to determine the residual swell. 

Edge Water Absorption 

A 24-h edge water absorption test was performed on 20 

specimens (4 by 8 in., 102 by 204 mm) of each type, as 

described by Kelly and others (1984). The specimens were 

suspended vertically with the bottom 1 in. (25 mm) of the 

board submerged in deionized water heated to 100°F (38°C). 

After immersion for 24 h, the bottom 4 in. (102 mm) of each 

specimen was cut into 16 slices approximately 1/4 in. 

(6 mm) wide, parallel to the submerged edge of the specimen. These slices were individually weighed and oven-dried 

to determine moisture content. Moisture content was 

recorded as a function of distance from the submerged 

specimen edge. 

Water Vapor Sorption 

We determined equilibrium moisture content, also known as 

water vapor sorption isotherms, using a methodology similar 

to that described by Richards and others (1992). For each 

class of siding, we selected 15 boards and cut two specimens 

from each board. The 15 pairs of specimens were then divided into two groups of 15 specimens each. Specimen size 

was approximately 2 by 1/16 in. (51 by 1.6 mm), and specimen weight ranged from 0.5 to 0.7 g. The procedure generally complied with, or exceeded, the requirements in ASTM 

Standard C 1498-01, Standard Test for Hygroscopic 

Sorption Isotherms of Building Materials (ASTM 2001), 

except that our samples weighed less than 10 g. The specimens were placed in jars containing saturated salt solutions 

at 73°F (23°C). Temperature in the room was controlled 

within 1°F (0.6°C). The following four salts were used: 

Salt 

Equilibrium RH 

(at 73°F, 23°C) (%) 

Magnesium chloride (MgCl2·6H2O) 33

Sodium bromide (NaBr) 58 

Ammonium chloride (NH4Cl) 79 

Potassium sulfate (KSO4) 97 

Each specimen was weighed with a precision balance with 

an estimated reading error of 0.0002 g. Specimens were 

weighed immediately after removal from the jar. Weight 

measurements were made a minimum of 4 days apart, except 

for measurements performed at 97% RH. We assumed equilibrium had been reached when the weight change between 

two consecutive measurements was undetectable (that is, 

less than 0.0002 g.). This translates to an equilibrium criterion of about 0.007% of specimen weight per 24 h. This is 

more stringent than the weight change criterion of 1% of 

specimen weight over a 2-week period used by Richards and 

others (1992) and probably more stringent than the criterion 

in ASTM 1498 of 0.1% of specimen weight in five successive readings at 24-h intervals. 

The specimens were first dried by placing them in jars with 

calcium chloride desiccant. To obtain a vapor adsorption 

isotherm, 15 specimens from each siding class were then 

placed in jars at sequentially higher humidities, up to 79% 

RH, and allowed to come to equilibrium at each humidity 

level. After reaching equilibrium at 79% RH, the specimens 

were weighed and placed in jars at sequentially lower humidities (58% RH, 33% RH, and desiccant) to obtain a 

desorption isotherm. Due to problems with mold growth at 

97% RH (potassium sulfate), we followed a different procedure to obtain equilibrium data at 97% RH: A second group 

of 15 specimens from each siding class was directly moved 

from the desiccant to the 97% RH environment. To inhibit 

mold growth, this group of specimens was exposed to 185°F 

(85°C) for 2 h in the 97% RH conditioning jars. The jars 

were then returned to the 73°F (23°C) room, and the specimens were allowed to reach equilibrium by leaving them in 

the closed jars for 11 weeks or more, without intermittent 

removal and weighing. After weighing, they were moved to 

the 79% RH jars, and subsequently a desorption isotherm 

was obtained as with the other group of specimens. 

After final exposure to the desiccant environment at the end 

of the cycle, specimens were oven-dried at 215°F (102°C) 

for 3 h to determine ovendry weight (compared with desiccant-dry weight). Because a large number of specimens were 

removed simultaneously from the oven and could not all be 
8 

weighed immediately, it was necessary to minimize vapor 

adsorption of specimens between removal from the oven and 

weighing. A 1-in.- (25-mm-) thick aluminum plate measuring 2 by 10 in. (0.05 by 0.25 m) was placed in the oven with 

the specimens on top. The plate and specimens were 

removed from the oven and placed next to the balance. The 

residual heat from the plate kept the specimens close to oven 

temperature, minimizing vapor sorption from the ambient air 

until each sample was weighed. 

Density and Thickness 

We determined the ovendry density of 10 samples of each 

class of hardboard. We used the same oven-drying technique 

that we used for the sorption measurements. 

Data Analysis 

With the exception of edge water absorption, all data were 

recorded by sample ID to allow cross correlation between 

measured parameters on a sample-by-sample basis. 

Results 

Laboratory Tests 

Weatherability of Substrate 

Results of weatherability tests for the non-commercial 

boards are shown in Table 2. The RTS of classes 1 to 6 

cover a broad range. Classes 1 and 2 exhibited very little 

RTS, whereas most boards in classes 5 and 6 did not meet 

the minimum industry standard of 20%. 

The RTS results for the commercial boards (Table 3) show 

considerable variation between classes. On average, all 

classes meet the industry minimum standard, but a significant minority of the test specimens in class 8 failed to meet 

the standard. 

The commercial OSB siding specimens (class 9) had an 

average RTS of 19.3% (standard deviation 3.2%), with 44% 

of the specimens failing the 20% criterion for hardboard. Of 

course, OSB siding is not required to meet the specifications 

in ANSI/AHA A135.6, which are specifically for hardboard 

siding. 

Edge Water Absorption 

Figure 8 shows the moisture distribution in the first 4 in. 

(0.1 m) of the specimens, averaged by siding class. Class 14 

absorbed much more water than did any other class. Classes 

10 and 12 showed the least water absorption. There does not 

appear to be a direct correlation between RTS and water 

absorption behavior. 

To arrive at a single index for water absorption, we 

calculated average moisture content of the 16 slices and 

subtracted the moisture content (MC) of the top slice, which 

was 3 in. above the waterline during immersion. The data 

indicate that water did not penetrate that far up in the sample. The calculated value thus represents the increase in 

average MC of the first 4 in. of the specimen due to water 

absorption. Table 4 shows the calculated values, along with 

other results from the test. Class 14 boards had an average 

MC increase of 28.9%, over three times as much as that of 

the board class (class 6) with the next highest average MC 

increase (8.0%). When class 14 and all non-commercial 

boards (classes 1 to 6) are ignored, considerable variation in 

water absorption remains: Class 8 showed more than twice 

the water absorption of the two least-absorptive classes of 

commercial board (classes 10 and 12). A statistical analysis 

(Tukey multiple range test) of individual measurements 

indicated that individual classes or groups of classes differed 

from each other with respect to MC increase (Table 5). 

The extremely high water absorption of class 14 boards led 

us to question whether wax sizing had been accidentally 

omitted during manufacture. The FPL analytical chemistry 

laboratory performed room-temperature toluene extractions 

on two specimens of class 14 board. As a reference, they 

also performed toluene extractions on two specimens of 

Table 2óWeatherability of substrate, final results for 

non-commercial boards

Class Average 

RTS (%)a Standard 

deviation (%) 

Specimens exceeding 

20% RTS (%) 

1 5.9 1.8 0 

2 6.7 1.3 0 

3 8.9 2.0 0 

4 12.0 2.5 0 

5 22.7 3.2 80 

6 30.1 5.6 99 

a

Average residual thickness swell of 80 samples. 

Table 3óWeatherability of substrate, final results for 

commercial boards

Class Average 

RTS (%)a Standard 

deviation (%) 

Specimens exceeding 

20% RTS (%) 

7 11.5 1.2 0 

8 15.0 4.4 14 

10 4.9 1.0 0 

11 7.5 2.9 0 

12 9.1 3.7 0 

13 12.4 1.4 0 

14 11.0 2.0 0 

a

Average residual thickness swell of 80 samples. 
9

Figure 8óAverage moisture distribution in bottom 4 in. of the specimens after 24-h vertical immersion. 

Table 4óResults of 24-h edge water absorption tests, 

average of 20 specimens

Increase in MC (%) 

Class 

Maximum 

MC 

(%) 

Average 

MC 

(%) 

MC at 3 in. 

above 

water 

surface 

(%) Average 

Standard 

deviation 

1 50.15 13.53 6.25 7.28 0.36 

2 48.47 13.01 5.80 7.21 0.24 

3 48.54 12.29 5.30 6.99 0.31 

4 50.65 12.56 5.34 7.22 0.22 

5 55.54 13.18 5.24 7.94 0.36 

6 58.71 13.32 5.27 8.05 0.36 

7 45.89 11.77 5.77 6.00 0.39 

8 53.15 16.01 7.98 8.03 0.72 

9a 44.28 11.55 5.43 6.12 1.15 

10 19.90 8.85 6.05 2.80 0.20 

11 46.24 11.83 5.99 5.77 0.76 

12 30.01 9.54 5.69 3.85 0.57 

13 40.42 10.39 5.03 5.36 0.38 

14 82.82 35.21 6.34 28.87 3.89 

a

OSB siding. 

Table 5óResults of Tukey multiple range testing 

on ranked values of individual specimen MC 

increase values

Class 

Average 

increase in MC 

(%) Groupingsa

14 28.9 A 

6 8.0 B 

8 8.0 B 

5 7.9 B 

1 7.3 C 

4 7.2 C 

2 7.2 C 

3 7.0 C D 

9 6.1 D E 

7 6.0 E F 

11 5.8 E F 

13 5.4 F 

12 3.8 G 

10 2.8 H 

a

 Classes with the same letter grouping do not differ 

in MC increase value at = 0.05. 

0

20 

40 

60 

80 

100

Moisture content (%) 

-1 0 1 2 3 

Center in relation to water line (in.)

 3

 4

 5

 6

 7

 1 8 

 2 9 

 10 

 11 

 12 

 13 

 14 

Class

4
10 

class 11 board.3

 The toluene-extracted material was solid at 

room temperature (after evaporation of the toluene solvent). 

Class 14 boards contained approximately 2.7% tolueneextractible material on an ovendry mass basis; for class 11 

boards, the corresponding value was 2.6% extractible 

material.4

The chemistry lab also analyzed the extract by infrared (IR) 

spectroscopy. Absorbance spectra were similar for the extracts for the two board classes. In summary, board classes 

11 and 14, which had very different water absorptivities, 

contained similar mass proportions of toluene-extractible 

material with similar IR absorbance spectra. These findings 

do not suggest an omission of wax sizing from class 14 

boards. 

Water Vapor Sorption 

Sorption measurements progressed slowly, and even with the 

modified procedures described earlier, we encountered 

 

3

Class 11 and 14 boards were evidently made by the same 

manufacturing process. 

4

The mass percentages stated are based on a single extraction. A 

second extraction yielded approximately another 0.5% of tolueneextractible material. 

significant problems with mold growth at 97% RH exposure. 

The mold species on samples we submitted for mold identification were Paecilomyces variotii, Penicillium spp., 

Taleromyces flavus, Penicillium corylophilum, Penicillium

glabrum, and Aspergillus niger. We observed no mold 

growth on specimens at equilibrium with 79% RH exposure. 

Because of mold growth, we were able to complete less than 

half the desorption measurements from 97% RH. The data in 

Table 6 are therefore incomplete. The table shows the results 

for adsorption measurements (samples are moved from dryer 

to wetter conditions) and desorption (samples are moved 

from wetter to dryer conditions). 

Variation in equilibrium moisture content (EMC) between 

classes is relatively minor (Table 6). Class 8 has the highest 

EMC values, followed by class 9 (OSB). The EMC of all 

other classes is very similar. Figure 9 shows the average 

EMC for all hardboard siding classes (excluding OSB). 

Variation between samples within each class is also relatively small. The within-class standard deviation is 0.1% to 

0.2% MC for most of the measurements but rises to around 

1% MC for the EMC data at 97% RH. 

Density 

Average density for each class is presented in Table 7. Each 

density value shown is an average of 10 specimens from 

Table 6óAverage equilibrium moisture content obtained at various relative humidities at 73°F (23°C)

Equilibrium moisture content (%) 

Adsorption Desorption Adsorption Desorption 

Class 

Dessic. 33 58 79 58 33 Dessic. Dessic. 97 79 58 33 Dessic 

1 1.5 4.9 7.0 9.3 7.8 5.4 1.5 1.3 18.3 13.3 9.7 6.1 1.9 

2 1.5 4.8 7.0 9.8 8.2 5.6 1.7 1.1 19.6 14.1 10.0 6.2 1.7 

3 1.3 4.4 6.5 9.4 7.9 5.4 1.6 

4 1.0 4.1 6.1 9.0 7.6 5.0 1.4 

5 1.4 4.5 6.5 9.5 7.9 5.4 1.5 

6 1.1 4.2 6.2 9.2 7.7 5.1 1.7 1.3 18.9 13.6 9.9 6.2 2.2 

7 1.6 4.6 6.8 9.6 8.0 5.4 1.9 1.4 19.0 13.6 10.2 6.1 2.2 

8 2.2 6.2 8.8 11.9 9.9 6.9 2.8 

9a 1.7 4.7 7.1 10.9 8.9 5.9 2.3 1.8 21.1 14.2 9.9 6.3 1.7 

10 1.6 4.6 6.8 9.0 7.6 5.4 2.1 

11 1.7 4.8 6.7 8.7 7.2 5.2 2.1 

12 1.3 4.2 6.1 9.0 7.6 5.2 2.5 

13 1.6 4.1 5.7 8.1 7.0 4.4 1.5 

14 2.1 5.1 6.9 9.4 8.1 5.6 1.8 

Avg.b

 1.5 4.7 6.7 9.5 8.0 5.4 1.9 1.4 19.4 13.7 9.9 6.2 1.9 

Avg.c 1.5 4.7 6.7 9.4 7.9 5.4 1.9 1.4 19.4 13.7 9.9 6.2 1.9 

a

OSB siding. 

b

Average of all classes for which measurements are available. c

Average of all hardboard classes for which measurements are available (excluding OSB). 
11

Figure 9óSorption isotherms for hardboard, average 

of all classes (excludes OSB). The graph shows 

adsorptionñdesorption from desiccant to 79% RH and 

back, adsorption directly from desiccant to 97% RH, and 

desorption back down to desiccant on two separate 

sets of specimens. All data are for 73°F (23°C). 

Table 7óAverage density 

Class 

Average density 

(lb/ft3

 (kg/m3)) 

Standard 

deviation (%) 

1 46.8 (750) 3.2 

2 47.0 (753) 3.2 

3 44.9 (720) 2.3 

4 45.6 (730) 4.1 

5 47.8 (766) 2.9 

6 47.4 (758) 2.7 

7 48.4 (776) 1.8 

8 52.8 (846) 2.9 

9a 44.7 (717) 4.7 

10 55.9 (896) 1.9 

11 52.5 (841) 2.3 

12 52.3 (838) 4.3 

13 47.2 (756) 2.8 

14 45.8 (733) 2.4 

a

OSB siding 

each class. The non-commercial boards (classes 1 to 6) show 

no significant differences in density. The commercial hardboards fall into two categories: classes 7, 13, and 14 have 

similar densities, and the densities of classes 8, 10, 11, and 

12 are somewhat higher. 

Outdoor Exposure Tests 

Most of the results in this report are based on the last site 

inspections between September 26 and October 4, 2000. 

Class averages are reported, as are selected results of a 

board-by-board statistical analysis and statistical correlations 

between selected performance indicators as recorded on the 

test fences and laboratory test results. 

Paint performance on all test specimens was generally good. 

No paint peeling, flaking, or erosion was observed. We 

observed only localized paint cracking near nail heads and 

on some drip edges, which was largely associated with inservice substrate swelling or installation damage. Our observation of good paint performance on hardboard siding agrees 

with previous research results (Feist 1982, 1990). 

There was no visual evidence of decay in any of the boards 

on either fence, and we saw only minor amounts of mildew 

on some of the boards. Regarding mildew growth, it should 

be noted that the spray fence exposure employed in this 

study probably was not conducive to surface mildew growth. 

The fences were exposed to significant direct solar radiation, 

and the water spray was sufficient to flush the board surfaces. Surfaces exposed to extended periods of clinging 

moisture and little or no direct solar radiation are more prone 

to mildew growth (Forintek 2003). 

Observed board warp ranged from wholly imperceptible to 

barely perceptible by visual estimate. However, in some 

cases it was enough to complicate in-place thickness measurements. 

We observed no noticeable buckling during our inspections. 

The combination of relatively short test specimen length 

(0.57 m), industry-prescribed distance between nailing 

points (0.41 m), and wide gaps at each end of each specimen 

(6 mm) limited the likelihood of specimen buckling. 

Thickness Measurements 

Figure 10 shows the average thickness of all boards as 

measured in-place on the two test fences and the test 

Figure 10óAverage siding thickness as measured in 

place on the test fences and the building. 0 20 40 60 80 100

2

4

6

8

10

12

14

16

18

20

Relative humidity (%)

Equilibrium MC (%)

Adsorption 

Desorption 

0.46 

0.48 

0.50 

0.52 

0.54 

Average thickness (in.)

Jun 97 Jan 98 Jul 98 Feb 99 Aug 99 Mar 00 Oct 00 

Fence 1

Fence 2

Building
12 

building. Although the average board thickness on fence 1 

(cut drip edges) was less than that on test fence 2 during the 

early stage of the exposure, it continued to increase and 

surpassed the average thickness on fence 2 in 1998. In comparison, boards on fence 2 (drip edges intact) remained 

relatively stable. The siding on the building, which is not 

sprinkled daily, experienced no significant thickness swell 

over approximately 37 months of exposure. 

Table 8 shows the class-average thickness swell, as measured on the fence during the final inspection (autumn 2000) 

and the thickness swell after reconditioning at 50% RH. The 

standard deviations within each class are also shown. It is 

difficult to draw conclusions from the measurements on the 

fence, given the large board-to-board variations. Apparent 

differences between classes are generally not statistically 

significant. The thickness swell values for classes 2 and 11 

on fence 2, as measured on the fence, were negative. We 

believe these negative thickness swell values were due to 

large additive errors in measurement on the fence, both in 

initial and final on-fence thickness measurements. We feel 

that on-fence thickness measurements, despite the inherent 

individual measurement errors, when averaged over many 

specimens, are of some use for observing time trends in 

board thickness (as shown in Fig. 10). The magnitude and 

consistency of the differences in on-fence swelling between 

fences (which can be seen for individual board classes in 

Table 8) suggest a real between-fence difference (which can 

also be seen in Fig. 10). As discussed in following sections, 

edge welt data and data for paint cracks at nail heads further 

suggest that there was a real difference between fences. 

However, the errors in on-fence measurements were sufficiently large that we did not attempt to use these data for 

making comparisons between board classes. 

Laboratory thickness measurements of the reconditioned 

specimens, after removal from the fence, and initial thickness measurements in the laboratory were far more accurate 

than the on-fence measurements. In contrast to on-fence 

swelling data, the reconditioned thickness swelling data 

show neither consistent nor substantial influence of painted 

drip edges. Board-to-board variation in the reconditioned 

thickness data, although less than in the on-fence thickness 

data, is still large enough that we did not attempt to use it to 

draw distinctions between board classes. 

Edge Welt 

Welt is a term sometimes used in the hardboard siding industry to describe localized swelling along panel edges (Baldwin 1988). The swollen area projects beyond the normal 

panel surface plane, and the surface within the swollen area 

is irregular (that is, not flat) (Fig. 11). Some degree of fiber 

raising can be expected in welted areas. 

Some degree of edge welting at the horizontal drip edges 

developed in all classes of board, with the exception of 

class 9 (OSB siding). The textured surface of the OSB acted 

to obscure or hide localized edge swelling or roughening of 

the surface. We thus were unable to assess edge welt on the 

OSB siding. 

We recorded the approximate area and severity of edge welt. 

Edge welt severity rating was a visual estimate, which was 

influenced by the degree to which the welt projected beyond 

the board plane, the abruptness with which it projected, and 

Table 8óAverage thickness swell as measured on the fence during inspection between September 26 

and October 4, 2000, and after reconditioning at 50% RH

Thickness swell (%), fence 1 Thickness swell (%), fence 2 

On fence Reconditioned On fence Reconditioned 

Class Average 

Standard 

deviation Average 

Standard 

deviation Average 

Standard 

deviation Average 

Standard 

deviation 

1 7.7 4.7 8.2 2.2 1.8 5.6 8.5 6.5 

2 5.0 4.4 7.0 1.6 ñ0.9 5.2 4.1 2.2 

3 12.5 5.1 10.4 2.3 4.4 6.9 9.4 3.5 

4 14.5 4.5 12.3 2.5 5.5 6.1 10.0 4.6 

5 23.0 7.1 18.0 4.5 21.7 8.1 23.5 6.1 

6 28.0 6.6 22.0 4.0 18.5 7.7 22.5 6.3 

7 10.4 4.7 9.3 1.6 2.9 4.3 7.9 4.9 

8 15.3 7.1 21.2 3.0 9.2 4.5 17.4 7.9 

9 13.0 6.6 13.5 3.9 5.7 6.1 12.4 3.4 

10 13.0 6.7 12.8 3.4 0.4 6.2 8.5 4.2 

11 3.7 6.1 5.6 3.0 ñ2.4 5.5 3.9 2.0 

12 12.2 6.5 10.4 3.0 9.1 6.2 9.8 4.0 

13 6.3 4.6 10.9 4.6 2.4 4.9 11.3 4.7 

14 13.2 5.4 11.9 2.9 7.5 8.6 11.3 5.6 
13

surface roughness within the welt area. A rating of 10 indicates no perceptible welt; 9.5, welt could be perceived when 

the surface was viewed at an oblique angle; 9, welt was 

perceptible when the surface was viewed from a position 

normal to the board surface. Ratings lower than 9 indicate 

progressively more noticeable welt. 

We combined area and severity into a single edge welt index 

that reflects both the extent and severity of edge welt, as 

follows: 

I = Area × (10 ñ Severity) (1) 

The results in Table 9 show that all classes showed some 

welting but that some classes (11, 12, and 13) consistently 

had considerably less welting than other classes. Classes 2 

and 10 experienced very little welting with the drip edge 

intact (fence 2) but fared less well on fence 1. 

Drip edge welting always occurred earlier and to a more 

extensive degree than did welting along vertical edges, even 

on fence 2, where drip edges were painted and vertical cut 

edges were not. Welting along vertical cut edges did not 

become discernable in any of the specimens until May 2000. 

In contrast, most specimens that showed welting along their 

vertical edges in May 2000 had shown easily discernable 

drip edge welting by July 1998.5

 Where welting along vertical cut edges occurred, its presence was noted, but the degree of welting along vertical edges was not quantified. 

Values presented in Table 9 reflect welting along drip edges 

only. For board classes where welting along vertical edges 

occurred, the edge welt index values in Table 9 thus slightly 

understate the total extent of welting. 

Visible fiber raising was common in welted areas, although 

in no cases did fiber raising result in rupture of the paint 

film. We could commonly observe fiber ìtelegraphing,î in 

which fiber or fiber bundle outlines could be seen through 

intact paint surface films. 

 

5

This suggests that water retention in the laps played a role in welt 

development. 

Light conditions influenced the degree to which a welt was 

perceivable. We therefore ensured that light conditions were 

similar at all welt inspections. Welt inspection was performed on sunny days, preferably when some degree of 

upper level cloudiness or haze was present. On days when 

sun exposure was judged to be too direct or intense, moveable translucent shading was used to limit light intensity; the 

west edge of the shading device was placed approximately 

even with the left (west) edge of the column of specimens 

being inspected. When moveable shading was used, the 

location of its edge varied with sun intensity and time of day. 

Welt inspections were always performed between 10:30 a.m. 

and 4:00 p.m. 

Time trends in edge welt development are shown in Figure 12 for all classes combined. Edge welt generally became 

progressively worse over time, although at some intermediate inspection times welt was less perceivable than it was 

during the preceding inspection. These ìreversalsî in edge 

welt behavior coincide with in-place thickness measurement 

data, as shown in Figure 10. A decrease in thickness on both 

fences and an improvement in edge welt rating on fence 2 

occurred over the winter of 1998ñ1999. This can be explained by the fact that the specimens were not sprayed over 

winter months, and thus were most likely drier in May 1999 

than they were in November 1998. A decrease in thickness 

of specimens on the test building (Fig. 10) over the winter of 

1998ñ1999 also suggests drying over that period. A decrease 

in thickness and an improvement in welt rating occurred on 

both fences between the mid-summer and autumn inspections in 1999. The likely explanation is that between August 

and October 1999, there was a 6-week period during which 

approximately 40% of daily spray events did not occur 

because of intermittent malfunction of the well pump. 

From November 1998 through the final inspections in SeptemberñOctober 2000, the rate at which edge welting increased apparently accelerated. The greatest increase in 

welting occurred during the last year of exposure. By the end 

of the spray season in 1999, the specimens had experienced 

in excess of 400 spray events. Edge welting became noticeably more severe over the following year. 

Also evident from Table 9 and Figure 12 is that a painted 

drip edge retarded development of edge welting. 

Mildew 

We saw only minor amounts of fungal discoloration of 

painted surfaces of some boards. The fungal species was 

identified as Aureobasidium pullulans. The paint and coatings industry generally refers to A. pullulans as a mildew 

fungus (Bussjaeger and others 1999, Zabel and Morrell 

Figure 11óExample of edge welt on fence 2 (drip 

edges intact). Specimen was rated on August 25, 1999, 

as having a welt severity of 9.0 and a welt area of 

21 in2

 (EWI = 21). 
14 

1992).6

 We recorded mildew in a similar manner as edge 

welt (area and severity) and calculated a mildew index with 

Equation (1). The results for the last inspection during fall 

2000 are reported in Table 10. Mildew was more common 

on the non-commercial boards, perhaps because they were 

hand-primed with an alkyd primer.7

 Class 5 boards had more 

mildew than any of the other classes. We attribute the lack of 

mildew on the OSB siding to its zinc borate treatment. 

Mildew growth on specimen surfaces was concentrated in 

distinct areas. Even on specimens that had intense patches of 

mildew growth, there were appreciable areas that were devoid of mildew. We therefore characterized mildew growth 

by measuring the surface area within which mildew was 

observed and assigning an intensity rating within that area. 

We used an adaptation of ASTM Standard D 3274 

 ìStandard Test Method for Evaluating Degree of Surface 

 

6

Also, see the sidebar entitled ìWhat is mildew?î in Forintek 

(2003). 

7

The alkyd primer contained soya alkyd resin. Soya alkyd resins 

are significantly more resistant to mildew growth than are unmodified plant-derived oils such as linseed oil, but they are generally 

viewed as having lower resistance to mildew growth than acrylic 

resins. The commercial boards were factory-primed with thermosetting primer, in which the paint resins were likely acrylic. Field 

application of acrylic emulsion paints is sometimes accompanied 

by problems stemming from the surfactants contained in them, but 

such problems would not be anticipated in factory-applied thermosetting emulsion paints. 

Table 9óSeverity of drip edge welt and welt index, by class, as measured September 26 to October 4, 2000a

Fence 1 Fence 2 

Severity (10ñ0)b Edge welt index Severity (10ñ0)b Edge welt index 

Class Average 

Standard 

deviation Average 

Standard 

deviation Average 

Standard 

deviation Average 

Standard 

deviation 

1 8.3 0.5 48.2 17.1 9.5 0.7 4.6 8.4 

2 7.9 0.6 30.4 14.8 9.8 0.3 0.5 1.1 

3 7.9 0.4 64.8 17.2 8.8 0.6 13.3 11.6 

4 7.7 0.3 66.6 14.5 8.8 0.8 12.2 14.2 

5 7.8 0.2 71.1 12.8 7.6 0.4 51.0 13.5 

6 7.6 0.5 72.6 21.6 8.0 0.6 32.9 12.6 

7 7.6 0.5 51.3 17.0 9.4 0.6 3.3 7.2 

8 8.0 0.4 50.8 13.1 8.5 0.5 23.6 12.8 

9 NA NA NA NA NA NA NA NA 

10 7.8 0.5 23.1 10.7 9.7 0.5 0.2 0.4 

11 8.2 0.4 9.8 4.0 9.6 0.6 0.3 0.8 

12 8.6 0.5 13.9 7.1 9.0 0.3 5.9 4.1 

13 8.8 0.4 4.7 3.8 9.4 0.5 2.1 3.3 

14 8.7 0.6 44.9 54.2 9.1 0.6 24.9 40.6 

a

Does not include welting along vertical edges. 

b

Lower numbers mean greater severity. 

Figure 12óEdge welt behavior over time. (a) Fence 1 

(drip edges cut); (b) fence 2 (drip edges intact). 
15

Disfigurement of Paint by Microbial Growth or Soil and Dirt 

Accumulationsî (ASTM 1995b) for judging intensity rating 

within face areas containing mildew. While D 3274 appears 

to have been developed for surfaces on which mildew distribution is more uniform than it was on our specimens, we 

used the same approach to judge severity where mildew was 

present. 

Although mildew growth was more severe on drip edges 

than on board faces, we did not attempt to measure mildew 

growth on drip edges. Mildew growth on drip edges was in 

some cases difficult to distinguish with certainty from dirt 

accumulation. Mildew growth on drip edges was also sometimes accompanied by appreciable algal growth, which made 

assessment of the degree and severity of mildew growth 

difficult. On fence 1 the drip edges were unpainted and dark, 

which made it difficult to see the dark gray or black mildew 

growth. On fence 2, the drip edge surfaces of board in 

classes 1 to 6 and 9 were significantly rougher than the 

preformed drip edges of boards in classes 7, 8, and 10 to 14, 

which would have confounded any attempt to compare 

mildew growth between classes. On board surfaces, mildew 

growth was somewhat more common in the proximity of lap 

areas than elsewhere. A plausible explanation is water retention in lap areas. 

Paint Cracks 

We documented the occurrence of paint cracks at the nails 

on fences 1 and 2 and on the drip edge on fence 2. Ratings 

for paint cracks around the nail heads were either ìnoî (with 

an assigned value of 0) or ìyesî (assigned value of 1). We 

determined an average for each board by dividing the sum of 

the ratings by two (two nail heads). Thus, average values 

range from 0 to 1. Cracks at the drip edge were assessed 

according to severity: 0 (no cracks), 0.5 (minor, visible with 

a 10× hand lens), 1 (significant, visible without magnification at about 15 in. from the drip edge), and 2 (severe, visible without magnification at about 3 ft from the drip edge). 

Table 11 summarizes the results from the final inspection 

(SeptemberñOctober 2000) by siding class. 

Differences in degree of drip edge cracking between classes 

were substantial, with some classes showing very little 

cracking, and other classes, namely classes 5, 6, 8, and 12, 

showing drip edge cracking that was significant or severe. 

The nature of drip edge paint cracking that we observed in 

class 12 boards differed substantially from that in boards in 

classes 5, 6, and 8, but they received the same drip edge 

crack rating with our evaluation criteria. Drip edge paint 

cracks in class 12 boards were substantially fewer in number 

but generally extended for longer distances along the drip 

edge. Board classes 5, 6, and 8 showed significant edge 

welting, as well as drip edge paint cracking, whereas boards 

in class 12 showed modest levels of edge welting (Table 9). 

A close inspection of spare unexposed boards of class 12 

revealed that these boards contained inconspicuous press 

blows as shipped from the manufacturer. The press blows 

evidently opened during board exposure on the fence and 

showed up as cracks in the drip edge paint film. 

Drip edge paint cracking seems to correlate with on-fence 

swelling (Table 8) and with moisture content at time of 

removal (Table 12). As discussed in a later section of this 

report, performance in the substrate weatherability test 

Table 10óSeverity of mildew and mildew index, by class, as measured September 26 to October 4, 2000

Fence 1 Fence 2 

Severity (10ñ0)a Mildew index Severity (10ñ0)a Mildew index 

Class Average 

Standard 

deviation Average 

Standard 

deviation Average 

Standard 

deviation Average 

Standard 

deviation 

1 9.0 1.8 2.6 5.0 9.8 0.9 0.4 1.8 

2 9.4 0.8 0.7 1.6 9.9 0.4 0.3 1.5 

3 7.8 1.8 4.3 4.4 8.6 1.6 2.9 7.1 

4 8.2 1.6 4.5 6.1 8.7 1.9 3.0 6.1 

5 7.1 2.2 11.3 14.1 6.1 1.5 16.6 14.7 

6 7.9 1.3 4.0 4.2 6.9 2.1 7.6 8.5 

7 9.6 0.6 0.5 1.0 10.0 0 0 0 

8 8.7 1.0 4.1 5.4 9.5 0.8 0.9 1.4 

9b 10.0 0 0 0 10.0 0 0 0 

10 9.0 0.7 1.5 1.7 10.0 0 0 0 

11 9.8 0.5 0.1 0.3 10.0 0 0 0 

12 10.0 0 0 0 10.0 0 0 0 

13 10.0 0 0 0 10.0 0 0 0 

14 10.0 0 0 0 10.0 0 0 0 

a

Lower numbers indicate greater severity. 

b

OSB siding.
16 

Table 11óPaint cracks at the nail heads and drip edge, averaged by class, 

as recorded September 26 to October 4, 2000 

Fence 1 Fence 2 

Cracks at nail heads Cracks at nail heads Drip edge cracks 

Class Averagea Standard 

deviation Averagea Standard deviation Averageb Standard deviation 

1 0.70 0.22 0.44 0.24 0.33 0.44 

2 0.55 0.34 0.19 0.20 0.08 0.18 

3 0.73 0.33 0.44 0.24 0.88 0.28 

4 0.66 0.32 0.50 0.29 0.60 0.45 

5 0.73 0.31 0.39 0.29 1.60 0.50 

6 0.83 0.20 0.31 0.27 1.56 0.51 

7 0.46 0.37 0.20 0.24 0.03 0.11 

8 0.09 0.25 0 0 1.15 0.49 

9c

 0.88 0.25 0.69 0.32 0.93 0.18 

10 0.26 0.25 0.08 0.14 0.55 0.32 

11 0.29 0.35 0.19 0.25 0.28 0.38 

12 0.68 0.27 0.31 0.30 1.40 0.50 

13 0.78 0.26 0.59 0.26 0.60 0.38 

14 0.59 0.32 0.30 0.28 0.82 0.30 

a

Values range from 0 (no cracks) to 1. 

b

Values range from 0 (no cracks) to 2 (most severe). 

c

OSB siding. 

Table 12óMoisture content at time of removal (October 12 and 13, 2000) and after 

reconditioning at 70oF, 50%RH 

Moisture content (%), fence 1 Moisture content (%), fence 2 

At time of 

removal

Reconditioned at 

70oF, 50%RH 

At time of 

removal

Reconditioned at 

70oF, 50%RH 

Class 

AverageStandard 

deviation Average

Standard 

deviation Average

Standard 

deviation Average

Standard 

deviation 

1 8.1 0.6 8.1 0.3 8.8 2.6 8.2 0.2 

2 8.2 0.4 8.5 0.3 8.8 1.0 8.7 0.2 

3 8.7 0.5 8.4 0.2 8.9 0.6 8.5 0.3 

4 8.6 0.4 8.5 0.2 8.6 0.5 8.4 0.3 

5 9.0 0.7 8.5 0.3 9.4 0.6 8.7 0.3 

6 8.9 0.4 8.6 0.2 9.0 0.7 8.7 0.3 

7 9.2 0.5 8.5 0.3 9.3 0.7 8.6 0.3 

8 13.9 2.4 10.3 0.4 13.1 0.9 10.3 0.3 

9a 9.7 0.3 8.3 0.3 10.1 0.6 8.7 0.3 

10 8.0 0.5 7.8 0.2 8.2 0.5 7.9 0.2 

11 7.8 1.0 7.6 1.0 8.0 0.7 7.7 0.3 

12 9.8 0.6 8.7 0.4 10.1 0.9 8.9 0.3 

13 7.0 0.7 7.0 0.3 7.4 0.6 7.2 0.3 

14 18.7 6.5 9.5 0.5 16.4 6.0 9.6 0.4 

a

OSB siding. 
17

procedure correlated reasonably well with drip edge paint 

cracking. In contrast, the data for paint cracking at nail heads 

show no clear relationship with performance in any laboratory test or with any other behavior observed on the test 

fences. 

We observed cracking of the paint film at some nail heads 

on the test fences and the house during the fall of 1997. On 

the house, we concluded that installation had resulted in 

some paint cracks at nail heads because the cracks not only 

occurred early on specimens that were never sprayed but 

also occurred without perceptible specimen swelling. The 

nail head areas of specimens on the test house were therefore 

touched-up with paint in spring 1998 when the weather 

became suitable for painting with latex paint. Similar paint 

touch-up was not done on the test fences, because we were 

less certain that cracking had resulted from installation damage. Because of uncertainty regarding the origin of some 

paint film cracking around nail heads, these data should be 

treated with circumspection.8

Paint cracking at a nail head, regardless of how minor, was 

registered as a crack occurrence. Initially, we did not record 

the severity of cracking. By the autumn inspection in 1998, 

we also began to record whether the nail head was pulled 

through the surface paint film and the degree to which pullthrough had occurred. If less than half the circumference of 

the nail head had pulled through the film, the observation 

was rated as a ìborderlineî case and assigned a rating of 0.5. 

If half or more of the circumference of the nail head was 

pulled through the paint film, the observation was rated as a 

ìyesî and the observation assigned a rating of 1.0. 

Figure 13 shows trends for presence of paint cracking at nail 

heads on fences 1 and 2 and the degree to which nail heads 

are pulled through the paint film (severity) over the period 

from the autumn inspection in 1998 through the final inspection in autumn 2000. Ratings, in general, became progressively worse over this period. In contrast to edge welt ratings, which began accelerating after November 1998, the 

paint crack ratings increased at an approximately constant 

rate. Figure 13 indicates that presence and severity values 

parallel each other over time. 

Paint cracking at nail heads does not seem to correlate with 

thickness swelling, edge welting, or cracking of paint on drip 

edges. Specimens in classes 5 and 6 underwent appreciable 

in-service swelling (Table 8), showed the highest values of 

edge welt index (Table 9), and showed the highest incidence 

of drip edge paint cracking (Table 11), but cracking at the 

nail heads was no worse than for classes 1 to 4 or for two of 

 

8

It is also plausible that some paint cracking at nail heads was 

associated with nail head deflection caused by board linear expansion. We did not document nail head deflection out of the surface 

plane. This is a further reason to treat the data for paint film cracking at nail heads with circumspection. 

the seven classes of commercial hardboards. In addition, 

boards of class 8 performed poorly overall, and showed 

appreciable paint cracking on painted drip edges, but never 

developed paint cracks at nail heads on the same fence 

(fence 2). Boards in classes 5, 6, and 8 often showed distinct 

dimpling around the nail heads (that is, swelling around the 

nail head without cracking of the paint). Because the swelling in a dimple increased gradually with radial distance from 

the nail head, the strain on the paint film was moderate, 

which, in turn, kept it from failing. Consumers would probably object to dimpling of board surfaces around nail heads, 

even though dimpling does not result in paint film rupture. 

Table 11 and Figure 13 indicate that a painted drip edge 

helped inhibit development of paint cracks at nail heads. 

Moisture Content 

Moisture contents at the time of removal (October 12 and 

13, 2000) and after equilibrating the boards at 70oF, 50% RH 

are shown in Table 12. Boards in classes 8 and 14 had appreciably higher final moisture contents than boards of the 

other classes. The higher final MC of class 8 correlates with 

higher MC values after reconditioning and with high equilibrium MCs in the vapor sorption tests (Table 6). Compared 

with other classes, class 14 displayed much higher water 

absorption in the 24-h edge water absorption tests (Tables 4 

and 5), and class 8 also exhibited higher water absorption 

than most other classes. As discussed in the next section, 

boards in each of these classes showed more back-face water 

staining than did boards of the other classes. The backFigure 13óPaint cracks at nail heads. (a) Average of all 

boards on fence 1; (b) average of all boards on fence 2. 
18 

face staining observations and the results of water absorption 

tests suggest that liquid water absorption played an important role in the high on-fence MCs of both of these board 

classes. 

For all classes except 8 and 14, boards on fence 1 (which did 

not have painted drip edges) were slightly drier at time of 

removal than corresponding boards on fence 2. The water 

sprays had been shut off at the completion of the final inspection, roughly a week before specimen removal, and 

there had been virtually no rain during that week. If boards 

lacking a painted drip edge were wetter than corresponding 

boards with a painted drip edge at the time of our last onfence thickness measurement and last inspection, the difference had vanished by the time boards were removed from 

the fence a week later. Only in the two classes of commercial board with the highest levels of liquid water absorptivity 

(classes 8 and 14) were boards on fence 1 wetter at time of 

removal than boards on fence 2. 

Staining of Back Face 

Specimens removed from the spray fences were evaluated 

for back face staining by measuring how far water stains 

extended in from the ends at four locations on each specimen 

and how far up from the drip edge at mid-length. Figure 14 

shows the measurement locations. 

It was generally possible to identify water stain marks that 

resulted from water coming from the back face of the specimen above, and these were ignored. Water staining at the top 

of a specimen appeared to be largely the result of water on 

the back face of the specimen immediately above it and was 

therefore not included. 

Demarcation between stained and unstained areas was sometimes clear as an easily visible line that contrasted in shade 

with both the stained and unstained areas. In other cases, no 

clear demarcation line was visible. Stained areas were usually darker than unstained areas, but for some board classes 

with dark-colored back surfaces, the water-stained areas 

were of a lighter shade than the unstained areas. Where a 

measurement value for the border between stained and unstained areas could be discerned for only 5 or fewer of the 

20 test specimens in a classñfence combination, measurement values are not reported. No attempt was made to measure stain intensity. 

Stain extent measurements are shown in Table 13. The data 

indicate that stain extent in from the sides was similar on the 

two fences. Stain extent up from the drip edge generally 

appears to be greater on boards with painted drip edges 

(fence 2), but it is difficult to interpret the modest betweenfence difference because of variability in the data and differences in the character of the laps. 

For almost all board classes, the extent of stain up from the 

drip edge was significantly less than the lap dimension 

(1.25 in. (32 mm) on average for fence 2 and 1.125 in. 

(29 mm) inches on average for fence 1). For classes 8 and 

14, the entire lap area was usually water-stained (Fig. 15), 

but we generally could not establish whether the water came 

inward from the sides or upward from the drip edge. Values 

for stain extent up from the drip edge for board classes 8 and 

14 are therefore, for the most part, not reported in Table 13. 

The degree of back-face staining varied appreciably with 

board class, with board classes 8 and 14 showing significantly more extensive staining than any of the other board 

classes and the OSB siding consistently showing the least 

staining. 

A cursory examination of relationships between MC at the 

time of removal and data on back stain suggested a stronger 

relation between the extent of staining inward from board 

ends and MC than between extent of staining upward from 

board drip edges and MC. This, along with the previously 

mentioned observation that water stains upward from drip 

edges usually remained well below the top of the lap area, 

suggests that no appreciable capillary rise of water in lap 

Figure 14óLocations of water stain measurements 
19

Figure 15óBack staining on class 14 and class 5 boards. 

Table 13óAverage extent of water staining of the back of siding boards 

Extent of stain on fence 1 (in.) Extent of stain on fence 2 (in.) 

In from sides Up from drip edge In from sides Up from drip edge 

Class Average Standard 

deviation 

Average Standard 

deviation 

Average Standard 

deviation 

Average Standard 

deviation 

1 0.93 0.74 0.11 0.26 0.92 0.63 0.06 0.15 

2 0.45 0.14 0 0.01 0.46 0.18 0.09 0.18 

3 1.31 0.34 0.14 0.12 1.26 0.49 0.23 0.26 

4 1.03 0.34 0.14 0.10 1.14 0.26 0.23 0.34 

5 1.78 0.62 0.23 0.21 1.96 0.59 0.52 0.13 

6 1.47 0.51 0.21 0.13 1.87 0.96 0.50 0.26 

7 0.90 0.61 0.01 0.03 0.90 0.63 0.27 0.24 

8 10.76 0.91 ó ó 9.57 1.42 ó ó 

9 0.45 0.17 0.03 0.08 0.40 0.25 0.03 0.15 

10 1.90 0.50 0.08 0.19 1.69 0.50 0.11 0.24 

11 0.41 0.48 0 0.02 0.70 0.71 0.04 0.11 

12 2.41 0.82 0.24 0.36 2.68 1.24 0.49 0.57 

13 0.78 0.56 ó ó 0.70 0.44 0.69 0.30 

14 8.20 2.09 ó ó 8.12 1.75 0.93 0.79 
20 

areas occurred. However, water tends to be retained longer 

in laps than on an open surface. The fact that we observed 

earlier and more extensive welting at drip edges than at 

vertical edges would be consistent with longer water retention times in laps. Our observations that mildew occurrence 

was (1) most extensive on drip edges and (2) somewhat 

more prevalent on board surfaces in the proximity of lap 

areas than at some distance from lap areas were also consistent with longer water retention times in laps. 

Within specially made non-commercial boards (classes 1 to 

6), stain extent tended to become greater with increasing 

(lab-measured) RTS value, although the relationship was far 

from monotonic. When all hardboards were considered, we 

detected no obvious correlation between (laboratorymeasured) RTS value and the extent of back-face water 

staining. In contrast, both classes 8 and 14, which showed 

the most extensive staining, had shown significantly higher 

levels of edge water absorption in laboratory testing than did 

all other classes of commercial board and most classes of 

non-commercial board (Table 5). 

Correlations With Residual 

Thickness Swell Test Results 

We performed a board-by-board correlation between 

selected performance characteristics on the two fences and 

the results of the weatherability of substrate tests (that is, 

residual thickness swell). Each specimen on the fence was 

associated with two adjacent RTS specimens (see Fig. 3), 

and in our analyses we correlated performance values of 

individual test fence specimens with the average RTS value 

of those two RTS specimens. 

Direct data plotting over the full data range usually showed 

only modest degrees of correlation and a great deal of scatter. This data scatter prevented us from establishing simple 

relationships that would allow us to quantify the potential 

effect of changes in the performance criteria in the 

ANSI/AHA standard. We therefore analyzed the data in 

terms of frequency (or probability) of occurrence of a failure 

or failure severity level within ranges of RTS values (RTS 

bins). This allowed us to determine the change in probability 

of occurrence of a certain value or value range for welt, 

mildew, or other failure, as a function of RTS. It also revealed in which RTS range the change in performance characteristics was the most rapid. To have a sufficient number 

of data points in each bin, we selected bins with an RTS 

range of 6%. For instance, the 15% RTS bin contains data 

for boards with an RTS between 12% and 18%. Table 14 

shows the number of specimens in each RTS bin. The number of specimens per RTS bin reached a maximum at around 

10% bin midpoint value and thereafter declined with increasing RTS. The number eventually became too small for meaningful data analysis. 

Edge Welt as a Function of RTS 

Figures 16 to 18 show the probability of occurrence of various levels of edge welt as a function of RTS on fence 1 at 

final inspection. Figure 16 depicts data for all hardboard 

classes on fence 1. The frequency of an edge welt index of 

over 50 (category D) increases steadily between the 5% and 

11% RTS bins (2% to 14% RTS), and steeply increases in 

the 18% and 19% RTS bins (15% to 22% RTS). The frequency of category C index value (30 < I < 50) steadily 

increases from the 13% to 18% RTS bins, where category D 

begins to predominate. The 30% RTS bin and the bins beyond (RTS > 27%) contain only boards with a welt index 

over 50. 

When consideration is confined to commercial board classes 

on fence 1 (Fig. 17), the data again show a steady increase in 

more severe welting (categories C and D) between the 8% 

and 18% RTS bins (5% to 21% RTS) and a steep increase in 

the most severe index level (category D) in the 19% and 

20% RTS bins (16% to 23% RTS). Above 17% RTS (20% 

RTS bin) there are no boards with a welt index below 30.A 

strong relationship between RTS and edge welt also exists 

for the non-commercial board classes on fence 1 (Fig. 18). 

With these boards, severe welting (I > 50, category D) 

steeply increases at much lower RTS ratings, specifically 

between the 5% and 13% RTS bins (2% to 16% RTS). With 

a few exceptions, non-commercial boards on fence 1 with 

RTS ratings over 11% (14% RTS bin and higher) had severe 

welting. 

Severe welting (category D) was less common on fence 2 

than on fence 1 (Figs. 19 to 21 compared with Figs. 16 to 

18); the painted drip edges on fence 2 evidently retarded the 

development of severe welts. Figures 19 to 21 more clearly 

show a decreasing frequency of boards with low welt index 

values as RTS increases. Figure 19 (all boards on fence 2) 

shows that the frequency of boards with the lowest edge welt 

index (category A, I = 0 to 10) steadily drops with RTS 

between the 5% and 14% RTS bins and then drops more 

rapidly in the 15% and 16% RTS bins. Higher edge welt 

index categories (C and D) begin to increase markedly at the 

16% RTS bin. No strong correlation seems to exist at RTS 

levels beyond 25% except that the number of boards with an 

edge welt index between 30 and 50 (category C) gradually 

increases with RTS. When non-commercial boards are excluded from the analysis (Fig. 20), the same gradual drop in 

number of boards with the lowest index (A) occurs at lower 

RTS ratings, with a sharp drop at the 15% and 16% RTS 

bins. More severe edge welt categories (C and D) begin to 

increase at RTS over 17% (14% RTS bin). 

As on fence 1, the non-commercial boards on fence 2 

show greater susceptibility to edge welt than do commercial 

boards at a given RTS level, even in the lower RTS bins 

(compare Figs. 20 and 21). The frequency of boards with 

little or no edge welt (category A) declines rapidly between 

the 14% and 18% RTS bins (11% to 21% RTS). 
21

Table 14óNumber of specimens in each RTS bin 

RTS bin Number of specimens per bin 

All boardsa Non-commercial Commerciala 

Center 

(%) 

Range 

(%) Fence 1 Fence 2 Fence 1 Fence 2 Fence 1 Fence 2 

5 2ñ8 88 81 43 37 45 44 

6 3ñ9 103 102 52 49 51 53 

7 4ñ10 111 119 58 60 53 59 

8 5ñ11 114 119 54 55 60 64 

9 6ñ12 118 116 50 50 68 66 

10 7ñ13 118 120 47 43 71 77 

11 8ñ14 111 115 35 39 76 76 

12 9ñ15 103 102 26 31 77 71 

13 10ñ16 98 88 22 19 76 69 

14 11ñ17 76 69 15 15 61 54 

15 12ñ18 51 49 10 9 41 40 

16 13ñ19 34 32 5 7 29 25 

17 14ñ20 22 26 5 8 17 18 

18 15ñ21 21 19 11 4 10 15 

19 16ñ22 19 14 12 7 7 7 

20 17ñ23 21 18 16 12 5 6 

21 18ñ24 21 22 18 18 3 4 

22 19ñ25 22 20 20 18 2 2 

23 20ñ26 23 18 21 16 2 2 

24 21ñ27 21 22 19 21 2 1 

25 22ñ28 17 22 16 21 1 1 

26 23ñ29 13 19 13 18 ó 1 

27 24ñ30 12 14 12 14 ó ó 

28 25ñ31 11 15 11 15 ó ó 

29 26ñ32 9 16 9 16 ó ó 

30 27ñ33 6 11 6 11 ó ó 

31 28ñ34 7 8 7 8 ó ó 

32 29ñ35 8 7 8 7 ó ó 

33 30ñ36 7 6 7 6 ó ó 

34 31ñ37 5 3 5 3 ó ó 

35 32ñ38 5 3 5 3 ó ó 

36 33ñ39 5 4 5 4 ó ó 

37 34ñ40 4 4 4 4 ó ó 

38 35ñ41 3 3 3 3 ó ó 

39 36ñ42 2 2 2 2 ó ó 

40 37ñ43 2 2 2 2 ó ó 

41 38ñ44 2 1 2 1 ó ó 

42 39ñ45 1 ó 1 ó ó ó 

43 40ñ46 1 ó 1 ó ó ó 

a

Does not include OSB boards. 
22 

Figure 16óEdge welt index as a function of residual 

thickness swell (RTS), fence 1, all classes except OSB 

(class 9). Lower index means less welt. 

Figure 17óEdge welt index as a function of residual 

thickness swell (RTS), fence 1, commercial classes only 

(except OSB). Lower index means less welt. 

Figure 18óEdge welt index as a function of residual 

thickness swell (RTS), fence 1, non-commercial classes. 

Figure 19óEdge welt index as a function of residual 

thickness swell (RTS), fence 2, all classes except OSB 

(class 9). 

Figure 20. Edge welt index as a function of residual 

thickness swell (RTS), fence 2, commercial classes only, 

except OSB (class 9). 

Figure 21óEdge welt index as a function of residual 

thickness swell (RTS), fence 2, non-commercial classes. 

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B

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CCC

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D

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A: Index = 0ñ10 

B: Index = 10ñ30 

C: Index = 30ñ50 

D: Index > 50 

Center of the RTS bin (%) 

0 10 20 30 40 50

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

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Center of the RTS bin (%) 

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Probability of occurrence 

A: Index = 0ñ10 

B: Index = 10ñ30 

C: Index = 30ñ50 

D: Index > 50 

0 10 20 30 40 50

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D

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DD

DDDD

1.0 DDDDDDDDDDDDDD

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

A: Index = 0ñ10 

B: Index = 10ñ30 

C: Index = 30ñ50 

D: Index > 50 

Center of the RTS bin (%) 

0 10 20 30 40 50

AA

A

AAA

A

A

AA

A

AAA

A

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A

AAAAA BB

B

BBBBBBBB

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CCCCCCCCCC

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D CCCCCC DDDDDDDDDD

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A: Index = 0ñ10 

B: Index = 10ñ30 

C: Index = 30ñ50 

D: Index > 50 

Center of the RTS bin (%)

0 10 20 30 40 50

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

Center of the RTS bin (%)

0 10 20 30 40 50

A

A

A

AA

AA

AA

A

A

AA

AAAAAAAA

A

AAA

A

A

A

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B

B

B

BB

BBBB

B

B

B

B

BBBB

BBBBB

B

B

BB

B

B

BB

BBBBBBB CCCCCC

CCC

CC

C

C

C

C

CC

C

C

CCCC

C

CC

C

C

C

C

C

CC

CCCC

DDDDDDDDDDDD

D

D

D

DDD

D

DD

D

DDD

D

D

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A: Index = 0ñ10 

B: Index = 10ñ30 

C: Index = 30ñ50 

D: Index > 50 

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

A

A

A

AAA

A

AAA

A

A

AA

A

AA

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A

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A

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B

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CC BBBBBBB CCCCCCCC

C

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DDDDDDDDDDD

D

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Probability of occurrence 

A: Index = 0ñ10 

B: Index = 10ñ30 

C: Index = 30ñ50 

D: Index > 50 

1.0 

0.8 

0.6 

0.4 

0.2 

Center of the RTS bin (%)

0 10 20 30 40 50
23

Figure 22óProbability of boards developing edge welt as 

a function of RTS, fence 2, all classes except OSB 

(class 9). 

Figure 23óProbability of boards developing edge welt as 

a function of RTS, fence 2, commercial classes only, 

except OSB (class 9). 

Another way of viewing the results is to graph the probability of a board showing any edge welt as a function of its 

RTS rating. Figures 22 and 23 show the results of this analysis for fence 2, with and without non-commercial classes, 

respectively. Figure 22 shows an increase with RTS in the 

occurrence of welted boards in RTS bins below 19%, with 

the steepest increase between the 5% and 9% RTS bins and a 

more gradual increase between the 10% to 18% bins. Above 

RTS = 16% (19% RTS bin) all boards have edge welts. The 

data for commercial boards (Fig. 23) show similar trends. 

In summary, the data indicate that the current 20% RTS 

criterion in the ANSI/AHA standard does exclude boards 

that are prone to serious edge welting but that the incidence 

and severity of edge welting may be further reduced, and to 

Figure 24óProbability of boards developing mildew as a 

function of RTS, fence 1, all classes except OSB 

(class 9). 

Figure 25óProbability of boards developing mildew as a 

function of RTS, fence 1, commercial classes only, 

except OSB (class 9). 

a significant degree, if the RTS criterion were lowered from 

20% to 16% or 17%. Even further reduction in the probability of welt incidence and severity would be expected if the 

level were to be reduced still more. 

Mildew as a Function of RTS 

The mildew index does not correlate as well with RTS as 

does the edge welt index. Figures 24 to 29 show the occurrence of mildew as a function of RTS ratings. When all 

classes on fence 1 are included (Fig. 24), the data show a 

sharp increase in the occurrence of mildew after the 16% 

RTS bin (13% to 19% RTS) and mildew on all boards with 

an RTS rating of 27% or greater (30% RTS bin). Without 

the non-commercial classes (Fig. 25), the data show similar 

trends but not as clearly. Mildew occurrence increases 

Center of the RTS bin (%) 

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

0 10 20 30 40 50

Center of the RTS bin (%) 

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

0 10 20 30 40 50

Center of the RTS bin (%)

1.0

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

0 10 20 30 40 50

Center of the RTS bin (%)

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

0 10 20 30 40 50
24 

rapidly at the RTS = 17% bin, but the correlation at lower 

RTS values is not clear. Commercial boards on fence 1 with 

an RTS rating of 19% (22% RTS bin) or higher all had 

mildew. Mildew occurrence on the non-commercial classes 

on fence 1 increases more gradually and consistently with 

RTS (Fig. 26) between 2% RTS and 27% RTS. All noncommercial boards with an RTS rating over 27% (30% RTS 

bin and higher) had mildew. 

The data for fence 2 (drip edges intact, Fig. 27) give somewhat different results. The likelihood that boards had mildew 

rapidly increases after the 13% RTS bin (10% to 16% RTS), 

but after the 20 RTS bin (17% to 23% RTS) there appears to 

be no clear correlation. When non-commercial classes are 

excluded from the data (Fig. 28), a somewhat clearer picture 

emerges. Occurrence of mildew increases sharply after the 

14% RTS bin (11% to 17% RTS). However, the correlation 

fails at RTS values of 24% and higher (21% RTS and higher 

bins) because the sample size is reduced to four specimens 

or fewer, and the specimen with the highest RTS value had 

no mildew. Mildew on the non-commercial boards on 

fence 2 increases rapidly with RTS between 2% RTS and 

21% RTS (5% RTS and 18% RTS bins). Mildew occurrence 

remains high at RTS values over 21% until it seems to drop 

off after the 34% RTS bin. However, only four or fewer 

specimens are in these higher bins, and the results are 

skewed by one specimen without mildew having an extremely high RTS rating. 

In summary, although occurrence of mildew is obviously 

related to multiple factors, mildew correlates reasonably well 

with RTS, and the data suggest that the incidence of mildew 

Figure 26óProbability of boards developing mildew as 

a function of RTS, fence 1, non-commercial classes 

only. 

Figure 27óProbability of boards developing mildew as 

a function of RTS, fence 2, all classes except OSB 

(class 9) 

Figure 28óProbability of boards developing mildew as 

a function of RTS, fence 2, commercial classes only, 

except OSB (class 9). 

Figure 29óProbability of boards developing mold or 

mildew as a function of RTS, fence 2, non-commercial 

classes only. 

Center of the RTS bin (%)

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

0 10 20 30 40 50

Center of the RTS bin (%)

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

0 10 20 30 40 50

Center of the RTS bin (%)

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

0 10 20 30 40 50

Center of the RTS bin (%)

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

0 10 20 30 40 50
25

may be reduced by lowering the RTS requirement in the 

ANSI/AHA standard from 20% to 16% or 17%. However, 

the data do not show that further significant improvement 

can be expected by dropping the criterion below 16% RTS. 

Paint Cracks at Nail Heads as a Function of RTS 

Figure 30 shows the occurrence of paint cracks around the 

two nail heads of boards on fence 1 as a function of RTS. 

All board classes are included. The results show no clear 

correlation, except that there are no boards without cracks if 

the RTS is 17% or higher (20% RTS bin). 

Figure 31, which depicts corresponding data for fence 2, also 

shows little correlation between the occurrence of cracks 

around nail heads and RTS. On fence 2, the frequency of 

boards without cracks declines for RTS bins beyond 21% 

(RTS > 18%). Above RTS of 27% (30% RTS bin and 

higher), all boards from fence 2 showed paint cracks. 

In summary, paint cracks around nail heads did not correlate 

well with RTS, although boards with RTS over 20% tended 

to have more cracks. We thus have no evidence that lowering the current RTS requirement of 20% would decrease the 

incidence of paint cracking around nail heads. This conclusion may be limited to prefinished siding, where paint cracking due to installation may obscure any effect of thickness 

stability (RTS rating). With site-finished hardboard siding, 

RTS level might have a greater influence on occurrence of 

paint film cracking at nail heads than we observed in this 

study. 

Nail Heads Through the Surface as a 

Function of RTS 

Figure 32 shows the correlation of occurrence of nail heads 

penetrating the paint surface with RTS for all classes on 

fence 1. This was also referred to previously in this report as 

ìseverityî of paint cracks around the nail head. ìBorderlineî 

indicates that one nail head was rated borderline (0.5). 

ìModerateî penetration means that one nail had fully pulled 

through (rated at 1) or that two nail heads were rated borderline. ìSignificantî indicates one complete pull through and 

one borderline, and ìsevereî means that both nail heads had 

completely pulled through the surface. No clear correlation 

appears to exist below 20% RTS, but above that level the 

number of boards without penetration begins to drop off. 

The RTS bins of 26% and higher contain no boards without 

penetration, which means that all boards with a RTS of over 

23% showed nail head surface penetration. 

There was no apparent correlation between nail head penetration through the paint surface and RTS on fence 2 

(Fig. 33), even at high RTS ratings. Some boards with very 

high RTS ratings did not have any nail heads penetrating the 

surface. As discussed previously in this report, these boards 

often had considerable thickness swelling around the nail 

heads, but the swelling was gradual and the paint film was 

evidently capable of accommodating the dispersed strain. 

In summary, as with paint cracks around nail heads, nail 

head penetration of the paint surface did not correlate well 

with RTS. The same limitation that applies to our findings 

concerning paint cracks at nail heads also applies to our 

findings concerning nail head penetration. 

Drip Edge Paint Cracks as a Function of RTS 

Figure 34 shows the correlation of probability of occurrence 

of various drip edge crack ratings with RTS. The data show 

a strong inverse correlation between frequency of boards 

without drip edge cracks and RTS. The frequency of such 

Figure 30óOccurrence of cracks at nail heads as a 

function of residual thickness swell, fence 1, all classes 

except OSB (class 9). 

Figure 31óOccurrence of cracks at nail heads as a 

function of residual thickness swell, fence 2, all classes 

except OSB (class 9). 

OOOOOO

OOO

OO

OO

O

O

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Center of the RTS bin (%) 

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

0 10 20 30 40 50

o No crack

* One nail

 Both nails

o No crack

* One nail

 Both nails

OOOOOOOO

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O

O

O

O

O

O

O

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Center of the RTS bin (%) 

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

0 10 20 30 40 50
26 

boards steadily declines with increasing RTS until, at 16% 

RTS and above (19% RTS bin and higher), no board specimens are without drip edge cracks. At 32% RTS and above 

(35% RTS bin and higher), all cracking is severe, but the 

number of specimens in those bins is very small (four or 

fewer). 

When only commercial boards are considered (Fig. 35), the 

correlation between the number of boards without cracks and 

RTS is not as clear for RTS values below 15%, but a rapid 

decline is apparent when RTS increases from the 15% RTS 

bin to the 19% RTS bin. However, the occurrence of significant drip edge cracking increases steadily from extremely 

low RTS values up to the 20% RTS bin, with an especially 

rapid increase after the 15% RTS bin (RTS > 18%). Boards 

with RTS ratings of over 16 % (19% RTS bin) all had drip 

edge cracks, and the occurrence of severe cracking begins to 

increase significantly beyond the 19% RTS bin. 

In summary, paint cracking on the drip edge correlated well 

with RTS, showing a marked increase for boards with RTS 

ratings over 18%. The data also suggest that lowering the 

RTS requirement beyond 18% is likely to yield additional 

improvement in drip edge performance. 

Summary of Correlations 

Correlations of test fence performance with residual thickness swell (RTS) test results can be summarized as follows: 

• Edge welt correlated well with RTS. The data suggest 

that lowering the RTS test criterion to 16% or 17% would 

Figure 32óOccurrence of nail heads penetrating the 

paint surface as a function of residual thickness swell, 

fence 1, all classes except OSB (class 9). 

Figure 33óOccurrence of nail heads penetrating the 

paint surface as a function of residual thickness swell, 

fence 2, all classes except OSB (class 9). 

Figure 34óOccurrence of cracks on the drip edge as a 

function of residual thickness swell, fence 2, all classes 

except OSB (class 9). 

Figure 35óOccurrence of cracks on the drip edge as a 

function of residual thickness swell, fence 2, commercial 

classes only, except OSB (class 9). 

o No 

* Borderline

 Moderate 

x Significant 

+ Severe 

O

OOOO

OO

OOO

O

OO

O

OO

OOOO

O

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Center of the RTS bin (%) 

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

0 10 20 30 40 50

o No 

* Borderline

 Moderate 

x Significant

+ Severe 

Center of the RTS bin (%)

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

0 10 20 30 40 50

O

O

OOO

OOO

OO

O

O

O

O

O

O

O

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OO

O

O

O

O

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O

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o No crack 

* Minor 

 Significant 

x Severe 

OOOOO

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O

O

O

O

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Center of the RTS bin (%)

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

0 10 20 30 40 50

 Significant 

x Severe

O No crack

* Minor 

OOO

OOOOOO

OO

O

OO

OOOOOOO

Center of the RTS bin (%)

1.0 

0.8 

0.6 

0.4 

0.2 

Probability of occurrence 

0 10 20 30 40 50
27

be beneficial and that lowering it still further would have 

additional benefits. 

• Mildew growth correlated reasonably well with RTS. 

Lowering the RTS criterion to 16% or 17% would likely 

reduce mildew growth on boards, but further reductions 

are unlikely to be beneficial. 

• Paint cracking of the drip edge correlated well with RTS. 

The data suggest that lowering the RTS criterion to 18% 

would have a beneficial effect and that further reductions 

would yield additional benefits. 

Issues Related to Standard 

ANSI/AHA 135.6 

Besides passñfail criteria, other issues related to the weatherability of substrate test procedures in the ANSI/AHA 

A135.6-1998 standard are worth noting. The standard does 

not specify any statistically based procedures that would 

ensure, at a specific level of confidence, that a product meets 

the standardís performance requirements. Instead, it implies 

that all specimens (in a sample selected for purposes of 

certification testing) must meet the test criteria, without 

specifying sample size, frequency of sampling, or how 

specimens should be selected. It appears that in practice, 

sample size for certification testing is sometimes very small 

(Carll and others 2000).9

In this study, we found that a significant minority of specimens of one class of commercial board failed to meet 

ANSI/AHA requirements, even though the board was gradestamped as conforming to the standard. Biblis (1989) apparently had a similar experience. The 100% passage rate requirement for specimens in the certification sample, implied 

in the ANSI/AHA standard, does not guarantee that all 

boards in the population associated with that sample also 

meet test criteria requirements. 

We believe that the standard would be more meaningful if it 

specified a statistical confidence level with which the product can be expected to meet the standardís RTS requirement. 

If the sample size were 10 randomly selected specimens and 

we assumed a normal distribution, a 95% confidence level 

for the board population that the sample is assumed to represent would translate to the following passñfail criterion: 

Average + (1.8 × standard deviation) < maximum 

RTS requirement 

where standard deviation is the standard deviation of the 

individual measurements. If the required number of samples 

were lowered, the multiplier value for the standard deviation 

value would need to be raised in order to maintain the same 

confidence level. A similar approach could be followed for 

 

9

 See table 1 of Carll and others (2000). 

all other test requirements (such as linear expansion, water 

absorption) in the standard. 

The measures above, by themselves, address only the variability in the sample population, not variations in the product 

over time. Variability over time can be addressed only by an 

effective in-plant quality control program, frequent random 

sampling and testing, or both. 

Conclusions 

We observed the following in laboratory testing: 

• RTS and 24-h edge water absorption test results varied 

considerably between commercial hardboards. 

• Most of the commercial hardboard siding tested met the 

industry criterion of 20% maximum RTS. However, 14% 

of tested specimens from one of the classes of commercial hardboard exceeded 20% RTS. 

We observed the following on the water-sprayed test fences: 

• The most noticeable failure mode was surface welting, 

which occurred earlier and to a greater extent near drip 

edges (in lap areas) than it did on board end cuts. The 

next most noticeable failure modes were paint film breakage on drip edges and board swelling that resulted either 

in cracks in the paint film at the nail heads or in ìdimplingî around nail heads. 

• There was no noticeable buckling, but test specimen size 

and attachment to the fences was such that noticeable 

buckling would have been unlikely. 

• We found no visible evidence of decay in any of the 

siding. Open end joints at board ends evidently permitted 

water entry but also afforded drainage and drying. 

• We only found minor amounts of mildew on the painted 

surfaces of some boards. However, the exposure regimen 

(appreciable sun exposure and daily spraying with quantities of water sufficient to flush the board surfaces) may 

not have been conducive to mildew growth. 

• Paint performance was generally good, even on the test 

fence where drip edges had been removed. 

• There were substantial differences in performance of 

commercial boards, especially with regard to edge welting. 

• A painted drip edge substantially retarded development of 

surface welting at the drip edge, reduced the magnitude of 

measured in-service swelling, and reduced development 

of paint cracks at nail heads. 

Correlations between laboratory RTS data and performance 

on the fences showed the following: 

• Direct correlations of RTS and any given measurement of 

field performance were rarely strong. There was usually 
28 

substantial scatter in the data, but RTS nevertheless often 

was a useful indicator of field performance. 

• RTS proved to be a good indicator of the likelihood of 

edge welting. The data suggest that lowering the test criterion in AHA/ANSI Standard A135.6 to 16% or 17% 

would noticeably reduce the incidence of edge welting in 

service and that lowering it still further would have additional benefits. 

• RTS proved to be a reasonably good indicator of likelihood of mildew growth on painted surfaces. The data 

suggest that lowering the RTS criterion in the standard to 

16% or 17% would likely reduce the incidence of mildew 

growth in service but do not suggest that further reductions would be beneficial. 

• RTS proved to be a good indicator of the likelihood of 

paint cracking on drip edges. The data suggest that lowering the RTS criterion in the standard to 18% would noticeably lower the incidence of drip edge paint cracking 

and that further reductions would yield additional benefits. 

Our review of the current version of ANSI/AHA Standard 

A135.6 for hardboard siding led us to the following conclusions: 

• The standard lacks an appropriate statistical basis for 

sample selection. 

• The standard does not account for the effects of variability in the test results. 

Other conclusions and observations: 

• The degree of back-surface water staining varied appreciably between commercial hardboards. Despite significant water staining on the back surface of some boards, 

there is no convincing evidence that vertical capillary 

suction in the laps between boards played a significant 

role in back-surface wetting. With the short specimen 

length and open end joints, migration of water from board 

ends appeared to play a substantial role in back-surface 

wetting. However, retention of water in lap areas may be 

an important factor in board degradation. 

• We saw considerable mold growth on specimens exposed 

to 97% relative humidity (RH) during the sorption tests 

but no visible mold on any of the specimens maintained 

at 79% RH. 

Recommendations 

Based on our conclusions and observations, we make the 

following recommendations 

• ANSI/AHA Standard A135.6 should be revised to include 

statistically based criteria for sample selection. 

• Statistically based passñfail criteria should be added to 

the standard that account for the variability in the individual measurements and the number of samples measured. 

• The RTS passñfail criterion in the standard should be 

lowered to 17% or lower. Although there likely would be 

additional performance benefits from lowering the criterion further, we believe that such a decision should also 

be based on factors such as the effect on mill operation 

and economics, which are outside the scope of this report. 

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ASTM D 1037. West Conshohocken, PA: American Society 

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ASTM. 1995b. Standard test method for evaluating degree 

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