Durability of Hardboard 

Lap Siding

Performance in Laboratory Testing 

and in Long-Term Exterior Exposure

Charles G. Carll

Charles R. Boardman

Steve P. Verrill

Forest

Products

Laboratory

Research

Paper

FPL–RP–674

United States Department of Agriculture

Forest Service
September 2013

Carll, Charles G.; Boardman, Charles R.; Verrill, Steve P. 2013. Durability 

of hardboard lap siding—performance in laboratory testing and in longterm exterior exposure. Research Paper FPL-RP-674. Madison, WI: U.S. 

Department of Agriculture, Forest Service, Forest Products Laboratory. 

25 p.

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Abstract

This paper describes a study that was undertaken to evaluate 

the degree of correlation between in-service performance of 

hardboard siding and its performance in the industry standard test procedure for “weatherability of substrate.” The 

study included 13 different hardboard sidings: 6 noncommercial boards and 7 commercial products. All manufacturing plants operating in the United States in 1996–1997 were 

represented in the study. Twenty replicate specimens of each 

of the 13 different boards were exposed on the south side of 

an unconditioned building near Madison, Wisconsin, for 

155 months (almost 13 years). Although the climate at the 

exposure site was less challenging with regard to rainfall 

or decay hazard than most locations in the eastern United 

States, paint coating on test specimens was only about half 

as thick as recommended. Two failure modes were observed 

in service, drip-edge paint cracking and edge welting, 

although paint condition on board faces remained essentially 

perfect throughout the exposure period. With regard to the 

two observed failure modes, substantial differences were 

observed among the 13 sidings. Relative frequency of the 

two failure modes was related to residual thickness swelling (RTS) value of the board, as identified by the substrate 

weatherability test procedure. Lower RTS values were associated with lower relative frequency of failure and lower 

relative frequency of more intense failure. In general, there 

was no “plateau” value below which further reductions in 

RTS were not associated with greater relative frequency of 

improved in-service performance.

Keywords: Hardboard siding, long-term exposure, 

durability, edge welt, drip edges

Acknowledgments

This study was funded by the U.S. Department of Housing 

and Urban Development. Additional support was provided 

by the American Hardboard Association. Anton TenWolde 

(retired from FPL) played a lead role in design of the study. 

Gerry Laughinghouse, formerly of Georgia-Pacific Corporation, assisted in developing the study, and coordinated 

production of noncommercial board. Harry Warren (retired 

from Masonite Corporation) and Louis Wagner (formerly 

with the American Hardboard Association) provided advice and coordinated the acquisition of commercial board. 

Vyto Malinauskas (deceased) prepared the test building for 

installation of specimens and served as data recorder for 

inspections conducted during the early years of exposure. 

Robert Munson assumed the role of helper/recorder for the 

inspector from 2002 through 2007. Alex Wiedenhoeft performed the microscopic inspections of board back surfaces 

that identified that fungal colonizations had occurred. Flavio 

Ruffinato (visiting scholar at FPL, and PhD student at the 

University of Turin) produced the micrographs shown as 

Figures 7a, 7b, and 12. David Marr (formerly with FPL) and 

Neil Gribbins performed density distribution measurements, 

which yielded data presented in Table 4. Chris Hunt produced micrographs from which paint film thicknesses were 

measured and provided the lead author with information that 

permitted him to download and use the ImageJ software.

In Memorium 

Charlie Carll passed away shortly before the publication of 

this paper. His coauthors acknowledge his strong leadership 

in this research and recognize that his death represents a 

great loss to his family, his friends, and the wood research 

community.

Contents

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

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

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

Results..............................................................................10

Discussion and Analysis ..................................................18

Conclusions......................................................................22

Literature Cited ................................................................23

Appendix—Rainfall and Scheffer Index Values for 

One Location in Each State East of and Immediately 

West of the Mississippi River ..........................................25
Durability of Hardboard Lap Siding

Performance in Laboratory Testing 

and in Long-Term Exterior Exposure

Charles G. Carll, Research Forest Products Technologist, Retired

Charles R. Boardman, General Engineer

Steve P. Verrill, Mathematical Statistician

Forest Products Laboratory, Madison, Wisconsin

Background

During the 1990s, researchers at the USDA Forest Products 

Laboratory (FPL) undertook two studies concerning performance of hardboard lap siding. The first study involved exposure of hardboard lap siding on test buildings in southern 

Florida for a period of 29 months (Carll and others 2000). 

The study was intended to evaluate the effectiveness of a 

Local Acceptance Standard (LAS) promulgated by the Department of Housing and Urban Development (HUD) that 

would have required factory finishing of hardboard siding 

and priming of all surfaces (including the back surface). 

All siding in that study was produced at one manufacturing 

plant and had physical properties that were substantially better than required by the industry standard for hardboard siding (AHA 1990).1 Moisture content of the siding remained 

low over the course of the exposure period, regardless of 

whether the siding was site-finished or factory-finished or 

whether it was primed on its back surface. The siding remained in excellent condition, reflecting its consistently low 

in-service moisture content. No conclusions that might have 

been generically applicable to hardboard siding could be 

drawn from the study, because the study had included siding from only one manufacturing plant. Commercial hardboard siding available during the late 1980s and early 1990s 

showed substantial variation in physical properties (Biblis 

1989, 1991). A second study, which addressed performance 

of hardboard siding from all manufacturing plants in the 

United States (Carll and TenWolde 2004) was therefore 

initiated in 1997.

The American National Standard for Hardboard Siding, in 

its three most recent editions (AHA 1990, 1998; CPA 2006), 

outlined a number of test procedures for evaluation of the 

product and specified acceptance criteria related to the test 

procedures. Keplinger and Waldman (1998) and Baldwin 

(1988) indicated that when there are durability problems 

with hardboard siding, irreversible (or “residual”) thickness 

swelling is frequently a contributing factor to, or a cause of, 

1The study was performed under a Sponsored Research and 

Development Agreement between the USDA Forest Service, Forest 

Products Laboratory, and a manufacturer of hardboard siding. The 

manufacturer operated more than one plant, but produced siding 

for the Florida market at only one of their plants.

problems. This suggested that the procedure in the American 

National Standard that involves measurement of residual 

thickness swell (RTS), referred to as the “weatherability of 

substrate” test, would predict performance in service. As of 

the late 1990s, however, the literature did not contain evidence that performance in the substrate weatherability test 

procedure correlated with performance in service. 

A main objective of the Carll and TenWolde (2004) study 

was to evaluate the degree of correlation between performance in the industry standard test procedure for “weatherability of substrate” and performance in use. The study 

involved laboratory testing and field exposure of a variety 

of hardboard sidings with a wide range of residual thickness 

swelling. Painted siding specimens, matched to unpainted 

laboratory test 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 warm months (accelerated exterior exposure). 

The exposure site was FPL’s Valley View test site, just west 

of Madison, Wisconsin. Results from the 39-month accelerated exterior exposures were reported (Carll and TenWolde 

2004). RTS, as measured by the “weatherability of substrate” test procedure, proved to be a good indicator of the 

likelihood of three modes of problematic performance on 

the spray fences: edge welting, mildew growth, and paint 

cracking on drip edges. Each of these problematic performance modes was moisture related. The exposure period for 

specimens placed on the test building (nonaccelerated exterior exposure) was intended to be a decade or slightly more. 

At the time of publication of Carll and TenWolde (2004), the 

exposure period had been substantially less than a decade, 

so data for specimens on the test building were, for the most 

part, not reported. The exposure period reached 10 years in 

September 2007 and was continued until August 2010. 

The present paper addresses performance of specimens on 

the test building over their approximately 13 years of exposure, in other words, the completion of the study initiated in 

1997. Among other things, it reports on correlation between 

in-service performance of specimens and their performance 

in laboratory testing. It covers, in some detail, the two 

problematic behaviors (“failure modes”) that were observed 

during exposure.
Research Paper FPL–RP–674

2

Objectives

The study objectives, outlined by Carll and TenWolde 

(2004), were (1) to examine to what extent performance in 

the industry standard test procedure for “weatherability 

of substrate” correlated with siding performance in use, 

(2) to determine if change in the acceptance criterion for 

“substrate weatherability” in the American National Standard was justified, and (3) to examine to what degree results 

from other laboratory tests correlated with siding performance in use. The acceptance criterion in the American National Standard for substrate weatherability was changed in 

2006, with maximum allowable value for RTS lowered from 

20% to 17% (CPA 2006). This change was based on results 

from the part of the study involving accelerated exposure 

(Carll and TenWolde 2004) that indicated that RTS was an 

imperfect but nonetheless useful predictor of performance 

characteristics on test fences and that the likelihood of certain problematic behaviors (“failure modes”) was noticeably 

greater when RTS exceeded 17%. In light of the 2006 revision of the American National Standard, the second objective for the remaining phase of the study has been revised. 

The revised objective is to evaluate the now-current 

17% RTS criterion value in the industry standard. A fourth 

study objective has been added, which is to identify prevalence of different “failure modes” in long-term exterior exposure and to document the patterns in which they develop.

Approach and Methodology

The approach of the study and the methodologies it employed were described previously (Carll and Ten Wolde 

2004). Nevertheless, essential elements of the study’s approach and methods are reiterated here. Materials in the 

study included six noncommercial hardboard sidings and 

seven commercial hardboard sidings. One oriented strandboard (OSB) siding was also included as a reference material. All U.S. hardboard manufacturing plants producing hardboard siding in 1996–1997 were represented in the study.

Siding Classes

The 13 different hardboard sidings in the study were each 

assigned a “class” number. In the case of noncommercial 

boards, class number was based on level of RTS observed 

in preliminary testing. In the case of commercial boards, 

class number was based on the order in which shipments 

were received at FPL from production plants.

The six noncommercial classes of siding were selected from 

different lots of board produced for the study at an industrial 

plant. One of the lots was produced using the plant’s normal 

pressing procedures and was thus similar to commercial 

board produced at the plant. Three additional lots were 

specially produced at lower than usual press temperatures 

and shorter press times to yield test materials that exhibited 

a wide range of properties, in all cases inferior to those of 

boards produced using the plant’s normal pressing procedures. The noncommercial boards were shipped to FPL 

as unprimed 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. In preliminary testing, RTS value within 

individual strips was found to be largely predictable based 

on production lot and position of the strip from within the 

4- by 8-ft sheet. The six classes of noncommercial board 

were thus based on production lot and strip position.

Each of the seven commercial classes of hardboard siding 

was produced at a different production plant. The commercial hardboards were shipped to FPL as factory-primed 

strips of nominally 8-in.- (0.2-m-) wide lap siding with beveled or slightly rounded drip edges, which had been shaped 

prior to priming. All the hardboard sidings (commercial 

and noncommercial) had flat smooth (not textured) front 

surfaces.

Because OSB siding was, at the time of study initiation, not 

readily available as lap siding, 8-in.-wide by 8-ft-long strips 

of siding were sawn at FPL from 4- by 8-ft (1.2- by 2.4-m) 

sheets of panel siding obtained from a lumber retailer. The 

OSB siding had an embossed (textured) face. It contained 

zinc borate, which was confirmed by atomic emission spectroscopy. The OSB siding was assigned a class number of 9, 

reflecting that it was received at FPL after two of the commercial hardboards (classes 7 and 8) but before five of the 

commercial hardboards (classes 10 to 14).

Specimen Selection and Preparation

Twenty 8-ft-long strips were selected from each “class” 

(a total of 260 hardboard siding strips and 20 OSB siding 

strips) to provide specimens for laboratory testing and for 

outdoor exposure. Laboratory testing included substrate 

weatherability (residual thickness swell), edge water absorption, and water vapor sorption. The cutting diagram for 

specimens obtained from the selected strips was presented 

as figure 3 in Carll and TenWolde (2004). Three specimens 

for exterior exposure were obtained from each strip, two for 

accelerated exposure on spray fences and one for long-term 

exposure on a test building.

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 noncommercial hardboards, which were 

not factory primed, the latex paint was applied over brushapplied alkyd primer. 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. The 

OSB siding had a resin-impregnated paper face, intended to 

serve as a topcoat base and was painted with two coats of 

the 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 hardboard and 267 ft2/gal (6.6 m2/L) on 

the OSB. The solids content of the satin exterior paint was 

approximately 37% by volume. At the combined spread rate 
Durability of Hardboard Lap Siding—Performance in Laboratory Testing and in Long-Term Exterior Exposure

3

of 387 ft2/gal, dry thickness of the coating on the hardboard 

specimens would have been approximately 1.5 mils 

(0.0015 in., 0.0381 mm) (ITW Resin Technologies [no 

date]). By applying two thin coats of topcoat rather than 

one heavy coat, uniform and complete coverage was assured; the risk of depositing irregular globs or fillets of paint 

around specimen edges was avoided. The application of two 

thin coats, by virtually eliminating paint drips, also provided 

greater confidence in recorded paint application weights, 

but resulted in topcoat dry film thickness (1.5 mils) being 

considerably less than the industry-recommended value of 

approximately 3 mils, which would have been attained with 

two coats, each of approximately 1.5 mils dry film thickness, each applied at the paint manufacturer’s recommended 

spread rate of around 400 ft2/gal. All brush painting was 

performed in a conditioned laboratory. The drip edges and 

faces of specimens were painted, whereas specimen end cuts 

were left bare.

Test Building Exposure

Building and Exposure Characteristics

The test building, located at FPL’s Valley View test site, was 

not conditioned (it was not heated or humidified in winter, 

nor was it air-conditioned in summer). The building was 

48 ft (14.6 m) long and 8 ft (2.4 m) wide, with the long dimension in an east–west direction. Siding specimens were 

installed on the south wall of the building as lap siding 

courses in 28 columns of 10 boards per column. The exposure of each course was 6.625 in. (0.17 m) (as had been the 

case on the spray fences). The lowest course of siding was 

installed over a 3.375-in- (0.086-m-) wide strip of wood–

plastic composite lumber that had been planed to 0.375-in. 

(9.52-mm) thickness (the same thickness as some commercial hardboard siding). The lowest siding course overlapped 

this strip by approximately 1 in. (25 mm). The means of 

mounting specimens on the building was essentially the 

same as that used to mount specimens on the spray fences, 

schematically depicted in figure 4 of Carll and TenWolde 

(2004); the wall framing was sheathed with 3/4-in. (19-mm) 

plywood, to which plywood nailing strips and treated lumber column separators were attached. The spaces between 

nailing strips and column separators were then filled with 

extruded polystyrene foam. The mounting means, which 

involved two layers of 3/4-in. (19-mm) plywood provided 

for ample embedment of siding nails. (Nailing strips were 

secured to the underlying layer of plywood with flat-head 

screws, with the heads flush with the surface of the nailing 

strip.) The foam between the nailing strips and column separators meant that the siding was not installed as a rainscreen; 

there was no continuous vertical air gap behind the siding 

because the top edge of each course of siding was in contact 

with foam infill or a plywood nailing strip. The building 

had been erected in the late 1970s; its existing length and 

the need to accommodate 28 columns of specimens limited 

specimen length to 17.75 in. (0.45 m). Each specimen was 

secured with two hot-dip galvanized nails, each placed 

3/4 in. (19 mm) up from the drip edge. Given the exposure 

of each siding course, the nails penetrated the upper edge 

of the specimen one course below. The nails were laterally 

spaced at 12.5 in. (0.32 m).

Joints between the (unpainted) siding ends and the column 

separators were open (uncaulked), as had been the case on 

the spray fences. The gaps between specimen ends and column separators were roughly 0.25 in. (6 mm) wide. Open 

joints are contrary to industry installation instructions, as 

they allow water entry at board ends. Caulking the joints 

would have introduced a confounding variable. Caulked 

joints are subject to failure, particularly with lap siding, 

where depth of end joints inherently changes. Caulk joint 

failure is furthermore not predictable; in a study conducted 

in Florida (Carll and others 2000) some caulk joints allowed 

water entry while appearing intact. Had the joints been 

caulked, some of the joints would have failed, and failures 

would have occurred to varying degrees. The open joints 

in this study extended down past the bottom siding course, 

unlike an open joint or a failed caulk joint between siding 

and a window jamb casing. The extension of the open joints 

past the bottom siding course likely provided considerable 

drainage potential. The substantial width of open end gaps, 

coupled with the siding being true lap siding (as opposed to 

shiplap siding) and with the specimens being short, provided 

the opportunity for back face venting (from end to end of 

specimens). 

Finally, the amount of water deposited as wind-driven rain 

into each open end joint was estimated as being roughly 

15 L over the entire 155-month exposure period. The area 

of an open end joint was approximately 17 in2 (0.011 m2). 

A generous estimate of cumulative wind-driven rain intensity on the wall over the exposure period was 1,360 L/m2

(see section in this paper concerning climatic conditions). 

This is a modest amount, particularly in contrast with water 

amounts that are potentially present at the heads or sills of 

fenestration units (windows and doors). The watershed area 

of a fenestration unit can exceed a couple square meters, 

and that of a building wall above a fenestration unit can be 

even larger. The amounts of water that would be deposited 

in these watershed areas during wind-driven rains could thus 

be substantial. Some significant portion of the wind-driven 

rain deposited in the watershed area above a window or 

door unit would be expected to run down the wall surface to 

the joint at the head of the unit. Likewise, some significant 

portion of the wind-driven rain deposited against a window 

would be expected to reach the horizontal joints at the sill of 

the window. The amounts potentially delivered to horizontal 

joints at window heads and sills by cascade wetting are dramatically higher than the estimated amount of water potentially deposited by wind-driven rain into tall narrow joints, 

such as the open end joints in this study. 

In summary, the installation was expected to stress the 

(unpainted) ends of specimens and to result in some water 

entry behind the siding at specimen ends, but to allow each 
Research Paper FPL–RP–674

4

of these to occur in a largely consistent manner. Moreover, 

the amount of water deposited into the open end joints was 

calculated as being modest.

The test building had a shed roof that sloped to the south but 

had a 13-in. (0.33-m) overhang at the eave. The building’s 

narrow north–south dimension resulted in a limited roof 

watershed area per unit length of eave. The roof was not 

guttered. Water from the eave fell on grass-covered soil, and 

the lower edge of the bottom course of siding was approximately 23 in. above ground level, resulting in limited potential for splash wetting. During a heavy rainstorm, with wind 

from the east, a wetted area that extended approximately 

21 in. above ground level (about 2 in. below the drip edge of 

the lowest siding course) was observed. Because of the wind 

direction and puddling of water at the roof-edge drip-line, 

the wetted area was assumed to have resulted primarily from 

splash wetting. The observation was made in late winter, 

when the ground was saturated, and may have been partially 

frozen. The degree of puddling (a factor in splash wetting) 

was considerable, and was thus assumed to be greater than 

during most rainstorms. This observation largely confirmed 

that there was not great potential for splash wetting. The potential exposure of the building wall to wind-driven rain was 

influenced by wind direction during rainstorms, the building’s surroundings, and its orientation relative to those surroundings. The building was located on a southwest-facing 

hillside, but there was another long single-story test building 

located 60 ft directly south of the building. The building was 

supported by posts; there was an open airspace below the 

building’s wood-frame floor system.

Wall height (up to the soffit of the roof overhang) was 8 ft. 

The 10 courses of siding had a vertical combined exposure 

of 66.25 in. There were roughly 2 in. between the drip edge 

of the lowest siding course and the bottom of the 8-ft wall. 

There was thus approximately 28 in. of wall surface between the upper exposed surface of the top course of siding 

and the soffit of the roof overhang. This space was filled 

with an additional course of hardboard lap siding and, above 

that, plywood installed on the column separators (which 

served as furring strips for the plywood). During a rain 

event with a horizontal intensity of 0.2 in. per hour, with 

concurrent wind at 7 mph directly from the south, and assuming that the building provided no aerodynamic obstruction to wind, the uppermost course of siding would have 

been partially within a sheltered zone provided by the roof 

overhang.

These values for rainfall intensity (0.2 in h–1) and wind 

speed (7 mph) were chosen on the basis of an analysis of 

data collected at the nearest National Oceanic and Atmospheric Administration (NOAA) weather station over the 

period of exposure. They were common values during winddriven rains where horizontal rainfall intensity was substantially, but not dramatically, higher than average. 

The assumption that the building provided no aerodynamic 

obstruction to wind is made solely for reasons of 

simplification and is known to be incorrect. Under this assumption, the sheltering effect of the overhang is minimized 

(actual sheltering will be greater than assumed).

Nails for securing siding specimens to the building were 

manually driven. A few mis-strikes occurred during installation (hammer glancing off nail head). None of the nails were 

noticeably overdriven. Cracking of the paint film at some 

nail heads was nonetheless observed in the fall of 1997 

(shortly after installation) without the boards having undergone perceptible swelling. In late spring of 1998, the areas 

around all nail heads on the building, including areas where 

mis-strikes had occurred, were touch-painted with the same 

satin latex paint used to paint the specimens in the laboratory. Any paint cracking that occurred after the touch-up 

painting could thus have been attributed to behavior of the 

siding, rather than to installation issues. For the remainder 

of the exposure period (until removal in August 2010), no 

further paint cracking was observed at nail heads.

Specimen Placement

As indicated previously, three specimens for exterior exposure were obtained from each 8-ft strip of siding selected to 

provide test material. As cut, each of the three specimens 

was labeled to identify the strip from which it was cut and 

its position (left, middle, or right) in the strip. Assignment 

of left, middle, and right specimens to exposure (spray fence 

1, spray fence 2, or building) was random, and varied from 

strip to strip. Twenty specimens of each of the 14 board 

classes (13 classes of hardboard siding and one OSB siding) 

were installed on the building (a total of 280 specimens). As 

indicated previously, the specimens were installed in 28 columns, 10 specimens in each column. Two specimens of each 

board class were assigned to each of the 10 courses, with 

column locations within each course assigned randomly. 

Assignment of two specimens from each siding class to each 

course compensated for potential variation in water exposure from course to course. As discussed in the Discussion 

and Analysis section of this paper, water exposure evidently 

was course-dependent.

Climatic Conditions—Rainfall and Decay Hazard Index

Specimens were installed on the building in September 1997 

and were removed from the building in August 2010. Climatic conditions for each of the full years during the exposure period, at the nearest NOAA station to the Valley View 

site, are presented in Table 1. The weather station was located approximately 12 miles to the northeast at Dane County 

Regional Airport (weather station code KMSN). Precipitation is recorded in the data sets maintained by the National 

Climatic Data Center (NCDC) as water equivalent, whether 

the precipitation falls as rain, snow, or other varieties of 

freezing or frozen precipitation. Estimated rainfall was determined from monthly recorded values for precipitation 

and monthly recorded depth of combined fall of snow, 
Durability of Hardboard Lap Siding—Performance in Laboratory Testing and in Long-Term Exterior Exposure

5

ice pellets, and hail, using the calculation method used by 

Cornick and Lacasse (2009) for climate normal data. The 

rainfall and decay hazard index values varied considerably 

from year to year but were nonetheless moderate over the 

exposure period. During all years, decay hazard (Scheffer) 

index value was between 35 and 65, a range recognized as 

representing a moderate hazard for decay. The mean decay 

hazard index value was noticeably higher than calculated by 

Scheffer (1971) for Madison, and slightly above the index 

value calculated from climate normal data for the period 

1971–2000 (Carll 2009). The higher mean Scheffer Index 

value over the 12 years (1998–2009) than for earlier periods concurs with increases in the Index value over recent 

decades in the coldest regions of the eastern United States 

(Lebow and Carll 2010) and in Canada (Morris and Wang 

2008). For comparative purposes, rainfall and Scheffer index values for one location in each state east of the Mississippi and one location in each the states immediately west of 

the Mississippi over the period 1990–2009 are provided in 

the Appendix. A comparison of values in Table 1 with those 

in the Appendix indicates that average annual rainfall at the 

test site over the exposure period was lower than at most 

locations in the eastern half of the conterminous United 

States over the period 1990–2009. The exposure could thus 

be characterized as less challenging than average for the 

eastern states.

Climatic Conditions—Wind-Driven Rain

An estimate of cumulative wind-driven rain exposure on the 

south wall of the building over the period October 1, 1997, 

to August 15, 2010, was calculated from hourly data recorded at the KMSN weather station. Hourly values for winddriven rain were calculated and the hourly values summed. 

Calculation of hourly wind-driven rain was by the equation 

in chapter 12 of Straube and Burnett (2005):

where

 WDR is wind-driven rain (L/m2h)

 RDF rain deposition factor 

 (dimensionless)

 DRF driving rain factor (s/m)

 V(1.75) wind speed at 1.75-m height 

 (m/s)

 rh rainfall intensity (mm/h)

 Ѳ	 angle of wind relative to south 

 (or an azimuth of 180°)

The calculation was thus for wind-driven rain intensity 

at 1.75-m height, which was substantially above average 

height of the test specimens on the building but below the 

height of the uppermost specimen in each column. RDF was 

set to a value of 0.8, which is substantially higher than average for walls in low-rise buildings. DRF is inversely related 

to terminal (vertical fall) velocity of raindrops and is thus 

inversely related to rainfall intensity. DFR was calculated 

using the following equation from Straube and Burnett:

Wind speed at 1.75 m was calculated as

where V(10) is wind speed at 10 m (standard height at which 

wind records are collected) and α = 0.14 (the value for open 

terrain, Straube and Burnett, chapter 3, 2005).

Hourly rainfall intensity values were obtained from hourly 

precipitation data by reference to weather observation codes 

in the NCDC data sets, using a simplified version of the 

adjustment method outlined by Cornick and Lacasse (2009) 

Table 1. Estimated rainfall and decay hazard index values at Madison for each of 

the full years during building exposure 

Year 

NOAA-recorded annual 

precipitation 

(in. (mm) water equivalent) 

Estimated annual rainfall 

(in. (mm)) 

Decay hazard 

(Scheffer) 

index 

1998 40 (1010) 36 (920) 47 

1999 32 (810) 29 (730) 60 

2000 40 (1020) 36 (900) 42 

2001 38 (980) 37 (940) 57 

2002 26 (670) 23 (580) 36 

2003 32 (800) 29 (740) 40 

2004 39 (1000) 37 (930) 53 

2005 25 (630) 19 (480) 41 

2006 37 (930) 33 (850) 47 

2007 44 (1130) 38 (950) 43 

2008 44 (1120) 34 (860) 40 

2009 38 (970) 34 (850) 44 

12-y summary mean = 36 (920) 

COV = 18% 

mean = 32 (810) 

COV = 19% 

mean = 46 

COV = 16% 

WDR = RDF × DRF × V(1.75) × rh × cosѲ

DRF = 0.22(rh)–0.123

( ) V(1.75) = × V(10) 

1.75

10

a
Research Paper FPL–RP–674

6

for hourly data. Weather observation codes included in the 

hourly datasets were inputs for the adjustment method. The 

primary hourly weather observation codes that were pertinent were RA (rain), DZ (drizzle), and TS (thunderstorm). 

Eight other observation codes, which indicated various 

forms of freezing or frozen precipitation, were also relevant 

if they were present during the same hour as any of the three 

primary codes.

Cumulative calculated WDR was approximately 

1,360 L/m2 over the period October 1, 1997, through August 15, 2010. The cumulative wetting to which specimens 

on the spray fences had been exposed (from August 1997 

through early October 2000) had been substantially greater. 

Wetting on the spray fences had two components: winddriven rain exposure and spraying. Spraying had been by 

far the larger component. On spray fences, each of the 

28 columns of siding had been wetted by a spray nozzle that 

delivered water at a rate of 15 mL/s. Spraying had occurred 

for 1 h each day during warm months. Each spray event had 

thus delivered 54 L of water to a column of siding. There 

had been approximately 580 spray events on each of the test 

fences, which amounted to a cumulative spray exposure 

per column of roughly 31,000 L. Surface area of the 

10 siding courses on each column on the spray fences 

was approximately 0.96 m2. Cumulative area-normalized 

spray exposure on the fences had thus been roughly 

32,600 L/m2. By comparison, calculated wind-driven 

rain exposure (at 1.75-m height) had been approximately 

360 L/m2 over the fence exposure period (approximately 

August 1, 1997, through October 12, 2000). Water 

exposure during spray fence exposure had thus clearly 

been dominated by spray wetting. Water exposure in spray 

fence exposure had been well in excess of 20 times the 

cumulative calculated WDR exposure on the building 

(32,960 L/m2 versus 1,360 L/m2) even though spray fence 

exposure was of shorter duration. Moreover, note that the 

calculated WDR values were generous estimates as they 

were calculated for 1.75-m height, and assumed a rain 

deposition factor of 0.8.

The data used to calculate WDR over the period October 1, 

1997, through August 15, 2010, were further evaluated to 

identify the number of hours over the period during which 

there was wind-driven rain with some southerly component. 

A histogram showing the frequency (number of hours) of 

WDR at various levels of WDR intensity is presented as 

Figure 1. The figure shows that for most of the hours over 

the exposure period during which there was WDR, the 

WDR intensity was modest. The total number of hours 

during which there was wind-driven rain with a southerly 

component (total of all frequencies in the histogram) was 

approximately 1,800. By comparison, the total number of 

hours during which there had been wind-driven rain with 

some southerly component over the period August 1, 1997, 

through October 12, 2000, was 440. If these 440 h are assumed to have been in addition to the approximately 

580 h of spray wetting, the total number of hours of wetting 

on the spray fences would have been 1,020. Therefore, the 

specimens on the building were evidently subjected to more 

hours of wetting than the specimens on the spray fences had 

been (1,800 h versus 1,020 h), in keeping with the longer 

exposure period, even though the amount of water to which 

they were exposed over the longer period was dramatically 

less.

Inspections

Inspection of specimens on the building were made in 

November 1998, October 1999, May 2000, October 2000, 

September 2002, September 2003, November 2004, 

September 2006, and September 2007. After removal of 

specimens from the building in August 2010, they were 

inspected inside a laboratory. Inspection was always made 

by the same individual. Each board was inspected for discoloration of the painted face and of the painted drip edge 

by mildew, for evidence of decay, for condition of the paint 

on the face and on the drip edge, and for 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). Some degree of fiber raising occurs 

in welted areas. Figure 2 shows edge welting as observed 

in two adjacent welt-prone specimens part way through the 

exposure period.

At the end of the exposure period, specimens were removed 

from the building by cutting a plug around each nail with a 

hole saw. This allowed removal of specimens without 

subjecting them to prying or gouging. During the final 

Figure 1. Hourly wind-driven rain (WDR) intensity 

values for weather station KMSN Madison, Wisconsin, 

for the period October 1, 1997, to August 15, 2010, 

against a south-facing wall at 1.75-m height.
Durability of Hardboard Lap Siding—Performance in Laboratory Testing and in Long-Term Exterior Exposure

7

inspection, severity and extent of edge welting at board ends 

was recorded. During field inspections, if edge welting had 

been present at board ends, its presence had been recorded, 

but no attempt had been made to quantify its severity or 

extent.

After the final inspection was complete, the extent of water 

stains on the back surfaces of boards was measured. Water 

stains were not a failure mode, but their extents and patterns 

can be instructive in helping explain how failure modes 

were likely to have begun. Measurement of the extent of 

water stains was performed in two phases. The first phase 

involved a set of measurements taken at the same locations 

where measurements had been made on specimens removed 

from spray fence exposure. Locations of these measurements were as shown in figure 14 of Carll and TenWolde 

(2004). This set of measurements included the distance 

by which staining extended upward from the drip edge at 

midspecimen length, and the distance by which staining extended inward from the ends of the specimen at four different locations, all of them above the lap area. Staining in the 

lap area at midspecimen length (Fig. 3) was only observed 

occasionally, with the extent of this staining varying considerably in the limited number of specimens where it was 

observed. In contrast, most specimens showed some staining 

extending inward from specimen ends above the lap area, 

although in some board classes, the extent of this staining 

was modest and its intensity faint (Fig. 4). Staining was of 

modest extent and faint intensity in classes 7, 11, and 13. 

Boards in classes 7 and 13 had back surfaces of dark color; 

this made perception of staining difficult, although identification of the stain extent generally was possible. Figures 

3–5 are presented for illustrative purposes and thus do not 

Figure 2. Two adjacent specimens in a column, each 

showing edge welting. This photo was taken after roughly 

7 years of exposure. Welting is predominantly at midlength of specimen (between the nails). Note lack of 

perceptible swelling (or welting) at the nails. The points 

at which on-fence thickness was measured were slightly 

outboard of (toward specimens ends from) the nails.

Figure 3. Rear surfaces of five specimens with staining at the drip edge at mid-specimen length. All five 

specimens also have staining above the lap area extending inward from the board ends, which is more 

extensive. The distance by which staining extends 

upward from the drip edge at mid-specimens length 

is modest in specimens C3/58-5/2e/R, C5/1/1T/L, 

and C14/A/1/M. Intensity of staining at the drip edge 

is modest (faint) in specimens C3/58-5/2e/R and 

C5/1/1T/L, but is perceptible. In all these specimens, 

staining at the drip edge is continuous between 

at least one end of the specimen and specimen 

mid-length.
Research Paper FPL–RP–674

8

include specimens from board classes with dark-colored 

back surfaces. 

Staining in the lap area that was restricted to board ends was 

also sometimes observed (Fig. 5). The second evaluation 

phase involved documentation of sideways extent of backsurface stains from board ends, in the lap area; measurements of stain extent were made at the drip edge. 

Measurements made in the second evaluation, when compared with measurements made in the first phase, allowed a 

comparison of sideways extent of staining from board ends 

above the lap area with sideways extent of staining from 

board ends within the lap area.

Laboratory Tests

Moisture-Related Tests

As indicated previously, laboratory tests included weatherability of substrate, edge water absorption, and water vapor 

sorption. Description of the test procedures, and the test results, were previously reported (Carll and TenWolde 2004). 

Results of substrate weatherability testing are reiterated in 

Tables 2 and 3.

Figure 4. Rear surfaces of five specimens with 

staining wholly or mostly restricted to areas above 

the lap. In specimen C11/A/3/L, the extent of staining is modest, and the intensity so modest as to be 

barely perceptible. With the exception of specimen 

C14/C/1/L, these specimens had more extensive 

staining than average for their respective board 

classes.

Figure 5. Rear surfaces of four specimens with staining above the lap area, extending to varying degrees 

into the lap area, and as far down as the drip edge on at 

least one specimen end, but not extending to specimen 

mid-length at the drip edge.
Durability of Hardboard Lap Siding—Performance in Laboratory Testing and in Long-Term Exterior Exposure

9

At a criterion level of 17% RTS, differences between classes 

of commercial board were more apparent than at a criterion 

level of 20% RTS. At a criterion value of 20%, no specimens of class 7 or of classes 10–14 exceeded the criterion 

level. In contrast, at a criterion level of 17% at least one 

specimen in roughly half the classes of commercial board 

exceeded the criterion level. The proportion of test specimens in a class that exceeded a 17% RTS level was related 

to average RTS value for the class and variation in RTS 

value. The class 12 board showed the highest variation in 

RTS value. It had a higher proportion of specimens exceeding 17% RTS than did classes 7, 13, and 14, each of which 

had higher average RTS values. Class 12 boards had been 

mismanufactured; they contained press blows.2 As will be 

discussed later, class 12 boards performed in a different 

manner on the building than boards of the other classes. 

2Press blows are internal delaminations (voids) induced by internal 

steam pressure. They occur at press opening and are associated 

with higher than desirable mattress moisture content at press 

loading, a pressing cycle that does not adequately allow for steam 

dissipation, or a combination of these.

Significant proportions of specimens of class 8 board 

showed RTS in excess of either 20% or 17%.

Density Distribution Measurements

Density distribution through the board thickness was identified using a Quintek Measurement Systems QDP-01X 

Density Profiler (Knoxville, Tennessee) on specimens from 

all board classes except 9 and 12. Class 9 was excluded 

from density distribution evaluation on the grounds that it 

was OSB rather than hardboard. Class 12 was excluded on 

the grounds that, as stated previously, boards in this class 

contained press blows. The Profiler makes measurements 

on 2-in. (51-mm) square specimens. Six specimens of each 

class were evaluated. The specimens were cut from end portions of the (8-in. wide) strips that yielded outdoor exposure 

and laboratory test specimens; the specimens were cut from 

the inboard edges of the end portions. The material from 

which the specimens were cut had not been exposed to the 

weather, but instead had been maintained in long-term storage under controlled temperature and humidity.

Density was calculated by the instrument, based on attenuation of a collimated X-ray beam passing, in series, through 

the test specimen and a slit measuring approximately 

0.002 in. (0.05 mm) by 1.0 in. (25.4 mm). In the instrument, 

specimens are presented to the X-ray beam by a specimen 

holder. Position through the specimen thickness is determined by position of the specimen holder, which is in turn 

controlled by a stepper motor and its controller system. 

Scan values at steps near the board faces are potentially 

influenced by the X-ray beam passing in part through air 

adjacent to the specimen, and in part through the surface 

layer of the specimen. This is discussed in the user guide 

provided by the instrument manufacturer, largely in the context of “misaligned” specimens (specimens whose surfaces 

are not parallel). The specimens (which had limited length 

and width dimensions and which had been stored under controlled conditions) showed no perceptible taper or cup, and 

the effect of “misalignment” was thus not considered as being significant. Density readings taken at less than 0.004 in. 

from either specimen face were, however, ignored. Specimens of commercial board were stripped of primer paint. 

A first set of measurements had been made on specimens 

of commercial board as cut (with primer paint). It became 

evident from this set of measurements that the primer had 

a higher X-ray attenuation coefficient than the hardboard 

substrate, and that this interfered with obtaining accurate 

density distribution measurements in the substrate. Stripping 

of primer was performed in a series of steps with “no-wash” 

(volatile) liquid stripper, a freshly sharpened paint scraper, 

and 320-grit abrasive paper.

A summary of the density distribution measurements is presented in Table 4. Each “scan” in this process represented 

a prism of width equal to the slit (0.002 in.) For purposes 

of presentation, the individual density scans for a specimen 

Table 2. Residual thickness swelling of 

noncommercial hardboards in substrate 

weatherability testing 

Class 

Average 

RTSa

(%)

Standard

deviation

(%)

Specimens exceeding 

two RTS values (%) 

20%

RTS

17%

RTS

1 5.9 1.8 0 0 

2 6.7 1.3 0 0 

3 8.9 2.0 0 0 

4 12.0 2.5 0 9 

5 22.7 3.2 80 99 

6 30.1 5.6 99 99 

a

Average of 80 specimens. 

Table 3. Residual thickness swelling of 

commercial hardboards in substrate 

weatherability testing 

Class 

Average 

RTSa

(%)

Standard

deviation 

(%)

Specimens 

exceeding (%) 

20%

RTS

17%

RTS

7 11.5 1.2 0 0 

8b 15.3 3.4 14 23 

10 4.9 1.0 0 0 

11 7.5 2.9 0 1 

12 9.1 3.7 0 6 

13 12.4 1.4 0 0 

14 11.0 2.0 0 3 

a

Average of 80 specimens. 

b

Carll and TenWolde (2004) contains typographic errors in 

the average and standard deviation values for class 8 board 

in. The correct values are 15.3 and 3.4 (not 15.0 and 4.4).
Research Paper FPL–RP–674

10

(each for a prism as wide as the slit) were segregated into 

three “zones”: a face-layer zone (incorporating the outer 

quarter thickness of the specimen), a core-layer zone (incorporating all layers between quarter-thickness points), and a 

back-layer zone (incorporating the back quarter thickness of 

the specimen). The third column of Table 4 indicates that, 

with the exception of board classes 1, 5, and 11, average 

density in the face-layer zone exceeded that in the core-layer 

zone. For board class 1, average density within the facelayer zone was less than that in either core-layer or backlayer zones, while for classes 5 and 11 average densities in 

the face-layer and core-layer zones were essentially equal. 

The fourth column of Table 4 indicates that for commercial 

boards, with the exception of class 11, density in the facelayer zone exceeded that in the back-layer zone (column 4). 

The fifth column indicates that for commercial boards, again 

with the exception of class 11, the location of the highest individual density reading (scan) was in the face-layer 

zone. For noncommercial boards, density in the face-layer 

and back-layer zones tended to be more similar (column 4), 

and the zone in which the highest density scan was located, 

while more commonly in the back-layer zone than elsewhere, was somewhat variable (column 5). Values in column 6 of Table 4 indicate (where appropriate) within-zone 

variability in density. Values are listed in column 6 for those 

board classes where the location of the highest density scan 

was consistently in one zone. The values indicate considerable, but not extreme, density gradients within zone layers.

Class 10 boards had higher density in the face-layer zone 

than any other class (column 2), and consistently had higher 

density in the face-layer zone than in core-layer or backlayer zones (columns 3 and 4). On a proportional basis, uniformity of density in the face-layer zone was similar in 

class 10 boards to that in other classes of commercial 

board (Table 4, column 6). In absolute terms, however, 

class 10 boards had more density variation within the 

face-layer zone than other board classes. In boards of 

class 10, the highest density scan (which was within the 

face-layer zone) was consistently within 0.2 mm of the surface, whereas in some classes of commercial board (classes 

7 and 14) the highest density scan was typically 1 mm or 

more from the board’s outer surface.

For one specimen of each of commercial board classes 8, 

10, and 14, an imprecise gravimetric check of density distribution was also made. A stationary wide-belt sander was 

used to remove successive 0.003-in. (0.076-mm) passes 

from the surfaces of 8-ft strips of board. (The board strips 

were spares that had been stored under controlled conditions. In one case, thickness variation along the strip length 

precluded making measurements on the full length strip. 

In this case, the strip was crosscut into two 4-ft strips). The 

specimens were weighed between each successive pass. 

The gravimetric measurements were imprecise because of 

thickness variation, which was considerable in the (relatively) wide and long strips. The gravimetric measurements 

confirmed, however, that the densities of surface layers of 

these boards were higher than their respective average board 

densities.

Results

Two failure modes became evident over the exposure 

period, namely cracking of paint on drip edges and edge 

welting. With the exception of very limited mildew growth, 

other failure modes were not observed. In-service thickness, 

and the extent of back-surface water staining (measured 

after specimens were removed from the building) are also 

reported in this section of the paper. These moisture-related 

behaviors do not, of themselves, necessarily result in problems, although they may in some cases relate to the problematic behaviors (“failure modes”) observed.

In-Service Thickness

As can be seen in Figure 6, siding thickness, measured at 

the drip edge, did not progressively increase over the first 

Table 4. Summarized layer density measurements obtained by X-ray densitometry

Board class 

Average density 

in face-layer zone 

(kg/m3

)

Density ratios 

Layer zone 

containing

highest density 

scan for 

specimen 

Density ratio 

highest density 

scan to layer zone 

density

Face : core 

layer zones 

Face : back 

layer zones

1 726 0.86 0.92 variable 

2 824 1.04 1.00 back 1.12 

3 849 1.10 0.98 back 1.18 

4 815 1.10 1.01 back 1.21 

5 853 1.00 1.06 variable 

6 849 1.07 1.06 variable 

7 (com) 869 1.16 1.22 face 1.10 

8 (com) 970 1.16 1.06 face 1.06 

10 (com) 1009 1.09 1.08 face 1.10 

11 (com) 907 0.98 1.01 variable 

13 (com) 785 1.07 1.01 face 1.08 

14 (com) 809 1.10 1.11 face 1.08 
Durability of Hardboard Lap Siding—Performance in Laboratory Testing and in Long-Term Exterior Exposure

11

decade of exposure. Localized edge swelling often became 

visually apparent at the specimen ends. The swelling at 

specimen ends, being localized, did not affect measurements made at the drip edge roughly 2 in. (51 mm) from 

the specimen ends. As mentioned previously, after paint 

touch-ups in July 1998, cracking of paint at nail heads did 

not occur during the exposure period. At specimen removal 

in August 2010, there was no observable paint cracking or 

thickness swelling at nail heads. Cumulative wetting of the 

boards over the exposure period was not sufficient to cause 

measurable swelling near the nail heads (where thickness 

was measured). Figure 2 shows that swelling at the drip 

edge sometimes occurred. It did not, however, occur at the 

points where thickness was measured. On the spray fences, 

the same spatial pattern of drip edge swelling had occurred 

(least near the thickness measurement points) but measureable and progressive swelling had nonetheless been recorded (figure 10 of Carll and TenWolde 2004). The lack of measureable thickness swelling of specimens on the building (as 

opposed to measureable thickness swelling on the fences) is 

most likely attributable to less water exposure on the building than on the fences.

Staining of Back Surfaces

Measurements taken during the first evaluation phase are 

summarized in Table 5. As indicated previously, these 

included (a) lateral extent of stain from board ends, with 

measurements taken above the lap area, and (b) upward extent of stain from the drip edge, measured at midspecimen 

length. For purposes of comparison, mean lateral extent of 

stain from board ends as measured at the end of exposure on 

spray fences are included in the two rightmost columns of 

Table 5.

The average extent of back-surface staining in the area 

above the lap was, in most cases, less than had been observed on spray fences, although there were a few exceptions. Boards in classes 1, 2, and 13 on average showed 

slightly more staining above the lap area after 155 months 

of exposure on the house than they had after 39 months of 

exposure on the spray fences, while OSB siding showed 

similar extents of above-lap staining in the two exposures. 

Boards in all the other classes showed lesser extents of 

above-lap staining than they had in spray fence exposure; 

classes 8 and 14 showed dramatically less above-lap staining than they had in spray fence exposure. Similarity of 

mean and median values for lateral extension of stains from 

board ends in areas above the lap (Table 5) indicate that the 

data distributions for this parameter are not significantly 

skewed. A difference between the exposures (spray fences 

versus building) was more apparent in the degree of staining 

within the lap area. Whereas roughly 40% of the hardboard 

specimens removed from the spray fences had shown some 

staining extending upward from the drip edge at mid-length 

of the specimen, roughly 7% of the hardboard specimens 

(only 18 of the 260) removed from the test building showed 

such staining.

Measurements taken during the second evaluation phase are 

summarized in Table 6. Median values for lateral extent of 

staining from board ends were much lower at the drip edge 

than they were above the lap area (compare median values 

in Tables 5 and 6). The same was generally true, although 

to a less dramatic degree, for mean values. With the exception of the 18 specimens where staining at the drip edge was 

present at mid-specimen length, the lateral extent of staining 

from board ends at the drip edge was modest. Median values 

in Table 6 are, without exception, lower than mean values, 

indicating skewed data distributions. The median values also 

indicate that the majority of specimens showed no inward 

extent of staining from board ends at the drip edge.

Board classes 8 and 14 generally showed the most extensive 

back face staining, although there were as many (or more) 

specimens of board class 5 and class 6 with staining at the 

drip edge at mid-specimen length as there were for any other class (Table 5). Also, the mean inward extent of staining 

within the lap area was higher for classes 5 and 6 than for 

any of the other classes (Table 6). The maximum extent of 

staining above the lap area (observed in any one specimen) 

was highest for two specimens of class 8 board, and next 

highest for a specimen of class 14 board. Above the lap area, 

mean and median values for inward extent of staining from 

board ends were also higher for classes 8 and 14 than for 

classes 5 or 6. In spray fence exposure, board classes 

8 and 14 had likewise shown more extensive inward staining (above the lap area) than any of the other board classes. 

Board classes 5, 6, 8, and 14 had shown the highest values 

of edge water absorption in laboratory testing (Carll and 

TenWolde 2004), with class 14 showing by far the most 

0.45

0.455

0.46

0.465

0.47

0.475

07/24/98 04/19/01 01/14/04 10/10/06 07/06/09

Average thickness (in.)

Hardboard

OSB

Figure 6. Siding thickness, measured in-place at the drip 

edge, on seven dates between October 1999 and September 2007. Neither the hardboard sidings nor the OSB 

siding underwent progressive thickness swelling, despite 

nearness of open end joints to the points at which thickness was measured.
Research Paper FPL–RP–674

12

water absorption. All of these board classes had shown higher than average levels of RTS in laboratory testing, although 

class 14 had shown significantly less RTS than classes 5, 6, 

or 8 (Tables 2 and 3).

In summary, back-surface staining of specimens removed 

from the house was, with few exceptions, considerably less 

than had been observed on specimens removed from the 

spray fences. The generally more prevalent and extensive 

back-surface staining on boards that had been exposed on 

spray fences was in concurrence with the relatively high 

cumulative area-normalized water exposure values on the 

fences, which as stated previously had been over 20 times 

as great as on the building. The relative amounts of staining between different board classes were, for the most part, 

similar to what had been observed on the spray fences.

As indicated previously, only 18 of the specimens (slightly 

less than 7%) showed staining upward from the drip edge at 

specimen mid-length. In all except one of the 18 specimens, 

the water stained area was continuous with at least one end 

of the specimen, indicating that the water could have entered from specimen end(s), rather than by capillary rise in 

the lap. The single specimen where the stained area at midspecimen length did not appear to connect with specimen 

ends was a board of class 8. There was, with the exception 

of this specimen, lack of evidence of capillary water rise at 

hardboard siding laps. In addition, the extent of water-staining in lap areas was typically much less than in areas just 

above the lap (Figures 3–5 and Tables 5 and 6). Water that 

entered at specimen ends and was thus present at the upper 

edge of the lap near the board ends, was not pulled into the 

lap by capillary suction. The few exceptions were a limited 

number of specimens of classes 5, 6, 8 and 14. The typical 

lack of evidence for capillary wetting concurs with what had 

previously been observed on specimens removed from the 

Table 5. Parameters relating to water staining on backs of siding boards 

(evaluation phase 1) 

Class 

Inward staining from ends 

(measured above the lap area) 

(in.) 

Upward staining 

from drip edge 

(at mid-specimen length) 

Mean inward staining 

 (measured above the lap) 

in fence exposure (in.)a

Mean Medianb Max.b

Number

specimens 

showing stain

Maximum 

upward extent 

(in.) 

Fence 1 

drip edges 

not painted 

Fence 2 

drip edges 

painted 

1 1.84 1.63 3.56 0 0.93 0.92 

2 0.85 0.84 1.31 0 c 0.45 0.46 

3 1.24 0.98 5.25 1 0.125 1.31 1.26 

4 1.07 0.83 3.88 0 c

 1.03 1.14 

5 1.06 0.98 1.81 5 c

 0.375 1.78 1.96 

6 1.03 1.05 2.41 4 c

 0.25 1.47 1.87 

7 0.46 0.50 0.71 0 c

 0.90 0.90 

8 4.74 4.44 8.88 4 4.00 10.76 9.57 

10 0.80 0.77 1.22 0 1.90 1.69 

11 0.25 0.25 0.47 0 0.41 0.70 

12 1.56 1.56 2.06 1 0.75 2.41 2.68 

13 0.98 0.92 1.91 1d

 0.375 0.78 0.70 

14 1.96 1.55 6.50 2 2.00 8.20 8.12 

OSB 0.55 0.53 1.09 0 0.45 0.40 

a

From table 5 of Carll and TenWolde (2004). 

b

Median and maximum values are for specimen average values rather than for individual measurements (four 

measurements per specimen).

c

One specimen (not counted) with faint tinting close to the drip edge that could not be conclusively identified as water 

staining. 

d

Three specimens (not counted) with faint tinting close to the drip edge that could not be conclusively identified as water 

staining. 

Table 6. Summary of staining extent 

laterally from specimen ends, at 

specimen drip edges 

Board

class 

Inward extent of back-surface stain 

 at drip edge (in.) 

Mean Median Maximuma

1 0.39 0 6 

2 0 0 0 

3 0.38 0 10.38 

4 0.23 0 1.75 

5 2.32 0 8.88 

6 2.66 0 8.88 

7 0.01 0 0.25 

8 1.61 0.13 12 

10 0.03 0 0.5 

11 0 0 0 

12 0.66 0 4.5 

13 1.00 0 10.5 

14 1.30 0 11.38 

a

Maximum extent for any single stain. If staining is 

not continuous along the entire specimen length, and 

is predominantly from one end of the specimen, the 

extent can exceed half of specimen length. 
Durability of Hardboard Lap Siding—Performance in Laboratory Testing and in Long-Term Exterior Exposure

13

spray fences (Carll and TenWolde 2004), with investigations 

performed by Tsongas and others (1998, 2004), and with observations of back surfaces of hardboard lap siding removed 

from test buildings in Florida (Carll and others 2000).

Paint Performance

The satin paint did not perceptibly lose its sheen, nor did 

it chalk or erode to any obviously noticeable degree. Film 

thickness of the satin topcoat paint at the end of the exposure was measured using ImageJ software (National Institutes of Health [no date]) on micrographs. Film thickness 

was, on average 29 μm (1.2 mils) on board surfaces, and 

32 μm (1.3 mils) on drip edges, indicating that modest erosion of the paint film had occurred. With the exception of 

what occurred on drip edges, the paint did not crack, nor 

did it peel or flake. As discussed in detail under a separate 

heading, edge welting developed in somewhat over half of 

the hardboard specimens by the end of the exposure period. 

Paint remained adhered to welted areas on board faces, although rupture of the paint film adjacent to the welted area 

(at the intersection of the outer face and the drip edge) was 

common. Stated another way, paint performance on board 

faces was excellent, but paint cracking on drip edges was 

common, and was the most prevalent failure mode observed 

over the exposure period. Some degree of drip-edge paint 

cracking occurred in over half of the specimens of all board 

classes, except class 1, by the end of the exposure period. In 

class 12 boards, edge paint cracking commonly progressed 

to the degree that the paint flaked off the drip edge.

Localized drip-edge paint failures commonly occurred early 

during the exposure period at the ends of specimens where 

the drip edge intersected with the specimen end cuts. As 

indicated previously, the specimen end cuts were not painted 

and localized edge swelling occurred at end cuts. Where 

the end cut intersected the drip edge, the localized swelling 

resulted in rupture of the paint film (at the ends of the drip 

edge). The rate at which drip-edge paint cracks extended 

from the specimen ends progressed slowly. At the final 

inspection, at roughly 75% of specimen ends, extension of 

drip edge cracks from swollen ends did not exceed 15 mm. 

In specimens that developed prominent drip-edge paint 

cracking, the cracks typically developed in patches along the 

drip edge, often at some distance from the board ends. Until 

cracking progressed to advanced stages, the patches did not 

connect with cracks at the specimen ends. A summary of the 

extent to which drip-edge paint cracks extended from the 

swollen ends of boards at the final inspection is shown in 

Table 7. The linear extents were typically modest and thus 

most convenient to measure on a millimeter scale. In boards 

of classes 8 and 12, drip-edge cracking was typically continuous along the entire length of the drip edge at the final 

inspection. In all other classes of board, patches of drip-edge 

cracks, which were typically most prevalent near specimen 

mid-length, rarely or never connected with board ends at the 

final inspection.

During each of the in-field inspections, drip-edge paint 

cracks that were restricted to board ends were ignored. 

Table 8 summarizes observations made with regard dripedge paint cracking over the exposure period (observations 

that disregarded cracking if it was restricted to board ends). 

The table indicates a general increase in the prevalence of 

drip-edge paint cracking as the exposure period progressed. 

In the overwhelming majority of cases, the number of 

specimens with cracks became progressively larger as the 

exposure period progressed. There are a few cases where 

the recorded number of specimens with drip-edge cracks 

decreased between successive inspections. These cases reflected observation of a minor paint crack at an inspection, 

followed by failure to detect the crack at the next inspection. 

All except one specimen of class 12 board showed readily 

observable drip-edge paint cracking after 14 months of exposure (in November 1998), whereas in all other hardboard 

classes only two or fewer specimens showed drip-edge paint 

cracks at that time, which were typically minor and generally not detected at the next inspection. As indicated previously, boards of class 12 contained press blows. Drip-edge 

paint cracks developed at the press blows.

The prevalence and severity of drip-edge paint cracking after 155 months of exposure on the building generally corresponded with what was observed on one of the spray fences 

after 39 months exposure. Specimens on one of the spray 

fences had been installed with drip edges intact (not cut 

off). Values for prevalence and severity of drip-edge paint 

cracking on that fence at the end of exposure (39 months) 

are presented in Table 9. They are generally similar to the 

values in the right two columns of Table 8, although there 

are some moderate differences. Drip-edge paint cracking 

in noncommercial boards tended to be roughly as prevalent, but more severe in accelerated (spray fence) exposure 

Table 7. Extension of drip-edge paint 

cracking from unpainted specimen ends 

Board

class 

Extent of drip-edge cracks from 

swollen specimen ends (mm) 

Mean Median Maximum 

1 6 5 13 

2 6 5 14 

3 10 9 40 

4 11 10 42 

5 20 16 75 

6 26 22 55 

7 6 5 9 

8 Continuousa

 

10 8 8 10 

11 6 5 14 

12 Continuousa

 

13 6 5 15 

14 11 10 43 

a

Drip-edge cracks at specimens ends typically

(class 8) or always (class 12) connected with patches 

of drip edge cracks that extended along most of, or the 

entire, specimen length.
Research Paper FPL–RP–674

14

than in long-term exposure. This is probably because of the 

higher cumulative wetting of specimens on the spray fences, 

and that specimens in long-term exposure on the building, 

unlike specimens on the spray fences, had not undergone 

measurable thickness swelling. In contrast, drip-edge paint 

cracking in board classes 7 and 8 tended to be less prevalent and severe in accelerated exposure than in long-term 

exposure. This suggests that development of drip-edge paint 

cracking is to some degree time-dependent as well as dependent on the degree of in-service wetting, and that the relative 

influence of these two factors can vary between boards from 

different production plants. Time-dependent development of 

drip-edge cracks may be related to change in properties of 

the paint films over time.

Location of the most prominent drip-edge paint cracking, 

relative to the front or back of specimens, was identified 

for each specimen at the final inspection. A summary of the 

observations is presented in Table 10. The count values in 

this table, when taken by board class, summed across the 

columns do not always exactly match the count values in 

the column in Table 8 for August 2010. There thus were 

minor inconsistencies in the observations; the origin of the 

inconsistencies invariably involved small cracks that were 

not visible without a hand lens. Regardless of the minor inconsistencies, the values in Table 10 convincingly indicate 

that paint cracking on drip edges was more prevalent near 

the outer face of the specimen than near the back of the drip 

edge. Relatively rare were the cases where drip-edge cracking was as prevalent or more prevalent near the back of the 

drip edge as near the front of the drip edge. In these cases, 

water stains on the back surface of the board at the drip edge 

were present, and the stains were near the drip-edge cracks. 

These cases were in specimens of classes 5, 6, and 14. Water 

intrusion into lap areas was thus evidently occasionally associated with development of drip-edge paint cracking. In 

the majority of cases, however, development of drip-edge 

paint cracks was apparently associated with something other 

than water intrusion into laps.

Edge Welting

Edge welt was characterized by a visually estimated severity rating and an associated index rating. The severity rating 

was influenced by the degree to which the welt projected 

beyond the board plane, the abruptness with which it projected, and the surface roughness within the welted area. 

A rating of 10 indicated no perceptible welt. A rating of 

9.5 corresponded with welt that could be perceived when 

the surface was viewed obliquely, while a rating of 9 corresponded with welt perceptible when the surface was viewed 

Table 8. Prevalence of drip-edge paint cracking over exposure time, and average severity of drip-edge cracking at final 

inspection 

Board

class 

Number of specimens with drip edge paint cracks at various inspection datesa

Average severity 

11/1998 11/1999 5/2000 10/2000 9/2002 9/2003 11/2004 9/2006 9/2007 after 8/2010 

after 

8/10

1 0 0 1 0 1 0 0 1 3 2 very minor 

2 0 0 1 0 0 1 2 1 16 16 minor 

3 0 1 0 1 0 5 5 3 16 17 minor 

4 0 0 0 1 0 1 1 5 17 19 minor 

5 2 0 1 1 3 1 2 3 8 13 minor 

6 2 0 0 1 3 5 6 6 17 17 moderate 

7 1 0 0 0 2 7 4 7 18 20 moderate 

8 0 0 7 16 20 20 20 20 20 20 severe 

10 0 0 1 6 8 18 18 19 19 20 significant 

11 1 1 1 3 2 5 5 4 11 11 minor 

12 19 20 20 20 20 20 20 20 20 20 severe 

13 1 0 0 1 1 4 0 0 7 11 minor 

14 0 0 1 4 5 10 14 15 18 19 moderate 

OSB 4 1 1 3 6 12 11 11 17 18 moderate 

a

Specimens with drip-edge cracks only at specimen ends were not included in the count numbers. 

Table 9. Prevalence and severity of 

drip-edge paint cracking at 

termination of spray fence 

exposurea

Board

class 

Number of 

specimens with dripedge cracking 

Average 

cracking 

severity

1 11 minor 

2 5 minor 

3 18 moderate 

4 15 moderate 

5 17 significant 

6 19 significant 

7 5 minor 

8 16 significant 

10 18 significant 

11 10 minor 

12 20 significant 

13 14 minor 

14 16 moderate 

OSB 18 significant 

a

Inspection terminated October 2000. 
Durability of Hardboard Lap Siding—Performance in Laboratory Testing and in Long-Term Exterior Exposure

15

from a position normal to the board surface. Ratings lower 

than 9 indicated progressively more noticeable welt. As had 

been the case for specimens exposed on spray fences, an 

index value for edge welting was calculated for each specimen. The area of perceptible welt was combined with the 

severity rating into an index value as follows:

where area was in units of in2.

Edge welting on the specimen surface at the drip edge, typically near mid-length, developed to various degrees in all 

classes of hardboard siding, although very little welting was 

observed in boards of classes 1, 2, and 11. No perceptible 

edge welting occurred in any of the OSB siding specimens 

over the course of exposure. The textured surface of the 

OSB siding apparently obscured edge welting. Values 

presented in Table 11 indicate the degree to which edge welt 

developed in specimens as the exposure period progressed. 

In most cases, the number of specimens with perceptible 

edge welting progressively increased as the exposure period 

progressed. As was the case for development of drip-edge 

cracks, cases occurred where there were reversals in recorded number of specimens with perceptible welt between 

inspections. These invariably involved the perception of 

a welt on a specimen with a severity rating of 9.5 (visible 

only obliquely) at an inspection, followed by failure to perceive welting in the specimen at a subsequent inspection 

or at subsequent inspections. Light conditions influenced 

the degree to which a welt was perceptible. Although care 

was exercised to limit the degree to which light conditions 

varied during inspections, some variation inevitably occurred. At some inspections, moveable shading was used to 

control light conditions. This is described in greater detail 

on page 13 of Carll and TenWolde (2004). Recorded values 

were thus not always consistent, especially when welting 

was near the threshold of perceptibility. Inconsistency in 

the recorded prevalence of edge welting is most noticeable 

for class 13 boards, no specimens of which showed a welt 

severity rating worse than 9.5 until September 2006; class 

13 boards furthermore showed a low average value for welt 

index at the end of exposure. Despite imperfect consistency 

in the values found in Table 11, the values indicate an overall trend for edge welting to become progressively more 

prevalent with increased exposure time. A comparison of 

values in Tables 8 and 11 indicate that drip-edge paint cracking commonly preceded development of welt.

The values in Table 11 for average welt index are reported 

to three or four significant digits to show that the averages 

for classes 1, 2, and 11 were not zero. The values are, with 

only one exception, lower than previously reported (Carll 

and TenWolde 2004) for specimens exposed in accelerated 

exterior exposure (on spray fences) at the end of their 

Table 10. Location of drip edge paint cracks relative to 

specimen faces or backs. 

Board

class 

Prevalence of drip-edge paint cracking (no. of specimens)

Most prevalent 

near the 

outer face 

Equally prevalent 

near specimen 

back as near face 

Most prevalent

 near the 

specimen back 

1 1 0 1 

2 16 0 0 

3 16 1 0 

4 19 1 0 

5 7 3 7 

6 10 4 2 

7 19 0 0 

8 19 0 0 

10 20 0 0 

11 11 0 0 

12 20 0 0 

13 10 0 1 

14 16 1 2 

Observations made at final inspection. 

Table 11. Prevalence of drip-edge welting over exposure time, and average welt index rating at final inspection

Board

class 

Number of specimens with perceptible edge welt Average welt 

index values 

after 8/10 

At

11/98

At

11/99

At

5/00

At

10/00

At

9/02

At

9/03

At

11/04

At

9/06

At

9/07

After

8/10

1 0 0 0 1 0 0 0 3 2 1 0.001 

2 0 0 0 0 0 0 0 1 7 4 0.03 

3 0 0 0 0 0 0 1 8 11 13 0.50 

4 0 0 0 1 0 0 1 12 15 15 0.62 

5 0 0 0 2 2 1 3 4 10 9 0.44 

6 0 1 1 4 3 1 1 9 15 15 1.06 

7 0 0 0 0 1 0 3 9 16 19 0.88 

8 0 0 5 9 19 18 20 20 20 20 7.72 

10 0 0 0 0 0 2 4 14 17 19 0.50 

11 0 0 0 0 0 0 0 2 3 2 0.03 

12 0 0 0 0 0 0 0 10 15 17 0.97 

13 0 0 0 7 2 1 2 7 11 5 0.09 

14 0 0 0 0 0 0 0 7 8 9 0.16 

OSB NA NA NA NA NA NA NA NA NA NA NA 

Index = Area × (10 – severity)
Research Paper FPL–RP–674

16

exposure period. Without exception, the values are dramatically lower than those for specimens that had been exposed 

on a spray fence with their painted drip edges removed. 

Average class index values for spray fence specimens with 

painted drip edges removed ranged from 6.6 times higher to 

multiple orders of magnitude higher than the corresponding 

average class index values of specimens after building 

exposure (table 9 of Carll and TenWolde 2004). With one 

exception, the values are also lower than for specimens 

that had been exposed on a spray fence with their painted 

drip edges not removed (again, see table 9 of Carll and 

TenWolde 2004). The exception was class 10 board. Average welt severity and index values for class 10 boards at the 

end of fence exposure had been 9.7 and 0.2 respectively, 

better than the corresponding values (9.0 and 0.5) at the 

end of building exposure. Class 10 boards on the building 

showed prevalent drip-edge paint cracking in September 

2003, less than halfway through the 155-month exposure 

period. Welting evidently originated at the drip-edge cracks, 

becoming progressively worse from September 2003 onward. Edge welting in class 10 boards never extended far 

above the drip edge, but was nonetheless readily perceptible; it consistently had a severity rating of 9 at the end of 

building exposure.

Table 11 indicates substantial differences in welt performance between classes of boards. Board class 1 showed 

the best performance, followed by classes 11 and 2. Board 

class 8 showed by far the worst welting. Board classes 5, 6, 

and 14, which had shown poor (high) welt index values in 

fence exposure, showed reasonably good (low) welt index 

values in building exposure. These boards, however, showed 

prevalent drip-edge paint cracking at the end of exposure. 

Inasmuch as drip-edge paint cracking typically preceded the 

development of edge welting, it is reasonable to expect that 

welting near the drip edges of boards in classes 5, 6, and 14 

would have become progressively worse had their exposure 

continued.

Edge Welting at Specimen Ends

As indicated previously, localized swelling occurred at 

board ends. This often gave the ends a “broomed” appearance, which was most noticeable where board ends intersected with drip edges. Welted areas tended to eventually 

develop near the swollen ends, although welting at board 

ends was not noted prior to September 2006. As indicated 

previously, quantification of welting at board ends was not 

attempted until after the boards were removed from exposure. The orientation in which specimens were held during 

inspection influenced the degree to which welting at board 

ends was perceptible. During evaluation of board ends for 

welting, the specimens were thus rotated so that board ends 

were in the same orientation relative to the inspector as drip 

edges had been when the boards were evaluated for welting 

at the drip edge. Perceptible welting at board ends did not 

occur in OSB siding, even though localized swelling at ends 

of these boards was observable. Table 12 provides descriptors for welting at board ends, and for comparison, descriptors for welting at board drip edges.

Table 12 indicates that the upward extent of welting from 

drip edges generally exceeded sideways extent of welting from board ends, although for all board classes except 

class 8 it was always less than the lap dimension. For class 

8 boards, the upward extent (distance) of welting from drip 

edges was roughly three times greater than the sideways extent of welting from board ends. For all other board classes, 

differences between upward extent of welting from drip 

Table 12. Descriptors for welting at board ends and selected descriptors for welting at drip edges 

Board

class 

Edge welting at board ends Edge welting at drip edge 

Number of 

ends with 

welt

Sideways extent from 

end (in.) 

Welt 

 severity rating Mean index 

rating 

Upward extent from 

drip edge (in.) 

Welt 

severity rating 

Mean Mosta Mean Worstb Mean Mostc Mean Worstd

1 11 0.07 0.13 9.9 9 0.02 0.06 0.06 9.95 9 

2 23 0.09 0.13 9.7 9 0.09 0.19 0.50 9.9 9 

3 40 0.13 0.19 8.9 8 0.88 0.21 1.0 9.4 9 

4 40 0.13 0.25 9.0 8 0.87 0.17 0.38 9.3 9 

5 40 0.16 0.25 8.8 8 1.19 0.20 0.63 9.6 8 

6 40 0.17 0.38 8.7 8 1.33 0.20 0.38 9.3 8 

7 39 0.12 0.25 9.1 8 0.67 0.15 0.38 9.1 8.5 

8 40 0.29 0.50 8.6 7.5 2.73 0.83 1.38 9.1 8.5 

10 36 0.07 0.13 9.2 8.5 0.28 0.08 0.13 9.1 8.5 

11 15 0.07 0.13 9.7 9 0.06 0.09 0.13 9.9 9 

12 32 0.10 0.13 9.5 9 0.23 0.21 0.25 9.4 9 

13 37 0.10 0.25 9.2 8 0.54 0.10 0.13 9.8 9 

14 19 0.11 0.25 9.7 8.5 0.18 0.21 0.50 9.6 9 

a

 Greatest extent measured on any of 20 specimens (40 ends). b

 Lowest (worst) rating observed on any of 20 specimens (40 ends). c

 Greatest extent measured on any of 20 specimens (20 drip edges). b

 Lowest (worst) rating observed on any of 20 specimens (20 drip edges).
Durability of Hardboard Lap Siding—Performance in Laboratory Testing and in Long-Term Exterior Exposure

17

edges and sideways extent of welting from board ends were 

relatively minor, although upward extent of welting from 

drip edges nonetheless typically exceeded sideways extent 

from ends. In contrast to extent of welting, welt severity 

ratings tended to either be similar at board ends as at drip 

edges, or slightly higher at board ends. Regardless of the parameter chosen to express it (whether prevalence, measured 

extent, severity rating, or index value), welting at board 

ends was consistently worst in boards of class 8.

Decay and Mildew

There was no outward (macroscopically visible) indication 

of decay in any of the specimens. Mildew growth on painted 

surfaces was rarely noted, and where noted was confined 

to small patches, always close to the drip edge, usually in a 

welted area, or near drip-edge paint cracks, or both. Mildew 

growth had not been noted during any of the in-field inspections. At the final inspection, (performed in a humidity 

controlled laboratory) only seven specimens showed mildew 

patches. Mildew patches on these specimens were, as stated 

previously, small; the largest two mildew patches had an 

area of roughly 2.5 in2 (1.6 × 10–3 m2). Three of the seven 

boards with mildew patches, and the boards with the largest mildew patches, were boards of class 6. Limited patches 

of mildew, typically growing through the paint film, were 

present on the drip edges of a few specimens, always near 

drip-edge cracks (Figures 7a, b). These patches were easily 

visible with a 10 power hand lens, but were not readily perceptible without magnification.

The building wall, as indicated previously, faced south 

and was not shaded, so had ample sun exposure. Williams 

(2010) indicates that mildew growth on painted wood siding is most prevalent on walls that remain damp. Walls with 

direct solar exposure are expected to dry rapidly. Sherwood 

(1983) reported very low moisture contents at the sheathing/

siding interface in south-facing walls installed in this building during warm sunny weather. The spray fences had likewise faced south and were not shaded. On the spray fences 

where specimens had been installed with intact drip edges, 

specimens of OSB and of classes 10 through 14 had shown 

no mildew growth. For specimens of OSB and of classes 

10 through 14, there was essentially no difference with regard to mildew growth, between spray fence and building 

exposure. For the other board classes, which had shown 

some mildew growth in spray fence exposure, mildew 

growth was less prevalent on the building than it had 

been on spray fences.

To identify the presence of fungal colonization that was not 

visible from board faces, a sample of boards was selected 

for microscopic inspection of water-stained areas on their 

backs. Eight board specimens were selected for the sample. 

Specimens with the widest possible range of back-face 

staining were selected. Three of the eight specimens had 

extensive and intense back-face staining; these were specimens of board classes 8 and 14. Five of the eight specimens 

had distinctly more limited and less intense back-face staining than average. This sample of five specimens contained 

one class 2 board, one class 7 board, two class 11 boards, 

and one class 13 board. Observations were made using reflected light microscopy at low magnification, and transmitted light microscopy at various magnifications. Observation 

was made at the most intensely stained area on the back of 

each specimen. Significant colonization by mold/stain fungi 

was observed in the specimens of class 8 and class 14 board. 

Limited presence of hyphae of decay-type fungi was also 

observed in these specimens. In contrast, the specimens of 

board classes 2, 7, 11, and 13 showed limited presence of 

mold/stain fungi, and no hyphae of decay-type fungi. In 

Figure 7a. Magnified image of the end of the drip 

edge of a specimen of Class 1 board. Drip-edge paint 

cracking is prevalent near the end of the specimen, 

but does not extend more than 16 mm from the specimen end. Patches of mildew are present only near the 

inboard terminations of drip-edge cracks. This specimen was from a bottom course location, (where water 

exposure was greatest), and in the left-most column 

of specimens (at the southwest corner of the building). This specimen showed more drip-edge mildew 

than any other specimen. In this photograph the outer 

specimen face is at the top of the specimen.

Figure 7b. Magnified image of the drip edge of a specimen 

of class 4 board approximately at specimen mid-length. A 

patch of drip-edge cracks is present near the outer edge 

of the drip edge, and a small amount of mildew is present 

in the patch of drip-edge cracks. The specimen was from 

a bottom course location. In this photograph, the outer 

specimen face is at the bottom of the specimen.
Research Paper FPL–RP–674

18

summary, microscopic examination indicated that all boards 

had some degree of fungal colonization by the end of the exposure period, even if minor and essentially inconsequential. 

The examination also indicated that fungal colonization was 

more extensive, and potentially more serious, in specimens 

with more extensive and intense back-surface staining.

Discussion and Analysis

Spatial Variation in Back-Face Staining

Of the 18 boards with perceptible back-surface staining at 

mid-specimen length, 10 were in bottom-course locations. 

In all the other siding courses, either no, one, or two specimens had staining at mid-specimen length. The inward extent of back-surface staining from specimen ends also varied 

with the siding course, with more staining being present in 

lower siding courses. Some course-dependency of inward 

extent of staining at board ends (above the lap area) can be 

seen in Figure 8. In summary, staining on the back surfaces 

tended to be less extensive on boards in upper courses than 

on boards in lower courses, and most extensive on boards in 

the lowest course. The specimen placement scheme, which 

as mentioned previously assigned two specimens of each of 

the board classes to each siding course, accommodated for 

course-dependency of water exposure.

Notably higher water exposure at the lowest siding course 

might intuitively be explained by splash wetting. As mentioned previously, however, under a set of conditions that 

were believed to be particularly conducive for splash wetting, a wetted zone was observed that did not quite reach 

the lowest siding course. The observation suggested that 

splash wetting was not a major factor in wetting of the 

lowest siding course. Nevertheless, the possibility of some 

splash wetting over the 155-month exposure period cannot 

be dismissed. It is also possible that the open airspace below 

the building resulted in higher than anticipated wind-driven 

rain exposure at the lowest siding course. The open airspace 

resulted in the lowest course of siding being located, from 

the perspective of wind, near a building edge. Rose (2005, 

chapter 4, page 122) provides an explanation for concentrated deposition of wind-driven rain at building edges and 

corners. It is not, however, known if edge effect, associated 

with the airspace below the building, had a significant effect 

on wind-driven rain deposition at the lowest siding course.

Progressively greater water exposure on lower courses of 

siding than on upper courses of siding can be explained by 

“run-down” wetting (sometimes called “cascade” wetting) 

by water shed from areas higher on the wall. In addition, 

during a substantial number of the hours during which there 

was wind-driven rain with a southerly component, the overhang likely provided shelter preferentially to upper courses 

of siding.

Correlation of Failure Modes with 

Laboratory Test Results

As was the case for the portion of the study involving spray 

fence exposure (Carll and TenWolde 2004), we performed 

analyses of the relative frequency (or probability) of varying 

degrees of development of the two observed failure modes, 

within ranges of (laboratory-measured) RTS. The drip-edge 

paint crack rating and the welt index rating of each specimen at the end of its building exposure were each placed 

in a data set along with the average RTS value (determined 

previously) of two specimens, which had been next to the 

“building” specimen before specimens had been cut from 

the strip of material that provided specimens (see fig. 3 of 

Carll and TenWolde). The analytical methodology involved 

segregating the data set into “bins” based on average RTS 

value, and then developing relative frequency plots from the 

segregated data. An issue in the methodology was selection 

of bin widths (range of RTS values) such that there was adequate resolution (number of bins) while leaving a sufficient 

number of data points in the bins such that calculated frequencies for the bins were meaningful. For that reason, we 

selected a RTS bin range of 10%.

Because boards of class 12 had been mismanufactured, they 

were not included in these analyses. As mentioned previously, OSB had been included in the study as a reference 

material, and there was no supposition that test protocols in 

the American National Standard for hardboard siding might 

be predictive of field performance of OSB siding. OSB 

siding was therefore also excluded from these analyses. 

Table 13 shows the data segregation used to develop the 

frequency plots.

We found residual thickness swelling to be a reasonably 

good predictor of the likelihood of a board developing edge 

welting in service, both for the noncommercial siding 

(Fig. 9a) and commercial boards (Fig. 9b). We also found it 

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3 4 5 6 7 8 9 10

Inward extent of staining (in.) 

Row

Figure 8. Measurements of inward extents of water stains 

from board ends, in areas above the lap, by siding course 

(26 hardboard specimens per course).
Durability of Hardboard Lap Siding—Performance in Laboratory Testing and in Long-Term Exterior Exposure

19

to be a reasonably good predictor for the development of 

high levels of either drip-edge paint cracking or edge 

welting in service (Figs. 10a, b and 11a, b, respectively). 

These findings largely correspond with what was observed 

previously in spray fence exposure (Carll and TenWolde 

2004). Figures 9 and 11 indicate a continual improvement 

in welt performance as RTS value decreased. The figures 

indicate no value below 20% RTS, (or for that matter 17% 

RTS), beyond which further decreases in RTS value were 

not associated with higher relative frequency of improved 

welt performance. In a similar manner, Figure 10a indicates 

that below a bin center value of roughly 17% RTS, performance of noncommercial boards with respect to drip-edge 

paint cracking continually improved with reduction (lowering) of RTS value. In contrast, while Figure 10b indicates 

a trend of improved performance with respect to drip-edge 

paint cracking with improved (lowered) RTS value, it also 

indicates that the relative frequency of significant or severe 

drip-edge paint cracking was not lowered as bin center value 

fell below 9% RTS. Figure 10b also indicates that the relative frequency of intact or nearly intact drip edges did not 

increase as bin center value fell below 9% RTS.

As indicated previously, the 17% RTS criterion value promulgated in the 2006 revision of the American National 

Standard represented a reduction in the criterion value 

(the establishment of a more stringent requirement) than 

in previous versions of the standard. In spray fence exposure, there was no “plateau” value for RTS below which 

edge-welt performance or drip edge paint cracking did not 

continue to improve. In building exposure, a similar trend 

Table 13. Number of specimens in each 

RTS bin 

RTS bin Number of specimens per bin

Center 

 (% RTS) Range 

Noncommercial

boards

Commercial 

boards

5 0–10 61 47 

6 1–11 67 55 

7 2–12 74 77 

8 3–13 77 95 

9 4–14 78 96 

10 5–15 72 99 

11 6–16 59 90 

12 7–17 48 81 

13 8–18 38 81 

14 9–19 30 77 

15 10–20 21 69 

16 11–21 18 63 

17 12–22 13 41 

18 13–23 15 25 

19 14–24 19 19 

20 15–25 19 7 

21 16–26 22 6 

22 17–27 24 6 

23 18–28 27 4 

24 19–29 29 4 

25 20–30 28 4 

26 21–31 28 2 

27 22–32 27 1 

28 23–33 23 1 

29 24–34 19 0 

30 25–35 19 0 

31 26–36 16 0 

32 27–37 14 0 

33 28–38 10 0 

34 29–39 10 0 

35 30–40 9 0 

36 31–41 7 0 

37 32–42 6 0 

38 33–43 5 0 

39 34–44 4 0 

40 35–45 3 0 Figure 9b. Relative frequency in 155-month exposure of 

edge-welt development as a function of residual thickness swell (RTS) of matched specimens in laboratory 

testing; (a) noncommercial hardboard sidings, (b) commercial hardboard sidings.

a

b
Research Paper FPL–RP–674

20

recommendations; two related to statistical confidence in 

test results, while the third was that the pass-fail criterion be 

lowered (from 20% RTS) “to 17% or lower.” Of these recommendations, only the recommendation to lower the RTS 

criterion to 17% was adopted in the 2006 revision of the 

Standard. To summarize, in both spray fence and building 

exposures, with regard to the two common failure modes, 

there typically was continual improvement in performance 

as RTS value decreased. In the one case where continual 

improvement over the entire range of RTS values was not 

observed (performance with regard to drip-edge paint cracking of commercial boards in building exposure) there was 

improvement as RTS value decreased until it reached a value of well below the current criterion level in the Standard. 

Improved in-service performance would thus be expected if 

the criterion level in the Standard were to be lowered to 

a value well below its current (17%) level.

Edge Welting—Association with Drip-Edge 

Paint Cracking

As discussed in the Results section of this paper, drip-edge 

paint cracking typically preceded development of edge welting. As also indicated in the Results section, welt behavior 

was, without exception, substantially worse for specimens 

exposed (in the earlier part of this study) on spray fences 

with painted drip edges removed than it was in specimens 

exposed on spray fences with painted drip edges, or than it 

was (in this portion of the study) in building exposure. This 

suggests that maintenance of an intact paint film on drip 

edges is a requisite for avoiding edge-welt development in 

hardboard siding.

Class 10 board was anomalous relative to other board 

classes in two regards:

•	 It showed marginally poorer welt performance (see Results section) on the building than it had when installed 

on a spray fence with a painted drip edge, whereas all 

other board classes showed better welt performance on 

the building than on the spray fence.

•	 It showed the lowest average RTS values in substrate 

weatherability testing (Table 3), but at the end of building 

exposure showed, on average, a poorer welt index value 

than boards in classes 1, 2, 11, 13, or 14 (Table 8). This 

contrasts with the general trends indicated by Figures 9 

and 11.

Each of these anomalous behaviors can be explained in 

terms of drip-edge condition. When viewed from this perspective, the welt behavior of class 10 board relative to other board classes was reasonably similar across the exposures 

(accelerated or nonaccelerated). Class 10 board had shown a 

lower (better) average welt index value than any other board 

class in accelerated (fence) exposure when installed with 

intact drip edges, but, relative to other board classes, had 

shown unimpressive welt performance when painted drip 

Figure 10. Relative frequencies in 155-month exposure 

of three different levels of drip-edge paint cracking as 

a function of residual thickness swell (RTS) of matched 

specimens in laboratory testing; (a) noncommercial 

hardboard sidings, (b) commercial hardboard sidings. 

Legends have abbreviated text: squares indicate drip edges 

in intact or largely intact condition, crosses indicate dripedge paint cracking ranging from minor to moderate, and 

circles indicate paint cracking ranging from significant to 

severe. A specimen with drip-edge paint cracking restricted 

to board ends would have been classified as intact.

(lack of “plateau” value) was observed with regard to welt 

performance, and was sometimes observed with regard to 

drip-edge paint cracking. In the case where there was an 

apparent “plateau” value for RTS, below which further reduction in RTS did not result in improved performance with 

regard to drip-edge paint cracking, the plateau value was at 

9% RTS, (well below the 17% criterion in the 2006 revision 

of the Standard). Carll and TenWolde (2004) made three 

a

b
Durability of Hardboard Lap Siding—Performance in Laboratory Testing and in Long-Term Exterior Exposure

21

edges had been cut off (Carll and TenWolde 2004). In a similar manner, class 10 boards showed good welt performance 

during roughly the first half of the 155-month exposure 

period, but unimpressive welt performance during the later 

stages of the period. In the later stages of the exposure period, drip-edge paint cracks had been present in most class 

10 boards for a number of years, whereas in board classes 

1, 2, 11, and 13 drip-edge paint cracking was relatively rare 

(Table 6). Drip-edge cracking in class 10 boards was restricted to layers very close to the specimen face, which coincidentally, were the layers of highest density (Table 4). In 

class 10 board, in particular, swelling of layers very close to 

the surface was an apparent factor in initiation of drip-edge 

paint cracking, even though overall thickness swelling for 

this class of board was low.

Initiation of Drip-Edge Paint Cracking

Drip-edge paint cracking occurred in a majority, or a significant minority, of specimens of most classes of commercial board by 6 years of exposure (column for September 

2003 in Table 8). A significant minority of specimens of the 

poorer classes of noncommercial board also showed dripedge paint cracking by 6 years of exposure. Drip-edge paint 

cracking moreover occurred even though there was no measurable increase in in-service thickness (Fig. 6). This suggests that fluctuations in board thickness that were of fairly 

modest magnitude were sufficient to initiate paint cracking 

at drip edges.

As indicated previously, there is indication that water intrusion into lap areas was occasionally a factor in initiation of 

drip-edge paint cracks (in some specimen of board classes 

5, 6, and 14), but in most cases was not a factor in initiation of drip-edge paint cracking. The majority of specimens 

showed no water staining in lap areas (Tables 5 and 6), yet 

most developed drip-edge paint cracks (Table 8). The bulk 

of drip-edge paint cracking initiated near the mid-length of 

specimens (away from board ends), and the distances by 

which back-surface stains at drip edges extended from board 

ends (Table 6) were typically far short of specimen midlength. Finally, the most prevalent location for drip-edge 

cracks was toward the front of the specimen (Table 10), not 

at the location (near the back of the specimen) where water 

that intruded into specimen laps would have been present. 

In most cases, something other than water intrusion into lap 

areas must have been responsible for initiation of drip-edge 

paint cracking. The likely explanation is penetration of the 

paint film by modest amounts of water, near the interface of 

the drip edge and the specimen’s front surface. Initiation 

of drip-edge paint cracking at this location is shown in Figure 12. The interface of the drip edge and the face was a 

common location for initiation of drip-edge paint cracking.

Thickness swelling of wood composition panels, on a 

unitized basis, invariably exceeds along- or across-panel 

dimensional change. In addition, unitized layer thickness 

swelling in wood composition panels varies with layer 

position, typically being greatest in surface layers (Xu and 

Winistorfer 1995). The intersection of the drip edge with 

the board surface can therefore be viewed as a location 

where there is a considerable change (a local discontinuity) 

in dimensional stability of the substrate. At this location, 

paint films can thus be expected to undergo particular strain 

when there is a change in moisture content of the underlying board. An imperfection in the paint film at this location 

could provide a pathway for water penetration of the film, 

providing conditions for initiation of a paint crack. The 

micrographs taken for measurement of paint film thickness 

Figure 11b. Relative frequencies in 155-month exposure 

of three different levels of edge-welt index in as a 

function of residual thickness swell (RTS) of matched 

specimens in laboratory testing ; (a) noncommercial 

hardboard sidings, (b) commercial hardboard sidings.

a

b
Research Paper FPL–RP–674

22

indicated substantial variation in film thickness on drip 

edges (much more so than on specimen faces), with much 

of the variation being localized, and associated with unevenness of the substrate surface. Projecting fibers or fiber fragments were commonly present on drip-edge surfaces, which 

protruded partially (and occasionally completely) through 

the paint film.

In the micrographs taken for measurement of paint film 

thickness, a thinning of the paint film at the very intersection 

of the face and the drip edge was also commonly observed. 

This localized thinning of the paint film tended to be more 

noticeable in the noncommercial boards (classes 1–6), 

which as indicated previously, did not have shaped drip 

edges. This might logically have been expected to result in 

more extensive or severe drip-edge paint cracking in noncommercial boards than in commercial boards, but Table 8 

generally indicates the opposite. We hypothesize that a 

combination of the dimensional properties of the substrate 

at the drip edge (and in particular a discontinuity in these 

properties at the interface of the drip edge and the face) with 

the presence of projecting fibers at the drip edge (which 

resulted in variable effective coating thickness) explains 

the pattern of paint performance we observed (essentially 

perfect performance on board faces, while cracking on drip 

edges was widespread, and was sometimes severe).

That average initial topcoat paint film thickness (approximately 1.5 mils) was roughly half of what is commonly 

recommended, was almost certainly a factor in penetration 

of the paint film at the drip edge by water, and in turn, to 

initiation of drip edge paint cracking. 

Conclusions

The failure modes observed in 155 months of exterior exposure in Madison, Wisconsin, were development of drip-edge 

paint cracks, followed by development of edge welting. The 

extent and severity of these failure modes varied considerably by board class. Considerable variation between board 

classes was observed for both noncommercial and commercial boards. Inasmuch as development of drip-edge paint 

cracks preceded development of edge welting, drip-edge 

paint cracking was the most prevalent failure mode at the 

end of 155-month exposure. In this study, initiation of drip 

edge paint cracking was, in most cases, evidently associated 

with water penetration of the paint film at the drip edge.

Paint film on specimens was substantially thinner than recommended. On board faces, thinness of the paint film did 

not seem to matter, inasmuch as paint condition on board 

faces remained essentially perfect. On drip edges, however, 

thinness of the paint film was almost certainly a factor in 

initiation of drip edge paint cracking. One of the classes of 

noncommercial board performed nearly flawlessly, indicating its ability to withstand long-term exposure despite thinness of the paint coating, even on drip edges.

Open end joints evidently resulted in water entry behind siding at specimen ends, but in most cases did not appear to be 

a significant factor in development of drip-edge paint cracks 

nor in subsequent edge-welt development. This is probably 

because the exposure site was evidently less challenging 

(experienced less rainfall) than most exposure sites in the 

eastern states, and that water entry into the open end joints 

was largely restricted to water that entered them directly 

as wind-driven rain. The amounts of water that could enter 

open end joints along window casings might be expected 

to be considerably higher, particularly where window head 

flashings are imperfectly selected or installed, potentially 

spilling water into the joints from substantial watershed areas.

Back-surface water staining indicated that water exposure 

was greater on lower than on higher courses of siding. The 

stain patterns also suggest that capillary rise did not occur at 

siding laps. For most of the classes of hardboard siding there 

was evidence that water entering open end joints, and thus 

present on the board rear surface, was not drawn by capillary suction into siding laps.

Relative frequency of the two failure modes observed was 

related to residual thickness swelling (RTS) value of the 

board, as identified by the substrate weatherability test 

procedure in the American National Standard for hardboard 

siding. Specimens in exterior exposure whose matched 

laboratory test specimens had lower RTS values generally 

performed better. Lower RTS values were associated with 

lower relative frequency of failure and lower relative frequency of more intense failure. In most cases, there was no 

“plateau” value below which further reductions in RTS were 

Figure 12. Magnified image showing initiation of cracking 

at the interface of the drip edge and the outer specimen 

face in a specimen of class 2 board. With the exception of 

these observed cracks, some cracking near the unpainted 

end cuts, and some surface dirt, the specimen was in 

pristine condition after 155 months of exposure. This 

specimen did not have a shaped drip edge, inasmuch 

as it was a specimen of noncommercial board.
Durability of Hardboard Lap Siding—Performance in Laboratory Testing and in Long-Term Exterior Exposure

23

not associated with greater relative frequency of improved 

in-service performance.

Although the degree to which failures occurred in accelerated exterior exposure was typically greater than the degree 

to which they occurred in long-term (nonaccelerated) exterior exposure, the relative degrees, between board classes, 

to which they occurred were similar. Stated another way, the 

findings of Carll and TenWolde (2004), which were based 

on accelerated exposure, were found to largely be applicable 

to long-term exposure.

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Durability of Hardboard Lap Siding—Performance in Laboratory Testing and in Long-Term Exterior Exposure

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Appendix—Rainfall and Scheffer Index 

Values for One Location in Each State East 

of and Immediately West of the Mississippi 

River Rainfall and Scheffer Index values for one location in each state 

east of and immediately west of the Mississippi River

Location Scheffer Index 

(1990–2009)

Annual rainfall 

(1990–2009)

Mean 

value

COV

(% ) 

Mean 

(in.) 

Mean 

(mm) 

COV

(% ) 

Albany, NY 53 17 34 880 17 

Baltimore, MD 58 22 41 1050 17 

Baton Rouge, LA 89 15 56 1430 19 

Boston, MA 50 13 37 930 17 

Burlington, VT 56 15 28 720 25 

Charleston, WV 74 14 40 1020 7 

Columbia, SC 72 18 44 1120 22 

Concord, NH 47 16 35 890 23 

Dayton, OH 56 16 40 1010 18 

Des Moines, IA 51 17 34 850 25 

Hartford, CT 53 18 44 1130 15 

Indianapolis, IN 57 16 41 1040 18 

Jackson, MS 73 19 54 1380 15 

Lansing, MI 46 9 28 710 16 

Little Rock, AR 62 21 46 1160 24 

Louisville, KY 64 16 45 1150 14 

Macon, GA 72 19 46 1170 17 

Memphis, TN 65 17 55 1400 21 

Minneapolis, MN 46 18 25 640 17 

Montgomery, AL 71 18 48 1210 19 

Newark, NJ 61 19 43 1090 14 

Peoria, IL 49 20 35 880 27 

Pittsburgh, PA 59 14 35 890 20 

Portland, ME 43 16 42 1060 24 

Providence, RI 49 18 44 1110 13 

Raleigh, NC 71 13 43 1100 17 

Richmond, VA 65 20 43 1100 20 

St. Louis, MO 56 16 39 980 20 

Tampa, FL 99 12 46 1180 22 

Wilmington, DE 56 24 42 1080 15