John Leeke's Historic HomeWorks™
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John Leeke's Historic HomeWorks™ |
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207 773-2306 26 Higgins St. Portland, ME 04103 |
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(Return to the List of Articles)
"Wood Properties
Affecting Finish Service Life"
Joint Coatings/Forest Products Committee
Chairman: Sam Williams - USDA Forest Products Laboratory,
Madison, WI 1-608-231-9412 George Daisey, Rohm & Haas
Company, Springhouse, PA Charles Jourdain, California Redwood
Association, Novato, CA Robert Springate, Tru-Serv Manufacturing,
Chicago, IL
Abstract..........(Return to
Table of Contents)
Wood is a biological material having widely different properties
depending on species, geographic area where the tree grows, the
growth conditions, size of the tree at harvest, sawing, and other
manufacturing processes. Some of the more important wood
properties as they relate to wood finishing are discussed. These
properties include growth rate, density, knots, extractives,
juvenile wood, grain orientation, and weathering characteristics.
Introduction..........(Return to
Table of Contents)
Current knowledge on wood properties has been learned through
decades of research in laboratories located in many parts of the
world and is contained in a number of publications (1-7). Much of
this knowledge, particularly information on North American wood
species is summarized in the Wood Handbook: Wood as an
Engineering Material (8). Detailed information on wood properties
that relate to wood finishing are contained in Chapter 16 of this
publication, in Finishes for Exterior Wood (9), and a monograph
by Feist (10). The benchmark research conducted by Browne (11)
during the 1950's and 60's is still valid and stands out as the
authoritative work documenting the effect of wood properties on
finish performance, particularly oil-based film-forming finishes.
Detailed information on wood properties is contained in these
publications. Those wood properties that have the greatest effect
on finish performance, particularly the properties of the lumber
available from today's forests are discussed herein.
Although dramatic changes have occurred to both the country's
timber base (12) and to exterior wood finish formulations since
that time, the link between finish performance and wood
properties has not changed. It is the change in species mix, the
dependence on small fast-grown second and third growth timber and
the more widespread use of wood composites to meet lumber demands
that causes changes in finishing practices, not any inherent
changes in wood properties. Smaller diameter, more rapidly grown
trees yield higher percentages of flat grain lumber containing
knots and other natural characteristics including sapwood. The
ways that these changes affect finishing practices are discussed
in another publication (13). An overview of wood properties is
contained herein. Wood that has been chemically treated
(preservatives, fire retardants, and bulking agents) and wood
composites (plywood, flakeboard, and fiberboard) do not usually
have the same properties as the original wood. The properties of
these wood-based materials will be the subject of an additional
publication.
Natural Characteristics..........(Return to
Table of Contents)
Growth Rate
In temperate climates, tree species add one growth increment or
ring to their diameter each year (Figure 1). This ring shows two
distinct periods of growth and therefore two bands, called
earlywood (springwood) and latewood (summerwood). Latewood is
denser, harder, smoother, and darker than earlywood, and its
cells have thicker walls and smaller cavities. The proportion of
earlywood to latewood, the density differences, and the
transition from the less dense earlywood to the more dense
latewood during the growing season can vary, depending on species
and growing conditions. Many species have a rather gradual change
in density during the growing season. Some species, have rather
narrow latewood bands; others have wide latewood bands. For some
species the density differences are not very great; others have
large differences in density between the latewood and earlywood.
However, for all temperate species, the change in density from
the end of one growing season to the beginning of the next is
always abrupt. If the difference is large, the stresses induced
to a coating system, can be quite large when dimensional changes
occur at this transition zone. This abrupt transition in wood
density also causes differences in properties for the pith-side
and bark-side of flat-grained lumber. The dimensional changes at
the latewood/earlywood interface can cause cracks in film-forming
finishes at this zone. Paint failure on latewood often begins
with these cracks. If the bands of latewood are narrow enough, as
in slow growth trees, the stresses are decreased and there is
less tendency for paint to crack or peel than on the wide
latewood bands. Wide latewood bands are normally absent from
edge-grained cedar and redwood, improving the paintability of
these species. It has been well established that wide latewood
bands in wood give a surface that is difficult to finish (Figure
2). However, they are prominent in southern yellow pine and
Douglas-fir, two of the most common species used for general
construction and for the production of plywood. Coarse grain
(i.e., wide annual growth rings of alternating earlywood and
latewood) is a natural result of the rapid growth rate
experienced by young growth forests under the optimal growing
conditions of intensive silviculture. On the other hand, growth
rate does not seem to significantly affect the ability of
hardwoods to retain a film-forming finish. They are generally
more difficult to finish than softwoods (Table 1). Table
1--Characteristics of selected solid woods for painting and
finishing. Editors, Please see Table 2. Monograph by W.C. Feist.
Paint performance on hardwoods, such as oak, is more a function
of density and the size and placement of vessels rather than
distinctions between earlywood and latewood (Figure 3). These
vessels are common to only hardwoods and are sometimes much
larger than the surrounding wood cells. They give added texture
to the wood and sometimes make it more difficult to finish these
species.
Density
The density of wood is one of the most important factors that
affect finishing characteristics. Density varies tremendously
from species to species (Table 1), and it is important because
higher density woods shrink and swell more than do lower density
woods. This dimensional change occurs as wood, particularly in
exterior applications, gains or loses moisture with changes in
the relative humidity and from periodic wetting caused by rain
and dew. Excessive dimensional change in wood stresses
film-forming finishes and may cause cracking and/or flaking.
The amount of warping and checking that occurs as wood changes
dimensions and during the natural weathering process is also
directly related to wood density. Cupping is probably the most
common form of warp for siding. Cupping is the distortion of a
board that causes a deviation from flatness across the width of
the piece. Wide boards cup more than do narrow boards and
flat-grained boards cup more than vertical-grained boards. Boards
may also bow, crook, or twist from one end to the other. Warping
is generally caused by uneven shrinking or swelling within the
board. Furthermore, checks or small cracks along the grain may
develop from stress set up during the initial drying (shrinking)
or from stresses caused by the alternate shrinking and swelling
during service. High density (heavy) woods such as southern pine,
Douglas-fir, and oak tend to warp and check more than do the low
density (light) woods such as redwood and western red cedar
(Table 1).
Knots
A knot is that portion of a branch that has become incorporated
in the stem of a tree (Figure 4). The influence of knots on the
finishing properties of lumber depends on their size, location,
shape and soundness. When sawing lumber from a log, a nearly
round shaped knot results when a branch is sawn through at right
angles to its length as in flat grain lumber. Vertical grain
lumber may have spike knots which result from sawing a branch
lengthwise (Figure 5). Oval knots will result from sawing a
branch diagonally as in lumber that is intermediate between flat
grain and vertical grain. Knots are further classified as
intergrown or encased. Intergrown or tight knots result from the
growth of the stemwood around a living branch. After a branch has
died, additional growth on the stem encloses the dead limb. This
results in an encased knot where the fibers of the stem are not
continuous with the fibers of the branch. Because shrinkage is
greater across the knot than in the surrounding wood, encased
knots may loosen and fall out causing a knot-hole (Figure 5).
Encased knots often have bark surrounding the knot, which may
cause additional discoloration problems with light-colored
finishes (14).
Knots are usually considered defects in wood for a number of
reasons. The wood of the knot itself is different in density
(usually higher), and its grain orientation is more or less
perpendicular to the surrounding wood. The lack of grain
orientation around knots may affect machining properties to the
point where it is difficult to achieve an acceptable machined
finish on the surface of the wood. For structural lumber, the
size and location of knots are one of the factors that determine
grade and thus its design values. Knots also affect the grade for
appearance grades of lumber, which include siding and trim. While
usually considered defects, a number of species and grades that
exhibit knots with some degree of regularity such as knotty pine,
select tight knot (STK) cedar and rustic grades of redwood have
been successfully marketed to feature their knots.
From the standpoint of finishing, knotty grades of siding are
more adapted for natural finishes such as water repellent
preservatives and semi-transparent stains than for paint systems.
Paint adhesion is poor over dense knots, and resin bleeding of
knots can discolor most paints unless they are sealed with
shellac or a similar product prior to priming.
Moisture Content
The moisture content of wood is the amount of water contained in
the wood. Moisture content includes both water absorbed into the
wood cell wall and free water within the hollow center of the
cell, and it is expressed as a weight percentage.
MC (%) = Mass (undried) - Mass (oven dried) X 100 / Mass (oven
dried)
The amount of water that wood can absorb (that is, that can be
bound in the cell wall) depends on the wood species; most species
can absorb about 30% water. This limit to the amount of water
that can be bound in the wood cell wall is called the fiber
saturation point. Wood can reach the fiber saturation point by
absorbing either liquid water or water vapor. The amount of water
vapor that can be absorbed depends primarily on the relative
humidity (RH) of the surrounding air. If wood is stored at 0 %
RH, the moisture content will eventually reach 0%. If wood is
stored at 100% RH, it will eventually reach fiber saturation
(about 30% water). Of course, if it is kept at a constant RH
between these two extremes, the wood will reach a moisture
content between 0% and 30%. The RH controls the moisture content,
and when the moisture content is in balance with the RH, the wood
is at its equilibrium moisture content. This rarely happens
because as the RH changes so does the moisture content of the
wood, and atmospheric RH is almost always changing. It varies
through daily and seasonal cycles, thus driving the moisture
content of wood through daily and seasonal cycles. Equilibrium
moisture content cannot be changed through the application of
finishes. Finishes affect only the rate at which absorption
occurs. Finishes can decrease daily and seasonal moisture
absorption and desorption, but they do not change the equilibrium
moisture content. Wood exposed outdoors cycles around a moisture
content of about 12% in most areas of the United States. In the
Southeast, average moisture content can be slightly higher and in
the Southwest, the average can be lower (9%). Daily and annual
moisture content will vary from these average values. Even in
very humid areas, the RH is rarely high enough for long enough to
bring the moisture content of wood above 20%. Wood that is warmed
by the sun experiences a virtual RH far below the ambient RH.
Wood will dry faster and become drier than expected given the
ambient RH. This is why checking often occurs on decking boards;
the surface is much drier than the rest of the board. Shrinkage
of the top portion of the board commensurate with this dryness
goes beyond the elastic limit of the wood at the surface and
checks form parallel to the grain.
As mentioned, fiber saturation is the greatest amount of water
that can be absorbed by wood via water vapor absorption. This
absorption is rather slow compared with the moisture changes that
can occur through absorption of liquid water. Liquid water can
quickly cause the wood to reach fiber saturation, and it is the
only way to bring the moisture content of wood above fiber
saturation. As wood continues to absorb water above its fiber
saturation point, the water is stored in the hollow center of the
wood cell. When all the air in the hollow center has been
replaced by water, the wood is waterlogged and moisture content
can be as high as 200%. The sources and ways by which wood can
get wet sometimes seem endless! The result is always the
same-poor performance, both of the wood and of the finish.
Water also causes peeling of paint. Even if other factors are
involved, water accelerates paint degradation. If the moisture
content of the wood exceeds 20% when the wood is painted, the
risk of blistering and peeling is increased. Moreover, dark
water-soluble extractives in woods like redwood and western red
cedar may discolor the paint shortly after it is applied.
Fortunately, the moisture content of lumber can be controlled.
But all too often, this critical factor is neglected during the
construction and finishing processes. It is best to paint wood
when its average moisture content is about that expected to
prevail during its service life. The moisture content and thus
the dimensions of the piece will still fluctuate somewhat,
depending on the cyclic changes in atmospheric RH, but the
dimensional change will not be excessive. Therefore, film-forming
finishes (such as paints) will not be stressed unnecessarily, and
service life should be better.
Plywood, particleboard, hardboard, and other wood composites
undergo a significant change in moisture content during
manufacture. Frequently, the moisture content of these materials
is not known and may vary depending on the manufacturing process.
To improve the service life of the finish, wood composites should
also be conditioned prior to finishing.
Water-Soluble Extractives
In addition to the physical structure and moisture content of
wood, wood chemical composition will affect the performance of
paints and finishes. Old growth timber is characterized by a high
percentage of heartwood. Heartwood contains extraneous compounds
such as tannins and other polyphenolics. These compounds, called
extractives, impart natural decay and insect resistance to
heartwood of several species of wood, such as redwood and the
cedars. The presence of water soluble extractives can discolor
the surface of paints and finishes and is often referred to as
extractive bleeding (15). Although these extractives can cause
problems with discoloration of finishes if not sealed with a
stain blocking primer, the decay resistance that some of them
impart to wood greatly outweighs disadvantages caused by
discoloration. The amount of extractives within the heartwood
depends on the age of the tree, thus the heartwood of younger
trees will naturally have a lower extractive content than that
from older trees. Although lumber from these younger trees is
less durable than lumber from old-growth trees, extractive
bleeding through finishes is less problematic. For clear grades
of wood, extractive bleeding can be eliminated with proper
priming, good building design, and conscientious construction
practices.
Water-Insoluble Extractives
Water insoluble extractives such as pitch and resin may also
interfere with the appearance of a painted surface. In some
species, small amounts of pitch form in the wood. In other
species the pitch can form in large deposits called pitch pockets
(large deposits of pitch). If the wood is kiln dried at high
temperature, the pitch can be hardened or set in the wood.
Specific kiln schedules have been developed for many wood species
to accomplish this. Unfortu-nately, some of these schedules can
discolor the surface of the wood. If pitch is not set, it can
become fluid enough during periods of warm weather to flow to the
surface of the wood. If the wood has been painted, the pitch
tends to soften and discolor the paint. Young growth knotty
siding products that have been air dried rather than kiln dried
may be more prone to pitch bleeding. Young growth products will
typically have smaller pitch pockets than those found in old
growth. Additionally, knots of many softwood species contain an
abundance of resin that can sometimes cause paint to turn
yellow-brown over the knots. Primers formulated to block water
soluble extractives will not block these resins.
The resins in some softwoods sometimes occur in small ducts
(Figure 3). Depending on the grain orientation, a large number of
these ducts can intersect the surface of the lumber. Paint on
these surfaces may be more prone to discolor. If various cuts of
lumber ( i.e. grain orientation) are used to make finger-jointed
moldings, the discoloration of the various pieces of wood in a
particular piece of molding can discolor in varying amounts. This
sort of checker-board pattern of discoloration can also occur
with waterborne extractives, particularly with finger-jointed
western red cedar or redwood.
Weathering
Weathering is the general term used to describe the degradation
of materials exposed outdoors. This degradation occurs on the
surface of all organic materials, including wood and finishes
used on wood such as paints and stains. The process occurs
through photo-oxidation of the surface catalyzed by ultraviolet
(UV) radiation in sunlight, and it is augmented by other
processes such as washing by rain, changes in temperature,
changes in moisture content, and abrasion by windblown particles.
Although the weathering process can take many forms depending on
the exposed material, in general the process begins with a color
change, followed by slow erosion (loss of material) from the
surface. The surface initially develops slight checking; with
some materials, deep cracks may ultimately develop. Weathering is
dependent on the chemical makeup of the affected material.
Because the surface of a material may be composed of many
different chemicals, not all materials on the surface may erode
at the same rate.
The surface of wood consists of four types of organic materials:
cellulose, hemicellulose, lignin, and extractives. Each of these
materials is affected by the weathering process in a different
way. The extractives undergo changes upon exposure to sunlight
and lighten or darken in color. With some wood species, this
color change can take place within minutes of exposure. Changes
in the color of the surface are accompanied by other changes that
affect the wettability and surface chemistry of the wood. The
mechanism of these early changes is not very well understood, but
these changes can have a drastic effect on the surface chemistry
of wood and, thus, the wood's interaction with other chemicals,
such as paint and other finishes and adhesives.
Lignin comprises 20% to 30% of the wood surface. It is a
polymeric substance that glues the celluloses together. Because
lignin is affected by photodegradation more than are celluloses,
lignin degrades and cellulose fibers remain loosely attached to
the wood surface. Further weathering causes fibers to be lost
from the surface (a process called erosion); but this process is
so slow that on the average, only about 6 mm (1/4 in.) of wood is
lost in a century ( Figure 6) This erosion rate is slower for
most hardwoods. Biological attack of a wood surface by
micro-organisms is recognized as a contributing factor to color
change or graying of wood. This biological attack, commonly
called mildew, does not cause erosion of the surface, but it may
cause initial graying or an unsightly dark gray and blotchy
appearance. The micro-organisms primarily responsible for gray
discoloration of wood are commonly found on weathered wood.
Paint Adhesion to Weathered Wood
Although the erosion of the wood surface through weathering is a
slow process, the chemical changes that occur within a few weeks
of outdoor exposure can drastically decrease the adhesion of
paints subsequently applied to the weathered surface. It is
fairly obvious that a badly weathered, powdery wood surface
cannot hold paint very well. This fact is not so obvious for wood
that has weathered for only 2 to 3 weeks. The wood appears sound
and much the same as unexposed wood. However, when boards that
had been preweathered for 1, 2, 4, 8, or 16 weeks were painted,
the paint had a drastic loss in adhesive strength after 4 weeks
of preweathering.(16) Similar results are reported.(17-28) In
subsequent studies using similarly preweathered boards, the time
until the paint started to peel was directly related to the
preweathering time.(29) For panels that had been preweathered for
16 weeks, the paint started to peel within 3 years; for panels
preweathered for only 1 week, the paint started to peel after 13
years. Panels that were not preweathered showed no sign of
peeling after 13 years. The paint system was a commercial
oil-alkyd primer with two acrylic latex top-coats over planed
all-heartwood vertical-grained western red cedar.
The best remedy for restoring a weathered wood surface is to sand
it with 50- to 80-grit sandpaper. Even if wood has not been
weathered, scuff sanding provides a much better surface for
painting.
Juvenile Wood
The presence of juvenile wood, which is often characterized by
lower density and higher longitudinal shrinkage, is responsible
for much of the decrease in strength of young growth timber.
Juvenile wood is produced during the early growth and development
stages of trees. Much of the warp, crook, and bow in lumber
containing the pith or sawn close to the pith is caused by the
presence of juvenile wood. Cross-grained cracking of lumber is
often a sign of juvenile wood (Figure 7). Because of the younger
age and smaller diameter of trees harvested from young growth
forests, a higher proportion of the volume in a log consists of
juvenile wood. Juvenile wood is particularly problematic in the
short rotation (harvested at 20 to 30 years) plantation grown
softwoods.
Manufacturing
Characteristics..........(Return to Table of Contents)
Grain Orientation
Several properties of flat grain lumber differ from vertical
grain lumber, these include: appearance or figure, dimensional
stability, potential for grain raising and grain separation and
ability to hold film forming finishes. The broad alternating
bands of earlywood and latewood that make up the annual growth
rings are highly visible on the surface of flat grain lumber.
This grain figure is valued for applications such as indoor
paneling where maximum natural visible character of the wood is
desired. As previously mentioned, in flat grain lumber knots will
appear round or oval rather than spike as in vertical grain
lumber. Since tangential shrinkage is about twice the rate of
radial shrinkage, flat grain lumber can be expected to shrink
across its face much more than vertical grain lumber.
Perhaps the greatest difficulty associated with the use of flat
grain lumber is the increased potential for grain raising and
grain separation (Figure 8). Raised grain, which results in a
corrugated appearance on the surface of flat grain lumber,
usually occurs when the harder latewood portion of each annual
growth ring is projected above the level of the softer earlywood.
This usually occurs when dry flat grain lumber is allowed to pick
up moisture. When the interface between the latewood and
earlywood become loosened, the result is separated grain, also
known as loosened grain, shelling, feathering or lifting of the
grain. Raised and/or separated grain is much more pronounced on
the pith side than on the bark side of flat grain lumber. The
primary method of preventing problems with grain separation is to
orient the bark side rather than the pith side of the product to
the weather. Since grain raising and grain separation can be
exacerbated by poor manufacturing practices, closer attention to
machine set up and more frequent sharpening of knives and greater
attention to quality control procedures are required when
surfacing flat grain lumber.
Surface Texture
The effect of surface quality, and texture play an important role
in the performance of a finish, and the variables that influence
the surface characteristics of the wood are numerous. Rough sawn,
sanded, saw-textured, and smooth planed surfaces all influence
finishes differently.
Generally speaking, rough sawn and saw-textured surfaces require
more finish than smooth planed surfaces and they provide longer
lasting performance because more finish must be applied. Rougher
surfaces provide more tooth for finish adhesion and actually have
much more surface area. These types of surfaces also dramatically
decrease grain raising problems associated with flat grain lumber
by decreasing grain compression and other damage sometimes caused
by planing processes. As the timber resource changes from an old
growth to a young growth supply, the ratio of flat grain, to
vertical grain is increasing dramatically. For this reason, rough
sawn and saw-textured surfaces are being used more frequently in
an effort to decrease grain raising, and improve finish
performance on flat grain wood products.
The bark side of flat grain products weathers differently than
the pith side, therefore, lumber patterns that have an obvious
exposed face can be manufactured to leave the bark side exposed.
This is referred to as "graining the piece." Some
patterns are reversible and in these cases it is up to the
installer to grain the piece and apply the product with the bark
side exposed. In cases where the specifier knows which face of a
pattern will be exposed to weathering, the manufacturer should be
notified prior to manufacturing so that the pattern can be
grained and manufactured properly. Graining solid sawn lumber is
a common practice that can be done by an experienced planer or
molder operator. This practice, however, is not always practical
when manufacturing finger-jointed lumber, and the chances of
encountering grain raising and the associated finish problems may
be higher for these products. For this reason, the use of
saw-textured patterns is especially important with finger-jointed
wood substrates.
Recent studies have shown that sanded surfaces may provide
improved finish performance and durability. Sanding with 50-80
grit sandpaper not only cleans the surface by removing dirt, oil
and other contaminants, but it also roughens the surface slightly
and removes raised grain. The abrasion of the surface is
particularly important on the latewood portion of the growth
rings where the wood is more dense and more difficult to attain
finish adhesion. Sanding helps prepare this portion of the wood.
Under magnification, sanded surfaces emulate characteristics
similar to saw-textured surfaces but to a lessor degree. More
surface area is exposed in the sanding process creating better
conditions for finish adhesion.
Moisture Content and Drying
Significant regional geographic differences exist in lumber
drying practices resulting from species mix, building practices
and climate throughout North America. Wood products are available
green (unseasoned), S-Dry (Surfaced-Dry, 19% or less moisture
content) or kiln dried (typically 10% to 15% moisture content).
Only dimension and finish lumber two inches and thinner in
nominal thickness may be available S-Dry or kiln dried. Larger
dimensions of solid sawn lumber should be assumed to be green.
Higher grades of lumber such as those used for architectural
finish are normally kiln dried. Common grades of lumber may be
available S-Dry and less frequently kiln dried.
Wood moisture content is one of the most critical factors
governing the performance of paints and other film-forming
finishes. Typically, paints and other surface coatings can not be
successfully applied to wood that has a surface moisture content
in excess of approximately 20%. If the moisture content is too
high, continued movement of this moisture out of the wood may
result in extractive bleeding and in blistering and peeling of
film-forming finishes. For best results, the wood should be at a
moisture content typical of what it will have during its service;
for most areas of the United States and Canada, this is about
12%. Penetrating finishes such as water repellent preservatives
and semi-transparent stains may be applied to products that have
a dry surface but which still may contain a high enough internal
or core moisture content to be considered green. Since
penetrating finishes do not block moisture movement, they allow
the wood to dry and yet do not blister or peel.
Lumber manufacturing can also have an effect on the overall
performance of a coating on wood. As previously discussed, proper
drying plays an important role in producing a wood product that
will be both dimensionally stable and favorable towards better
finish durability. Proper kiln drying provides the best situation
for wood as it creates a balanced piece of lumber that has an
even moisture content throughout the piece. Kiln dried lumber has
less tendency to shrink or warp, but the most important
characteristic from the standpoint of finishing is that kiln
dried lumber develops less surface checking. Paint applications
are especially susceptible to performance failures when surface
checking of the wood substrate occurs. These checks initiate
cracking and peeling of the coating. Kiln drying dramatically
decreases this condition.
Air-seasoned or S-Dry lumber will also provide a better substrate
for finishing than green lumber. These partially dried products
may not, however, give the same quality of finish performance as
kiln dried lumber. While some of the moisture has been removed in
S-Dry and air-seasoned lumber, a percentage of excess moisture is
still present in the wood and this can be detrimental to film
forming finishes such as paint. Partially dried products offer
dried surfaces that are satisfactory for applying penetrating
finishes such as water repellents and semi-transparent stains.
Green lumber is generally not suitable for immediate finishing
with paint or other film forming finishes. Green lumber can be
used in situations where penetrating stains and finishes will be
applied, but the wood surface must first be allowed to dry
sufficiently to accept the finish. This can usually be
accomplished during a few days of dry weather. Other problems
associated with the use of green lumber products include the
natural shrinkage, which occurs as these products acclimate to
the ambient conditions. Checks, splits and warping may occur from
rapid, uneven shrinkage of lumber. Using higher grades of lumber
can minimize these characteristics.
Performance Expectations
Unlike building materials such as steel and masonry, wood is a
biological product. As such, individual pieces vary more in
appearance, properties and performance. The life expectancy of a
lumber product is related to a number of direct and indirect
factors. Direct factors include the properties of a particular
species, the characteristics of the grade, and the quality of
manufacture. Indirect factors include how the product is used
after it has left the manufacturing facility. These indirect
factors include: handling, storage, installation and maintenance.
Siding is but a single component of a building. Its performance
depends on many critical factors including climate, the design of
the structure, the skill of the trade-person doing the
installation, the interaction with other materials such as
fasteners, flashing, sheathing, and house-wrap or felt.
Under ideal conditions, wood can have an almost indefinite
service life. As a rule of thumb, most low rise residential
structures are designed and built with an intended service life
of 100 years. It is assumed that after 50 years the structure
will require extensive renovation that will include the
replacement of the exterior cladding. A service life for exterior
wood siding on the order of 50 to 100 years is, therefore,
generally considered acceptable. The National Association of Home
Builders (NAHB) (30) gives a life expectancy for wood siding of
only 10 years if constantly moistened, but up to 100 years if
properly maintained. Keeping wood and wood products dry is
essential for long term (centuries) service life. The 100 year
service life is easily obtained if the wood is kept dry and
maintained with a finish. There are many examples of wood siding
lasting over 200 years, for example, the siding on Mount Vernon
and numerous other historic structures throughout the east coast
of the USA.
Wood buildings, properly designed to shed exterior water and to
avoid trapping moisture from interior sources routinely give
service lives of much longer than 100 years. This is particularly
true if the wood is maintained with a paint finish. The roof of
the building protects the wood from much of the water. The finish
stops the weathering of the wood's surface because the pigments
in the paint completely block the ultraviolet (UV) radiation in
sunlight, which is the radiation that catalyzes the degradation
of the wood. The paint itself undergoes UV degradation and a
paint system comprised of a primer and two top coats lasts about
10 years in full sunlight. During this time, the top coats erode
and when the primer begins to show, the surface should be
repainted. By keeping an intact paint film on wood, the wood's
surface can be protected indefinitely.
Wood used in exterior landscaping projects such as decks and
fences is generally considered to have a much shorter expected
service life. There are a number of reasons for this. Decks and
fences are often in direct ground contact or in very close
proximity to the ground. This results in a higher risk for
termite and wood decay fungus infestations. The horizontal
exposure of decks results in a much higher degree of weathering
from UV radiation, precipitation, and other elements. Decks are
also directly exposed to the wear from foot traffic. For
preservative- treated wood, the service life is often determined
by the weathering of the decking boards. The NAHB estimates the
life expectancy of wood decks and fences to be 15 years and 12
years, respectively (30). We consider these life expectancies to
be conservative. There are many examples of decks and fences
lasting much longer when properly designed, constructed and
maintained. We believe that a reasonable expectation of service
life for these and other types of wood landscaping structures to
be 20 to 25 years.
Summary and Conclusions..........(Return to
Table of Contents)
Of the vast array of wood properties that affect the performance
of finishes for wood, several stand out as having a major effect
on long-term service life of finishes. The growth rate and the
grain pattern that results from the growth rate, particularly the
relative width and densities of the earlywood and latewood,
affect the dimensional stability and thus the amount of stress at
the wood/coating interphase. Coatings systems lacking sufficient
flexibility can crack as a result of these stresses. The grain
orientation of lumber is affected by the way it is cut from a
log. Flat-grained surfaces are much more difficult to finish than
are vertical-grained surfaces. High density of some wood species
and/or the latewood of some species have rather poor paint
adhesion properties. The extremely high longitudinal dimensional
changes of juvenile wood can often cause cross-grained cracking
of wood with resulting cracking of film-forming finishes. Water
soluble extractives and water insoluble resins must be blocked by
a primer or suitable sealer to avoid discoloration. This is
particularly problematic of knots, which often contain large
concentrations of both water soluble and insoluble compounds.
Through understanding the properties of wood and using finish
systems compatible with these properties on well designed and
constructed buildings, painted wood can last for centuries.
References..........(Return to
Table of Contents)
1) Browning, B.L. 1964. The Wood-Water Relationship. In:Browning,
B.L., ed: The Chemistry of Wood. London: Interscience.
1963.Buckley, D.G. Absorption of Water-Repellent Wood
Preservatives in Swedish Redwood Timber. TRADA Research Report
C/RR/21.
2) Kollmann, F.F.P. and W.A. Côté. 1968. Principles of Wood
Science and Technology, Vol. 1, Solid Wood. Springer-Verlag New
York Inc.
3) Miniutti, V.P., E.A. Mraz, and J.M. Black. 1961. Measuring the
Effectiveness of Water-Repellent Preservatives. Forest Products
J. 11(10):453-462.
4) Siau, J.F. 1984. Transport Processes in Wood. Springer-Verlag.
New York, NY.
5) Siau, J.F. 1971. Flow in Wood. Syracuse University Press,
Syracuse, NY.
6) Skaar, C. 1972. Water in Wood, Chapter 3. Syracuse University
Press, Syracuse, NY
7) Stamm, A.J. 1964b. Wood and Cellulose Science. Ronald Press,
New York.
8) Forest Products Laboratory. 1987. Wood handbook: Wood as an
Engineering Material. Agriculture Handbook 72. USDA, Washington,
D.C.
9) Williams, R.S., M.T. Knaebe, and W.C. Feist. 1996. Finishes
for Exterior Wood. Forest Products Society, Madison, WI. 125 pp
10) Feist, W.C. monograph
11) Browne, F.L. 1951. Wood Properties That Affect Paint
Performance. USDA Forest Service. FPL Report No. R1053.
12) MacCleery, D.W. 1992. American Forests: A History of
Resiliency and Recovery. USDA Forest Service. FS-540.
13) Jourdain, C.J., J. Dwyer, K. Kersell, D. Mall, K. McClelland,
R. Springate, and S. Williams. 1999 Changing Nature of Wood
Products-What Does it Mean for Coatings and Finish Performance.
J. Coatings Tech. 71(890):61-66.
14) Levitin, N. 1962. Extractives of Red and White Pine and Their
Effect on Painted Lumber. Timber of Canada, June:66-71.
15) Donegan, V., et. al. Understanding Extractive Bleeding. Joint
Coatings/Forest Products Committee. California Redwood
Association, Novato, CA.
16) Williams, R.S., J.E. Winandy, and W.C. Feist. Paint Adhesion
to Weathered Wood. J. Coatings Tech. 59(749):43-49, 1987.
17) Williams, R.S., Plantinga, P.L., and Feist, W.C.,
"Photodegradation of Wood Affects Paint Adhesion,"
Forest Prod. J., 40, No. 1, 45 (1990).
18) Arnold, A., Feist, W.C., and Williams, R.S., "Effect of
Weathering of New Wood on the Subsequent Performance of
Semitransparent Stains," Forest Prod. J., 42, No. 3, 10
(1992).
19) Williams, R.S. and Feist, W.C., "Effect of
Preweathering, Surface Roughness, and Wood Species on the
Performance of Paint and Stains," JOURNAL OF COATINGS
TECHNOLOGY, 66, No. 828, 109 (1994).
20) Evans, P.D., Thay, P.D., and Schmalzl, K.J.,
"Degradation of Wood Surfaces During Natural Weathering.
Effects on Lignin and Cellulose and on the Adhesion of Acrylic
Primers," Wood Sci. Technol., 30, No. 6, 411 (1996).
21) Underhaug, Å., Lund, T.J., and Kleive, K., "Wood
Protection-The Interaction Between Substrate and the Influence on
Durability," J. Oil & Colour Chemists' Assoc., 66(11),
345 (1983).
22) Miller, E.R., "Chemical Aspects of External Coatings for
Softwoods," Symposium on Chem. Aspects of Wood Tech.,
Swedish Forest Prod. Res. Lab., Södergam, Stockholm, Sweden,
1981.
23) Bravery, A.F. and Miller, E.R., "The Role of
Pre-treatment in the Finishing of Exterior Softwood," Proc.
of the Ann. Conv. of the British Wood Pres. Assoc., 1980, p.
14-23.
24) Boxall, J., "Painting Weathered Timber,"
Information Sheet 20/77. Building Research Establishment, Princes
Risborough Laboratory, Alyesbury, Bucks, England, 1977.
25) Shurr, G.G., "Proper Coatings for Wood Exteriors,"
Am. Painting Contractor, 12, 18 (1969).
26) Desai, R.L., "Coating Adhesion to Weathered Wood,"
Eastern Forest Products Laboratory, Ottawa, Canada, Bi-monthly
Research Notes, 23(5), 36-37 (1967).
27) Kleive, K., "Weathered Wooden Surfaces-Their Influence
on the Durability of Coating Systems," JOURNAL OF COATINGS
TECHNOLOGY, 58, No. 740, 39 (1986).
28) Richter, K., Feist, W.C., and Knaebe, M.T., "The Effect
of Surface Roughness on the Performance of Finishes. Part 1.
Roughness Characterization and Stain Performance," Forest
Prod. J. 45, No. 7/8, 91 (1995).
29) Williams, R.S. unpublished data
30) National Association of Home Builders. 1997 Housing Facts,
Figures and Trends. NAHB, Washington, D.C.
Webpage creation by Dan Mitchell, working in the Historic Home Works Assistantship Program. Dan is a preservation enthusiast on Cobbosseeconte Lake in Maine.
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