HOW TO BUILD AND MAINTAIN EXTERIOR WOOD STRUCTURES
In order to build something to last, we need to know the
reasons why it does not last. These can be learned by observing
structural elements and joints in various states of decomposition.
That is to say, look at failures.
Understanding the nature of each kind of failure allows us
to develop technology to repair or restore that failure, as
well as an understanding of how to prevent that failure. The
Smith & Co. application note, Restoration of Wood [our
product applications literature is available upon request
and will be mailed.] is an outline of a step-by-step procedure,
developed and field-tested over thirty years, for the repair
and restoration of partially deteriorated wood. This is illustrated
by the Six Steps of Restoration.
It was inevitable that such extensive experience in restoration
would lead to identification of the different kinds or categories
of failure. From tens of thousands of individual circumstances
it has been seen that there are only a few basic mechanisms
which bring about the degradation of appearance and progressive
loss of mechanical properties. These mechanisms, acting singly
or in combination, are relatively few. One would suspect that
if each of these failure mechanisms could be addressed, it
should be possible to design a structure with a much longer
Broadly speaking, we can divide the causes of degradation
into two classes. There are those causes which are the actions
of life, and those causes which are the actions and interactions
of the non-living components of the physical universe.
A classic example of the degradation of wood by non-living
components of the physical universe may be seen in the cracks
that develop in the wood of a deck, directly underneath a
potted plant. Such plants are watered, and excess water drains
out the bottom of the pot. The wood directly under the pot
is in a more humid environment, and expands more than the
exposed wood elsewhere. Being a natural material, (and thus
randomly non-uniform), the wood expands a bit more or less,
here and there. Cracks (also called checks) thus develop.
Even if the wood deck planks are painted, some water diffuses
through the paint film and may cause the wood to expand beyond
the elastic limit of the paint. When this happens, the paint
film tears and the wood immediately beneath the failed paint
film can now dry out a bit more rapidly or absorb water a
bit more rapidly than neighboring painted wood. As the days
and nights and seasons cause changes in atmospheric temperature
and humidity and liquid water comes and goes, cracks in wood
may thus develop and grow, and the paint tends to peel near
Organic life causes degradation of wood because wood contains,
one way or another, everything life needs to survive. In simple
terms, life needs air, water, warmth and food in order to
survive. These factors need to be in the correct ranges in
order to optimize the survival and expansion of life. Too
much water may be good for some life forms but bad for others.
Too little water is bad for all the life forms of this planet.
Most life needs some air, but if there is too much air then
the circulation of that air facilitates the evaporation of
water, and thus too much air can mean not enough water. Too
much or too little warmth may freeze or cook some life, but
there is some temperature range where any life form will flourish.
The wood itself serves as food; wood is mostly made of cellulose,
an organic (made from life, as opposed to inorganic material
such as rocks or metals which never were alive) compound whose
structure consists of many sugar molecules connected together.
Many life forms can break down the cellulose and utilize its
food value. Cellulose has a natural affinity for water, due
to its chemical structure, and can easily hold ten to thirty
percent of water by weight. Since wood has natural porosity
(the structure being long hollow tubes with some space between
the tubes) the wood has air within itself.
Consider that wood floats in water. Organic materials typically
have densities near that of water, and yet wood observably
floats with ten to forty percent of its volume above the water.
Wood thus, observably, has a typical density less than water,
and is about 1/3 air by volume. Because these tubes and spaces
extend for quite some length (the direction of the grain)
it is possible for air inside the wood to slowly interchange
with external air. Thus, the wood can "breathe",
and life forms can obtain shelter slightly inside the wood,
while having available food, adequate moisture, and warmth
Some life forms, such as fungi, require more than about ten
percent humidity in the wood to flourish, but prefer it not
be waterlogged. Some bacteria, on the other hand, may actually
prefer completely waterlogged wood. Insects such as ants,
termites and various beetles have their own preferences, although
insects which breathe air do not prefer completely soggy wood.
Fungi may eat away some of the cellulose, leaving the wood
more porous, such that it can easily become waterlogged. Bacteria
may then flourish in the water, eating away the remaining
Having observed wood in varying states of decay, with fungal
or bacterial attack leaving little more than crumbling debris,
or with ant nests having hollowed out a piece of wood and
leaving little more than a shell of paint, certain common
factors are obvious by their chronic presence.
The end grain of a piece of wood is sometimes exposed, meaning
not painted. Exposed end grain affords entry into the interior
of the wood by fungi. Rot is commonly found in exposed wood
ends. Where the end of a piece of wood is cut at an angle,
exposing more area at that end-grain section, the wood rots
more readily. Where that angle-cut end grain is facing upwards,
as are some decorative rafter tails, severe rot develops rapidly.
Two pieces of wood are somewhere in contact. This situation
commonly occurs in all nailed wood structures, but decay is
worst when a vertical nail pierces two horizontal pieces of
wood, such as a deck plank resting on a deck joist. The slight,
inevitable gap between some part of plank and joist holds
excess moisture while allowing enough air for life but not
so much air as to dry out the wood. The nail or screw penetrates
both pieces of wood, and the inevitable cracks that develop
in the wood on both sides of the fastener afford an entry
into the wood for fungi. Rot follows the nails and the contact
areas as dependably as death and taxes.
Where debris is allowed to accumulate between planks of a
deck, especially over the joists, there will be rot. The debris
acts as a sponge, holding water and preventing the top edges
of the joists from drying out by evaporation of the water
into the air circulating through the deck. This, as well as
enclosing the perimeter of a deck built low to the ground,
encourages rot by providing the high humidity environment
in which rot fungi thrive. Add to these factors the use of
plain lumber instead of redwood or pressure-treated wood (treated
under pressure with preservatives) and you have a recipe for
One must not underestimate the role of weather in this activity.
In Alaska a typically built structure might last 100 years
whereas in California it might last twenty, in Florida ten,
or in Panama five years. The difference, given equal humidity,
is temperature. All chemical reactions go faster at higher
temperatures. Organic life is based on chemical reactions,
and the rate of chemical reactions doubles (roughly) every
18 Fahrenheit degrees/every ten centigrade degrees. Common
garden lizards may be seen to move much faster in the warm
summer weather than the cold winter weather, for example.
One could thus conclude that a few simple principles would,
if applied, lead to longer-lasting structures.
1) Use a species or variety of wood known to be resistant
to rot. Do not use varieties of wood known to rot easily.
Use pressure-treated wood where possible.
2) Paint or otherwise seal with a strongly adherent material
all exposed end grain of wood.
3) When nailing or screwing two pieces of wood, allow an air
gap (a quarter inch to a half inch, for example) so that rain
water can evaporate easily and accumulating debris can be
4) When nailing or screwing two pieces of wood that must be
held in contact, use an elastomeric caulk between the two
pieces, so as to exclude all air and moisture.
5) Clean gaps between wood pieces regularly, so that debris
does not accumulate.
6) Paint everything wherever possible. Use coating technology
that is effective in maintaining a long-lasting adhesive bond
between the paint and the wood, in spite of humidity variations
in the wood from season to season.
7) Inspect the wood regularly. Wood is a natural product (made
by Mother Nature, and She makes each piece a little different
from the next). There will be random variations, and occasional
cracks will develop. Water can accumulate in those cracks,
and fungal and bacterial activity can begin to attack the
wood. A variety of insects can also bore holes in the wood,
and these provide further entry points for water, fungi and
bacteria. Wood that was perfectly sound one year may show
some cracks in the next year, or perhaps an area of flaking,
peeling paint. Such small failures dealt with promptly and
repeatedly, will prevent serious decay and major structural
damage. Such small failures, ignored for ten years, will suddenly
reveal a disaster.
8) Clean debris out of gaps between wood joints (e.g. deck
planks) regularly. Depending on when or how much neighboring
vegetation contributes, it may be necessary to clean gaps
weekly some seasons and only every few months during other
The #8 above may seem to be a repeat of (5). I deliberately
mentioned it again just in case you didn't think it overmuch
important when I first mentioned it.
Copyright © 2002 Steve Smith All rights reserved