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Why Wood Rots?

Fungi are one of the oldest and most primitive of forms on this planet. Fungi feed on just about anything, whether dead or alive. Fungi move by growing more fungal cells that spread further by growth; they don't have legs or wings or swimming fins as some other life forms do. When a cell of fungus is in contact with something, and it is hungry, the cell secretes digestive enzymes onto what it touches. The enzymes break down the surface and dissolve it. The cell then absorbs the digested material as food. The stomach of a fungal cell is its outside surface; if a human body worked that way, our stomach would be on the soles of our feet, and if we wore shoes all the time we'd either starve to death or learn to eat our shoes.

Plants reproduce either by scattering seeds, which are a kind of egg, or by sending out shoots, runners, buds or branches (some kind of extension of the main body which is capable of taking root and becoming independent). Fungi reproduce in two ways. First, they just grow, forming new cells at the tips of old ones, spreading in branching strands everywhere they can find something edible. Second, they reproduce by creating seeds, called spores. The spores are smaller than can be seen with the human eye. They are so small and light that they drift on the wind. There are many varieties of fungal spores drifting in the air all the time. When it rains, the spores are picked up by rain water and carried everywhere rain water leaks into. As the rain water evaporates, the humidity of that surface drops from 100% down past moist to damp to maybe-damp to sort-of-dry to completely-dry.

Fungi need both water and air. As the surface (or piece of wood) slowly dries out and gets into the moist-to-damp zone the fungal spores hatch and become very fast-growing baby fungal cells. These rapidly grow up into adult fungal cells and begin to spread over and into whatever surface they are on, and the life cycle continues. When the surface dries up and cannot support fungal life, the adult fungal cells make many spores and leave them scattered around where they were. Eventually someone comes along and kicks that piece of rotten wood and millions of fungal spores drift invisibly away on the wind.

There are also bacteria, a different kind of life than fungi, but still a microscopic single-cell life form. They are more like an animal than a plant, and more like a fish than an air-breathing animal. There are thousands of different kinds of bacteria, and many of those will also make spores when the region dries out. Bacteria tend to prefer the wood more damp than do fungi, but there is a humidity range where both can live. Many bacteria can survive drifting through the air until they contact a wet surface. Some bacteria live completely immersed in water. Most bacteria can move about. Some wiggle, others have many small legs they use to crawl about with, or to swim through the water in search of food or company.

When you look at a stump or a piece of wood you can see on the cut surface a series of concentric rings, usually a darker brown and a light tan for more common wood such as Fir, Pine or Spruce. The darker rings are the growth of the diameter of the tree in the winter and the lighter rings are the (much greater) growth of the diameter of the tree in the summer as seen by the greater width of the summer growth rings. Certain kinds of bacteria or fungi will feed enthusiastically on the harder parts of wood where other kinds of fungi prefer only to eat into and dissolve the softer, more porous parts of wood. Wood, before deterioration has started, has only a little porosity, but is about fifty percent empty space inside. If you put a piece of wood in water, it floats about half-above the water and half-below. This shows that wood is less dense than water. As wood deteriorates, it becomes "waterlogged". In this condition water has migrated into most of the empty space inside the wood, and it floats with less of its volume above the water, or may even sink when there is no air space left inside the wood. You may have seen waterlogged wood on the bottom of a pond, or you have handled waterlogged or damp wood and noticed how much heavier it was than ordinary dry wood.

Wood is said to breathe because the natural humidity of wood, perhaps five to fifteen percent (once it has sat around in your garage for six months) can go up and down a bit as the humidity of air varies. The air humidity ranges from maybe ten percent in a dry summer to perhaps ninety-five percent in a humid summer

Humidity (of air) mean how much water vapour is dissolved in the air. Ten percent humidity means the air is holding ten percent of its maximum capacity. At one hundred percent humidity it probably is raining! The humidity of wood is usually expressed as a percent (say, ten percent). What this means is the percentage by weight of the wood that is water. In the case of air the humidity is NOT the percentage by weight of air but rather the percent of capacity. The capacity of air is about one percent water by weight and it varies a lot with air temperature. A cubic foot of wood weighs maybe 35 pounds. A cubic foot of air weight .07 pounds (a cubic foot of water, for comparison weighs 62.4 pounds).

Wood holds a little water very strongly and more water with less strength and even more water rather casually. When there is less humidity in the air, wood loses some of its water to the air by evaporation. When atmospheric humidity is high, damp wood may lose some of its water but really dry wood will actually capture some water from the air. You may have noticed that small branches of plants are very flexible. That is because the wood is full of water. As wood dries out it becomes stiffer. Old wood found in the desert is not only hard but brittle.

Wood can actually be placed in a box and exposed to the hot steam from boiling water. After a few hours the wood becomes flexible and can be bent into a new shape. If the wood is held in that shape as it cools down and dries back to its natural humidity at room temperature, it holds its new shape. The curved ribs for small boats are made by this "steam" bending process.

Wood, microscopically, consists of bundles of large hollow tubes with doors across the tube every so often. These tubes are the walls of living cells, long since dead with only the skeleton remaining. The hollow tubes are the skeletons on those cells. As the fungi eat away those cells walls, they open up the spaces between those tubes, and as the fungi dissolve the doors between one wood cell and the next, the wood porosity is opened up more and more. This allows more rainwater to be more rapidly absorbed in the wood, thus providing more humid wood which is more favourable to rapid fungal growth thus accelerating the decay of the wood. As the wood become more porous it holds enough water to favour growth of not only fungi but bacteria, and between them they eat first the porous summer growth rings and then the darker winter growth rings , and finally there is nothing left.

Cracking and splitting of wood due to age and poor maintenance of this Log home has led to extensive rot damage of the above timbers. A repair was attempted without fully understanding why the timbers were decaying; this amplified the rate at which the Fungi and Bacteria attacked the wood fibers. The damage is severe however using CPES and following our proven techniques, restoration this beautiful Log home can be done with long lasting results.

What that does to the logs of log buildings is this: At first the butt ends are exposed to the weather, and are extremely porous. As water enters the exposed wood, fungi will begin to eat it. This develops abnormal porosity, allowing liquid water to be retained for at least a few days. Environmental bacteria make their homes in the water, and begin to eat the wood. As the water evaporates, fungi take over. As the wood finally dries out below about fifteen percent water, the fungi die of thirst. The remaining wood, now slightly more porous than before, will absorb more water the next time and the decay continues, at an accelerating rate.

Wood tends to check with age as it dries out and shrinks. Log buildings which are not constructed with the "double-cut long groove" method, according to a recent article in an Australian publication, will develop uncontrolled checks on the outer surface; with that construction method the checks are negligible few , and typically only a couple of millimetres deep. The liability of deep checks or cracks on the outside surface of the logs is that rain or dew will accumulate in them, and rot will start in the bottom of them where the water is the slowest to evaporate.

If the logs of which a building is constructed are painted on the outside with some clear finish, so as to provide a thick film of Ultraviolet-absorbing material to protect the wood colour of the logs, this will prevent the logs from drying out after water exposure. People living inside a log building evaporate water from their bodies, by perspiration, exhaled humid air, and the normal cooking, washing and bathing activities. This humidity will be absorbed on the inner surfaces of the logs and must be allowed to escape to the outer surfaces.

Preventing such evaporation will lead to an increase in the amount of moisture content of the logs and, with the entire length inside used for an absorption surface inside, plus all the checks on the outside as entry points for liquid water, and only the exposed butt ends for evaporation to the outside environment, rot becomes very active in the center of log. This sort of thing was done in Canada in the nineties, and it was normal after ten to fifteens years to find the entire inside of large logs rotted.

There is actually an treatment method for this situation in most cases. Which is further explained in RESOURCE BULLITINS in the home page .

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. restorations technology details a step-by-step procedure, developed and field-tested over twenty-five years, for the repair and restoration of partially deteriorated wood.

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 life.

Broadly speaking, we can divide the causes of degradation into two classes. There are those caused 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 neighbouring 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 such cracks.

Cracks in these log timbers eventually lead to extreme degradation of the logs interior. An attempt was made to repair the damage but sadly it only helped in the progress of the rot.

Organic life causes degradation of wood, because wood contains one way or another everything life needs to survive. In simple terms, life needs are, 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 "breath", and life forms can obtain shelter slightly inside the wood, while having available food, adequate moisture, and warmth (weather permitting).

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 wood more porous, such that it can easily become waterlogged. Bacteria may then flourish in the water, eating away the remaining wood.

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 factor are obvious by their chronic presence. The end grain of a piece of wood is sometimes exposed, meaning not painted. Exposed end grains afford entry into the interior of the wood by fungi. Rot is commonly found in exposed wood ends.

Two pieces of wood are somewhere in contact. This situation commonly occurs in all nailed wood structure, 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 fasteners afford an entry into the wood for fungi. Rot follows the nail and the contact areas as dependably as death and taxes.

These two photos illustrate how rot will transfer from one piece to another if it is not treated. The photo on the left is the doubler/reinforcement plate for the exhaust ports. The photo on the right is the original wood (the transom) where the bronze exhaust pipe was. These two pieces of wood were bolted together. See project link (exhaust port repair) for more information about that repair or click on either photo.

When 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 joist 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 preservative) and you have a recipe for disaster.

One must not under estimate the role of weather in this activity. In Alaska a typically built structure might last 100 years where as 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 reaction, and the rate of chemical reactions doubles (roughly) every 10 (ten degrees) centigrade. Common 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.

Use a species or variety of wood known to be resistant to rot. Do not use varieties of wood which rot easily. Use pressure-treated wood where possible.
Paint or otherwise seal with a strongly adherent material all exposed end grain of wood.
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 accumulated debris can be removed.
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.
Clean gaps between wood pieces regularly, so that debris does not accumulate.
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 variation in the wood from season to season.
Inspect 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 variation, and occasional cracks will develop. Water can accumulate in those cracks, and fungal and bacteria 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 repeatly will prevent serious decay and major structural damage. Such small failures, ignored for ten year will suddenly reveal a disaster.
Clean debris out of gaps between wood joints (e.i. deck planks) regularly. Depending on when or how much neighbouring vegetation contributes, it may be necessary to clean gaps weekly some seasons and only every few months during other seasons. (This is a repeat of step 5 as it's very important) I have seen areas of extreme rot start just because a leaf was left sitting on the inside of the hull of a wooden boat. The leaf was always wet due to water spray and a very extensive repair was carried out due to this one leaf.