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Building pathology: wood rot

This wood rot page is provided as a public service by
A. Sebastian Engineering and Investigation Services

General presentation of the decaying process

Wood rot is caused by any of a variety of fungi. In order to flourish, all these fungi require moisture (although very little some), oxygen, food (the cellulose and lignin in wood fibers), and proper temperature (most any climate in the USA, and Canada are conducive to rot fungi existence, including the deserts and most Northern regions). These organisms depend on the moisture in (or the condensation on) the wood to sustain them. On the other hand there is at least one type of building fungus that wets the wood itself from ground moisture through a long strand called a rizomorph. .

Sometimes, and only when the environmental conditions are right, the fungi multiply rapidly and produce spores. Generally, the fungi reproduce and move along the easiest routes, which is usually with the wood grain. This is why the rot is usually most extensive in the same dimension as the grain. Should the wood dry out completely (which in most cases is an unlikely scenario) the fungi will die out, yet they leave behind their reproductive spores for later regeneration. Whenever the conditions within the wood become again appropriate, the spores reproduce and the rotting action continues. .

This resurgence process may be relevant if the instead of removing and encapsulating the whole wood member, even if only slightly affected, the restoration process is limited in nature. In such a case, only the rotted zone is dealt with, either by localized removal and replacement with healthy wood or by filling with any of a number of epoxy resins, polyesters, etc. This may mean that even though the apparent wood has been dealt with, the rotting may restart from either undetected portions of rot or from spores. It is entirely possible that both the rot fungus in question and the accompanying spores exist for 12", or more, beyond the visible edges of the rotted zone. Any restoration process applied to the wood, other than total replacement, should therefore encapsulate the potential fungi and spores remaining the outlying areas.

In some cases I would rather avoid working with epoxy resins altogether. They usually have strong odors and there are many known hazards associated with breathing fumes from most of the solvents. Epoxies can be highly irritant to the skin as well, so proper ventilation, masks and gloves are a must. Even with these protectors, epoxies are better left to experienced tradespeople. Alternatives to epoxy compounds do exist. Acrylic resin sealers for example are popular in some circles (mostly with wooden boat restorers). Although many may harden the wood surface, generally they do not restore substantial strength for structural elements and might be chemically unstable during long-term exposure to UV radiation and to moisture.

Of course, there is something to be said in favor of surface sealers. Consider a wooden structure such as a deck or a plank-on-frame boat. Hopefully it is protected with a semi-permeable coating while it goes through wet and dry cycles. These cycles disrupt fungal activity, while the coating disrupts the normal wet-dry cycle by (hopefully) acting as barrier to water loss from the wood.

The added condition is that once an epoxy coat is used, the wood is protected from further exposure to water. This means that for wooden boats the use of epoxy products below the waterline is generally a bad idea, although most of the rot wooden boats is in wood that is usually above the waterline. Which brings us back to alternating cycles commonly found in house decks or boat superstructures: the said coats cannot be long-term effective if they are exposed to all weather conditions.


We often see softwoods used in contemporary construction showing extensive decaying after only a few short years of usage. I have seen myself complete failuress in as little as 24 months of usage. The scary part is that, although much less frequently, major damage can happen rapidly even in the case of treated lumber.

At the other end of the spectrum, I saw wooden objects found during archeological digs that are literally hundred of years old and yet perfectly preserved. In Europe at least, I have also seen wood trusses used to support roofs that were in great shape after a few hundred years of service.

Wood decaying can be temporarily or permanently prevented (or at least controlled) only by understanding the nature of the fungi attacking the wood and the environmental conditions necessary for their growth. Only then it may be possible to either totally change (or perhaps only slightly modify) the wood's service conditions or treat it in order to prevent the start of fungal growth.

In the begining: the subject of wood decay in the Bible

Wood decay is by no means a new problem. It is referred to in the Old Testament. Noah by the way is not only the first known design detailer (as you may recall, God was the chief design engineer) and builder of floating homes; he was also a successful wood treater and waterproofer. As described in Genesis 6:14-16, his Ark was constructed of "gopher wood" (probably cypress) elements bound together. The joints had standard backing rods made of reeds, and were sealed with bitumen, of which natural deposits were found throughout the Middle East. The whole envelope had a bitumen coating: not exactly beautiful, yet highly effective even by today's standards. Another Biblical example of highly effective construction and waterproofing is the Ark of Moses (Ex. 2:3-6): the body was made of reeds, plastered with slime (probably a mud of some sort) inside, and sealed with bitumen on the outside of the box, so that the odor would not be offensive to the baby. Unfortunately we do not know the name of the designer and builder.

Sometimes, when incipient decay is neglected or goes unnoticed, almost complete destruction may occur in only a few years. On the other hand, even for the expert it is usually difficult (if not impossible) to detect incipient rot and perform successfully any limited-scale repair of such damage. Often enough, the only solution may be the complete replacement of the building element involved.

"Rot" or "rotting" is not simply the presence of fungi, but rather the damage or disintegration (and ultimately dissolution) of the wood substance as the result of the presence (and growth) of fungi within the wood tissue. Wood does not decay simply because it is wet or ill maintained! The process may happen only if the wood has been attacked by certain fungi under some very specific environmental conditions of oxygen, temperature and moisture. As far as decay prevention is concerned, generally it is necessary to identify the attack, know the nature of the particular fungi, and the conditions necessary for their survival. Only then it may be possible to modify the wood's service conditions or perhaps treat the wood to prevent the development of fungal growth.

Fungicides are generally toxic substances, used either as a preventive against the growth of or to kill fungi that already infested the wood mass. Historically, the first effective fungicide was developed in 1882; it was made of slaked lime and copper sulfate, which is still one of the most effective means of controlling fungi.

Resources Law Offices of Thomas E. Miller: wood rot. Law Offices of Thomas E. Miller: construction defects RottenBoat 'Zine Wood Treatment and Preservation Products detection oft rot in wood & Forest Pathology: Decays Biological Improvement of Wood Properties data In this case a pulseEKKO 1000 GPR system with 1200 MHz antenna was used to map a hydr Encarta MYCOLOGY, FUNGI, SMUT, WOOD ROT Bremerton root rot workshop

To learn more about fungi and wood click here

Back to Building Pathology Home Page Biblical references to fungi on stone:

"... if on inspection he finds the patch on the walls consists of greenish and reddish depressions, apparently going deeper than the surface, he shall go out of the house and put it under quarantine ..... if the patch has spread on the walls, he shall order the infected stones to be pulled out and thrown away outside the city in an unclean place (Leviticus 13: 14)."

Encarta Concise Encyclopedia:

Dry Rot, decay of seasoned wood caused by the attacks of any of a number of species of fungi. These fungi penetrate to the interior of timbers and consume the cellulose in the wood fibers, leaving the timbers porous, although the wood may appear sound. Dry rot is so called because of the powdery appearance of the decayed wood.

Cellulose, complex carbohydrate, the chief substance of the cell wall of all plant cells. With some exceptions among insects, true cellulose is not found in animal tissues. Cellulose is insoluble in all ordinary solvents and may be readily separated from the other constituents of plants. Rayon and cellophane are cellulose regenerated from special solutions. Cellulose acetates are spun into fine filaments for the manufacture of some fabrics and are also used for photographic safety film, as a substitute for glass, and as a molding material. With mixtures of nitric and sulfuric acids, cellulose forms a series of flammable and explosive compounds known as cellulose nitrates, or nitrocelluloses.


I Introduction

Fungi, diverse group of single-celled or multicellular living things that obtain food by direct absorption of nutrients. Together with bacteria, fungi are responsible for the decay and decomposition of all organic matter. The study of fungi is called mycology. Fungi were traditionally classified as a division in the plant kingdom, but most scientists today view them as an entirely separate group and place them either in the protist kingdom (see Protista) or in their own kingdom. Approximately 100,000 species of fungi are known.

II Structure

Most fungi are formed of tubes known as hyphae, frequently partitioned by walls called septa. One or two nuclei are found in each hyphal cell. Hyphae grow at the tips and also by branching. The resulting profusion of hyphae is called the mycelium. Abundant mycelium may produce large fruiting structures such as mushrooms and puffballs. Some fungi form resistant masses of mycelium, called sclerotia, which may be smaller than grains of sand or as large as cantaloupes.

III Reproduction

Most fungi reproduce by spores, which are formed both sexually and asexually. The common mushroom may form 12 billion or more spores on its fruiting body; the giant puffball may produce several trillion. Spores form in one of two ways. One method of spore production involves the transformation of hyphae into numerous short segments or into various kinds of more complicated structures. In the other process, spores form after the union of two or more nuclei within a specialized cell. These spores typically have varying combinations of the hereditary characteristics of the parent nuclei. The four types of spores that are produced in this way-oospores, zygospores, ascospores, and basidiospores-identify the four principal groups of fungi. Oospores are formed by sexual union of a male and a female cell, zygospores by genetic exchange between two similar sex cells. Ascospores are spores contained in asci (sacs), and basidiospores are contained in basidia (clublike structures).

IV Fungus Ecology

Fungi are found wherever other forms of life exist. Many fungi have been discovered in polluted rivers and streams, and these fungi participate in the decomposition of sewage. Soil is a natural habitat for saprophytic fungi, which live on organic remains, as well as a reservoir for parasitic fungi, which infect living plants and animals. The water molds and downy mildews are common soil inhabitants. Many such fungi decompose cellulose and proteins and thus are active in the formation of humus.

Certain fungi live in a symbiotic association with algae (see Symbiosis), forming lichens. Mycorrhiza are fungi that are intimately associated with roots of higher plants, many of which are dependent on this relationship. Some fungi are capable of infecting plants, while others exist only as parasites of plants. Diseases caused by fungi include clubroot of cabbage, potato wart, white rusts, potato late blight, downy mildews, Dutch elm disease, and ergot. Some soil-inhabiting fungi trap microscopic living things such as amoebas and nematodes in their hyphae, then deplete the captives of protoplasm.

V Uses of Fungi

Fungi are useful for a number of industrial processes, including alcoholic fermentation. Synthetic resins are manufactured from fumaric acid formed by black bread mold. Fungi are used in the preparation of certain vitamins. Fungi are also used to produce cheese, and yeast is used in bread making. In medicine, an alkaloid obtained from ergot is used to produce uterine contractions in childbirth, and the use of antibiotics began with the discovery of penicillin, a product of a fungus. The immunosuppressant drug cyclosporine, used in medical transplantation, is obtained from fungi.

VI Classification

Mycologists commonly divide fungi into four main phyla: Oomycota, Zygomycota, Ascomycota, and Basidiomycota, which produce oospores, zygospores, ascospores, and basidiospores, respectively. A large variety of species, seemingly related to Ascomycota, are loosely placed in a fifth phylum, Deuteromycota (also called Fungi Imperfecti).

The oomycetes are algaelike fungi, ranging from a single cell to a complex mass of hyphae that are not walled off by septa. The phylum includes water molds and downy mildews. The zygomycetes form both sexual, thick-walled zygospores and asexual, nonswimming sporangiospores. Black bread mold is a well-known member of this group. Other members of the zygomycetes are parasitic on flies and other insects.

Ascomycetes bear a definite number of ascospores inside a sac called an ascus. The ascomycota include yeasts and similar fungi, Penicillium and Aspergillus, powdery mildews, morels, truffles, and the red bread mold, Neurospora. The basidiomycetes include fungi bearing four basidiospores on stalklike protrusions called basidia. The basidia may be club-shaped, cylindrical, or oval. One class of basidiomycetes includes rusts and smuts. Members of the other class of basidiomycetes mostly live on dead organic matter. They include mushrooms, coral fungi, bracket fungi, and puffballs. See also Fungicides; Fungus Infections.

Wood, hard, tough substance that forms the trunks of trees, and that has been used for thousands of years as a fuel and as a material of construction.

The markings called grain, found on all types of wood, are due to the structure of the wood. Wood consists essentially of fine cellular tubes, which carry water and dissolved minerals from the roots to the leaves, and which are arranged more or less vertically within the trunk. When the wood is cut parallel to the trunk, straight-grained lumber is usually produced.

Many woods have prominent annual growth rings. The trunk of a tree does not grow in length, except at its tip, but it does grow in width. The only portion of the trunk that grows is the cambium (a thin layer entirely surrounding the trunk). In most trees, early wood is lighter in color than wood growing later in the year. The trunk of a tree is thus surrounded each year by a new pair of concentric sheaths, one darker than the other. As the tree grows older the central portion of the trunk, called heartwood, dies completely. Knots are areas of the trunk in which the base of a branch is embedded. When the wood is sawed into planks, a knot becomes evident as a roughly circular discontinuity or irregularity in the grain structure. Knots are generally undesirable in lumber from the standpoint of appearance, apart from their negative effect on the strength of the wood.

Freshly cut wood contains water, from one-third to more than one-half by weight. The drying of wood before it is processed into lumber is called seasoning. Seasoned wood is far more resistant to decay. It is much lighter and therefore less expensive to ship, and it has much higher heating value. Wood changes shape during drying, and this change should be completed before the wood is sawed. Wood may be seasoned by drying in air or in a kiln.

The principal physical properties of wood are strength, hardness, stiffness, and density. Dense woods are usually hard and strong. The term strength covers a number of different properties. Strength varies greatly with the seasoning and with the direction of the grain; wood is much stronger when cut along the grain than when cut across it. Toughness is a measure of strength against sudden, repeated stress.

Wood is naturally very durable. If not attacked by living things, it will last for hundreds or even thousands of years. The most important of the living things attacking wood are the fungi that cause so-called dry rot. The heartwood of a few species is naturally resistant to these fungi. Other woods are resistant to various other types of attack. Most of these woods are aromatic, and the resistance is probably due to the resins and similar chemicals they contain. Wood may be preserved by protecting it chemically against deterioration.

Mycology 101: the nature of fungi

Fungi are a diverse class (approximately 100,000 species) of either highly specialized plants or, according to some researchers, something totally new: the protist kingdom. They differ greatly in their form, are extremely numerous, and are able to multiply rapidly. Fungi do not possess chlorophyll, and unlike green plants are unable to build up sugar and starch from the carbon dioxide available in the atmosphere. They feed on (and thus decompose) a wide variety of organic matter. Fungi obtain their food by direct absorption of nutrients

Most fungi reproduce by spores, which roughly correspond to the seeds of higher plants, and which are formed both sexually and asexually. The common mushroom for example may form around 12 billion spores on its fruiting body. There are four types of spores, which in turn identify the four principal groups of fungi: oospores, zygospores, ascospores, and basidiospores. Oospores are formed by sexual union of a male and a female cell, while zygospores by genetic exchange between two similar sex cells; ascospores are spores contained in asci (sacs), and finally there are basidiospores are contained in basidia (clublike structures).

Whenever the spores land on a suitable medium under at least somewhat moist conditions they germinate somewhat in the manner of seeds.