Submitted By: Mark W. Hunter, Building Specs of Portland Oregon

Decay of wood in buildings is a problem in moist, humid regions. Moisture in some form is necessary for the growth of fungi that cause rot. A fungus sends minute threads, called mycelia, through damp wood, taking its food from wood as it grows. The visible body of the fungus is on the outside surface of the wood. This is called the fruiting body, and, when mature, it produces millions of tiny spores (seeds) that are present in the air and soil. A musty odor may be encountered inside a building or crawl space. Decay enhancing building practices include indoor plumbing, weatherization, shorter roof overhangs, slab construction, building membranes and air conditioning. Changes affecting decay are decreased heartwood durability, increased composite use, changing designs and chemical attitudes. In many cases, with bathroom or kitchen updates moisture tends to be covered up rather than corrected. A thorough inspection is necessary to locate and identify this problem.

Two major types of fungi can attack wood: (1) Surface molds and (2) Rot Fungi. Fungi that discolor the wood are called surface molds, mildews, and sap or blue stain fungi. They do not reduce the strength of the wood they grow on. The decay or rot fungi either bleach wood (white rot) or make it brown and crumbly (brown rot). They do degrade wood and can be as destructive as termites and other wood destroying insects, many of which are also attracted to moist wood. The surface molds and stain fungi grow more rapidly than decay fungi and often appear on wood during construction. Fungi growth will not continue after construction if the wood dries out but may live 10-12 years. However, the presence of stain fungi indicates that conditions at one time were suitable for decay, and an inspection, with the use of a moisture meter may confirm to see if the wood is still moist enough to support decay fungi. Most decay fungi commonly starts at the end grain growing only on wood subject to wetting by contact with moist soil, rain, or faulty plumbing. At moisture levels of 40% it really starts to grow and can be sustained at moisture levels of 70%. Water-conducting fungi (Poria), however, can actually conduct water directly to wood up to 25 ft. away thus causing severe damage to large amounts of wood in 1-2 years even though the wood is not exposed directly to moisture.

I have observed many cases of excessive moisture at comers of buildings, entry ways and decks. Decaying wood with moisture content of 6-30% is subject to other Wood Destroying Organisms such as Termites, Carpenter Ants or Wood Boring Beetles.* Carpenter Ants have habits of lazy infestations in insulation, a hollow area or tend to rely on decaying wood to establish colonies. Subterranean Termites feed off the wood containing cellulose that they excavate versus Carpenter Ants scout for food outside the nest. Monitoring of Ant forage trails would then be suggested in order to locate the infestation and/or parent colony. Carpenter Ants and Termites are capable of infesting sound wood. Other infestations may include Wood Boring Beetle larvae hatching from fallen, fire damaged timber, improperly seasoned wood or even wood furniture incorporated in buildings. Beetle larva in the wood can survive several years before finally pupating and will emerge as Adults. Many Beetle species typically do not alter the structural properties of wood framing. The Golden Buprestidae family is capable of living and developing for more than 40 years. They will not reinfest seasoned wood, so damage can be limited to that inflicted by one generation unless severely retarded. Infestations associated with wood decay are the Anobiidae, Lyctidae or Bostrichidae families. Anobiid damage is often over looked, as these insects live in portions of the structure where people seldom see them. Wood break down from termite, carpenter ant and powder post beetle infestations can lead to serious structural weakness and usually require control efforts and/or routine observations to monitor activity.

It is most important to point out that the application of fungicides or insecticides to infected wood or soil is only a preventative measure against infestation, not a corrective measure. (Chemicals generally soak in wood 1/8 -1/4 inch) Only by eliminating the moisture source, decayed and/or infested wood, would it be the most effective preventative control. Therefore, the application of chemicals by pest management professionals is of minor importance in fungus control work. All badly rotted wood should be removed and replaced with sound, dry lumber. If the wood is to be used in a wall cavity then the application of kiln dried lumber is recommended. Scabbing is not accepted. When it is not possible to eliminate the source of moisture entirely, the replacement lumber should be pressure treated with a wood preservative before installation. Pressure treated lumber has only a 25 year rating at ground contact and must be treated at the end grain, especially when a cut is made. Wood applications to soil contact is not Suggested.

Wood in unventilated crawl spaces under houses is subject to dampness. Condensation of water after evaporation from soil is the most common source of subsequent decay problems in homes. This condition can be relieved by providing proper drainage, ventilation and/or installing a vapor barrier. In the attic, vents should be located both near the ridge and at the eaves to induce air flow. Installing a vapor barrier on the soil surface of unexcavated areas will cause soil moisture to condense on the barrier and return to the soil, rather than condensing on the insulation, floor and joists above. When moisture problems result from poorly constructed or poorly maintained buildings, these problems have to be corrected before wood decay control can begin. Problems such as faulty roofing, siding and plumbing, usually require the services of other specialists. If the source of moisture is removed and the wood dries out, existing decay will be dormant.

* Moisture content of wood less than 20% found in structures is considered dry.

Source: M.P. Levy, A guide to the inspection of existing homes for wood-inhabiting Fungi and Insects, US Department of Housing and Urban Development.