Wood-Destroying Fungi in Residential Construction

More than 5% of all construction lumber manufactured each year in the United States is used to replace wood that has decayed in service. This need not be the case. Damage to wood-frame buildings by mildew, mold, staining fungi, and decay fungi is entirely preventable. Their presence points to design flaws, poor workmanship, and neglected maintenance.

Microorganisms

The microscopic organisms that discolor and decay wood belong to a huge group of primitive plants known as fungi. Unable to produce their own food, fungi feed instead on natural substances that make up organic materials like leather, cloth, rattan, paper, and wood.

Mushrooms that spring from lawns and tree trunks are fungal "fruits". They release millions of dust-size spores that are scattered helter-skelter by wind. When the conditions of the surface they eventually settle upon are right, spores germinate, sending out thread-like filaments called hyphae. Enzymes secreted by hyphae break down organic matter so fungi can use it for food.

Before fungi can colonize wood, four requirements must be met: an oxygen supply, temperature in the 40 to 100 (F range, a supply of sufficient moisture, and a food source (wood). Infection can be prevented by eliminating any one of the requirements. Obviously, it's hard to limit oxygen. Temperature control is tough too, since most living things thrive in this range. And even at subfreezing temperatures, many fungi don't die, they just go dormant.

The most effective "method" of preventing fungal deterioration of wood is to keep it dry. Most fungi need a wood moisture content of at least 20% to carry on. With the moisture content of wood indoors over most of the United States cycling annually between 6% and 16%, it's too dry for most microorganisms to get started.

In exterior or other situations where wood can't be kept dry, decay has been delayed traditionally by using naturally rot-resistant woods like Western redcedar and redwood. Nature has partially protected these woods from fungi by depositing toxic extractives in their heartwood. But supplies of naturally durable woods are too small to meet today's demand at an ecologically and economically acceptable price. In imitation of Nature, less naturally durable woods are impregnated with pesticides like CCA (chromated copper arsenate) that extend their service life by 30 to 50 years or longer.

Defeating mildew

Occurring outside and inside homes, most mildews are black, but reds, greens, blues, and browns are possible. Even the familiar gray color of weathered wood is the work of mildew. Masses of dark spores and hyphae give mildews their characteristic splotchy look. Merely discoloring the surface they grow on, mildews have no appreciable effect on wood itself. Some mildews that feed on airborne organic matter can even grow on inorganic vinyl and aluminum sidings. Dew and rain supply needed moisture.

Mildews appear most often on unheated, projecting parts of buildings that cool quickly after sunset, like eaves, decks, and porch ceilings. North-facing walls and those shaded by foundation plantings, trees and other obstructions that restrict sunlight and airflow are also candidates. Mildew's location often mirrors a building's dew pattern. Absent where siding crosses "hot spots" over studs and other thermal bridges, mildew may thrive where dew persists over cooler, insulated bays in between.

Virtually all exterior finishes -paints, solid color and semi-transparent stains, and water repellants alike- are susceptible to mildew. Oil-base formulations, especially those with linseed oil, are particularly vulnerable. Among water-base coatings, acrylic latexes have proven the most mildew-resistant. Defend against mildew on siding and trim by using only primers and topcoats that contain mildewcide, or by mixing in the add-it-yourself types paint shops sell. Finishes with zinc oxide pigments also deter mildew. Beware, finishes applied over mildewed surfaces that are recoated without first killing the fungus will quickly discolor as the mildew grows though the new coating.

Ridding wood of mildew is easy. But first, do a simple test to see if splotches are mildew, or just plain dirt. Place a drop of fresh household bleach containing sodium hypochlorite on the suspect area. The dark color of mildew will fade in a minute or so, while dirt is unchanged. Clean surfaces by brushing or sponging with a solution of 1/3 cup household detergent, 1 to 2 quarts household bleach, and 2 to 3 quarts of warm water. Or use commercial cleaners. Wear eye protection and gloves, and rinse surfaces with water.

Mildew occurs indoors most frequently in baths, basements, and other areas prone to high relative humidity. It also shows up in places with poor air circulation such as behind furniture against exterior walls, and in closets and closed-off rooms. Mildew can form whenever the relative humidity of air near a surface exceeds 70%. This can happen when warm air near the ceiling cools as it flows down colder wall surfaces. The relative humidity of 70 (F air, for example, rises from 40% to 70% when it's cooled to about 52 (F. Spores and musty odors emitted by mildew growing in indoor microclimates can trigger allergic reactions.

Thermal bridges that lead to "hot spots" outside create "cold spots" inside. Exterior corners are notoriously mildew-prone because of poor air circulation inside and heat-robbing windwashing outside. In summer, water vapor from warm, humid air entering crawl spaces and basements below air conditioned rooms may condense on cooler joists and subflooring, creating conditions irresistible to mildew, as well as mold, and staining and decay fungi. Moisture condensed as ice from heated air leaking into attics in winter likewise wets rafters and sheathing when it melts.

Determined by the occupants' lifestyle, the amount of moisture generated inside a home is beyond a builder's control. But use of the bath exhaust fan, for example, can be encouraged by wiring it to the room light switch or to a timer. Installing louvered doors ensures airflow in closets. Use a soil cover, and vent and/or insulate crawl spaces as site and climatic conditions dictate. The same is true for ceiling vapor retarders, and attic insulation and ventilation levels.

Managing molds

Molds need a wood surface moisture content of about 20% to get started. To provide that, simply surround wood with air at 90% relative humidity at any temperature from 40 to 100 (F, and presto! That's why mold and mildew sometimes suddenly appear on furniture during the dog days of summer.

While most are green, black and orange molds are not uncommon. Color comes from spores strewn across surfaces. Though hyphae reach deeper into wood, discoloration in softwoods tends to be limited to the surface of the sapwood. It can usually be planed, sanded or even brushed off. Brown, gray, or black patches penetrate more deeply into hardwoods and can't be machined away. Discoloration aside, mold's effect on wood is generally inconsequential.

Some molds are surprisingly tolerant of wood preservatives. This explains the fuzzy growths occasionally seen between boards in banded shipments of solid-piled CCA-treated southern yellow pine. Molds die once lumber dries, but can be washed off beforehand with the same solution used for mildew.

Flourishing in damp crawl spaces and basements, and in poorly vented attics, molds form a living veneer on framing and sheathing. Prevention lies wholly in controlling air moisture levels and condensation potential through proper site drainage and dampproofing, and use of soil covers, vapor retarders, insulation, and ventilation as ambient conditions call for.

Sidestepping staining fungi

Discoloration of wood by staining fungi happens almost exclusively in logs and freshly sawn lumber. As a precaution, rough lumber is often dipped in a fungicidal bath immediately after sawing. Also called sap stains, these fungi are most troublesome in the pines. The steel gray to blue-black color they cause in softwoods - blue stain, and the brown hues in hardwoods, are due to dark hyphae that permeate sapwood in search of stored starches and sugars. Inactive blue stain can usually be spotted in doors, millwork and other pine products without looking too far. Live staining fungi sometimes discolor the bottom rails and corners of pine windows continuously wetted by condensation. Stains are indelible and don't wash off. In finding food, staining fungi destroy certain wood cells. Wood becomes substantially more permeable, and more susceptible to decay as a result. Its strength and toughness are slightly reduced as well.

Guarding against decay fungi

While discoloration by mildew, mold, and staining fungi poses an appearance problem, attack of wood by decay fungi threatens its structural integrity. Aptly termed the "slow fire", wood decays or rots because these fungi eat the very cellulose and lignin of which wood cells are made.

>Moisture content is the critical factor determining wood's susceptibility to decay. It must exceed 28%, and liquid water must be present in cell cavities before fungi can gain a toehold. Once established, some fungi can carry on their destruction at a moisture content as low as 20%. When moisture content falls below this level, all fungal activity ceases. That's one reason why framing lumber is dried to 19% moisture content or less.

In its early or incipient stages, decay can be difficult to detect, even with a microscope. Strength loss can be appreciable even at this stage. As the slow fire advances, wood's luster fades. Surfaces become lifeless, dull, and discolored. A musty odor is often evident. The rate at which decay progresses depends on moisture content, temperature, and the specific fungus.

It doesn't take a trained eye to recognize decay in its advanced stages. Wood is visibly discolored, spongy, and musty. Surfaces may be stringy, shrunken, or split across the grain. Cottony masses of hyphae called mycelia, as well as fruiting bodies, may be present. Decay extends deep into wood; strength loss is significant.

Brown rots and white rots

Decay fungi fall into three major groups: brown rots, white rots, and soft rots. The latter are rarely found inside homes, though they occasionally degrade wood shakes and shingles on heavily shaded roofs in wet climates.

Brown rots are so-named because infected wood turns dark brown. Most commonly colonizing softwoods, brown rots consume cellulose, hardly touching the darker lignin. Mycelia appear as white sheet-like or fluffy growths on wood surfaces. Brown-rotted wood shrinks excessively and splits across the grain as it dries. Friable and crumbly, surfaces then show brown rots' hallmark cubical checking.

Water-conducting fungi are a special type of brown rot that show up infrequently in the southeast, northeast, and Pacific northwest. Sometimes called dry rot fungi, the name unfortunately suggests that dry wood can decay. Dry wood can't decay, period. What builders, inspectors, and homeowners alike routinely mislabel "dry rot" is almost always, in reality, wood that got wet, rotted, and dried out before discovery. Unique in their ability to pipe moisture from the soil over long distances through root-like rhizomorphs, water-conducting fungi wet otherwise dry wood in advance of their attack. Infecting softwoods and hardwoods, their light-colored mycelia look like large, papery, fan-shaped sheets. Dirt-filled porches, damp crawl spaces, and wood in ground contact are avenues for entry.

White rots impart a white, gray-white, yellow-white, or otherwise bleached appearance to wood. Most often infecting hardwoods, they feed on both cellulose and lignin. In advanced stages of decay, white-rotted wood is spongy, has a stringy texture, and lacks the cubical checking of brown-rotted wood. A thin black line often marks the advancing edge of incipient white rot in hardwoods. Ironically, this partially decayed or spalted wood is coveted by woodworkers for its unique figure.

Detecting Decay

When wood is suspiciously wet or discolored, but otherwise looks okay, determine its subsurface moisture content with a moisture meter. If it's 20% or below, there's no active decay present. If it's between 20 and 28%, existing decay can continue merrily on its way. If it's over 28%, conditions are ripe for fungi to get started.

The pick test is also useful. Here the soundness of wood is judged from the way a large splinter breaks when pried from it with an awl or ice pick. Sound wood emits a sharp crack as the splinter is pried up. The splinter is typically long, with one end still attached to the wood. Sometimes it breaks in the middle over the tool, but the fracture will still be splintery. A splinter pried from wood with incipient decay lifts quietly from the surface and almost always fails brashly directly over the tool, with both ends anchored to the wood. The pick test is highly subjective; natural characteristics of sound wood can produce misleading results. Accurate interpretation comes only with experience and consideration of other clues.

Decay hidden inside timbers can be revealed by examining turnings ejected from a medium diameter boring. Discolored, wet, and musty shavings signal decay. Plug the hole with a preservative treated dowel.

Dealing With Decay

The first and most important thing to do once decay is discovered is to figure out where the water is coming from. Check for the obvious - roof and plumbing leaks, and missing or punctured flashing. Look for stains and drip tracks caused by ice dams. Are eaves wide enough to prevent water from cascading down sidewalls? Are gutters poorly maintained or missing? Do finish grades slope towards or away from the foundation? Are foundation cracks admitting water? Is untreated wood in direct contact with concrete, masonry, or soil? Check to see if crawl spaces have soil covers, and if venting and/or insulation is present, adequate, and properly placed. The same goes for attics. Peeling and blistering paint often signal inadequate interior ventilation, or a missing vapor retarder. Water stains on framing and sheathing inside walls suggest condensation. Remember that to make the remedy permanent, you've got to cure the disease -water infiltration, not just treat the symptoms- mildew, mold, and decay.

Once the source of water has been shut off, remove as much decayed wood as is practical and economical. This is especially important with girders, columns, and other critical members whose load-carrying ability may have been compromised. There's no known way of accurately determining the remaining strength of decayed wood left in place. Cut back rotted members to sound wood, keeping in mind that difficult-to-detect incipient decay can extend well beyond visibly rotted areas. When a partially decayed structural member can't be replaced, reinforce it with a "sister" anchored to sound wood. Decayed wood absorbs and holds water more readily than sound wood, so let rotted areas of members not removed dry out before making repairs and closing in. Otherwise, you're just adding fuel to the slow fire.

In damp crawl spaces or other places where water is likely to appear, replace decayed members with preservative treated wood. The major model building code agencies -BOCAI, ICBO, and SBCCI- require that treated wood be used for sills and sleepers on concrete or masonry in ground contact, for joists within 18 in. of the ground, for girders within 12 in. of the ground, and for columns embedded in the ground supporting permanent structures.

In-place treatment with borates

Dormant fungi can be reactivated when dry, infected wood is rewetted. Consider treating infected, but otherwise serviceable wood left in place with a water-borne borax-based preservative that will not only kill active fungi, but guard against future infection as well. Borates have low toxicity to humans and are even approved for interior use in food processing plants. They don't affect wood's strength, color, or finishability, don't corrode fasteners, and don't outgas vapors. Widely used in treating new timbers for log homes, they're the preservative of choice for remedial treatment of wood in service. Because of the decay hazard posed whenever wood bears on concrete or masonry, solid borate rods are often inserted into holes bored near contact areas. Should wood ever get wet, the rods dissolve and ward off infection.

Epoxy repair of decayed wood

Sometimes replacing rotted wood isn't an option. In conserving historic buildings, for example, the goal is to preserve as much of the original "architectural fabric" as possible. Stabilizing deteriorated wood with epoxy is often the only choice. Epoxies consist of resin and hardener that are mixed just before use. Liquids for injection and spatula-applied pastes are available. After curing, epoxy-stabilized wood can be shaped with regular woodworking tools and painted. Epoxies are useful for consolidating rotted wood, restoring lost portions of moldings and carvings, and for strengthening weakened structural members. In the last case, they're used to bond concealed metal reinforcement inside holes or channels cut into hidden faces. Epoxies aren't preservatives and won't stop existing decay or prevent future infection. They can be tricky to use; follow the manufacturer's mixing, application, and safety instructions to the letter.

Stephen Smulski, Ph.D., is President of Wood Science Specialists Inc., a consulting firm in Shutesbury, Massachusetts, that specializes in solving performance problems with wood products in all types of wood-frame structures.

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