An Ounce of Prevention

Thanks to Grant Kirker for writing this article spotlighting how homeowner’s can better maintain their wood decks. Kirker is a Research Forest Products Technologist at FPL in the Durability and Wood Protection Research unit.

The experimental test block setup used to examine the role of accumulated leaf litter on material performance and wood durability in aboveground exposure. The test block is surrounded by untreated pine and the channel between the block and frame is filled with leaf litter. The two black fittings on the top are able to take repeated moisture measurement using a pin-type moisture probe. USDA Forest Service Forest Products Laboratory

Benjamin Franklin is credited with the saying “an ounce of prevention is worth a pound of cure”, which suggests that taking steps to avert a problem before it starts is far better than taking corrective steps after the problem arises. Forest Products Laboratory (FPL) Wood Durability and Protection researchers, in collaboration with research partners at Oregon State University, attempted to apply this concept to a situation close to home for many homeowners—the wooden deck.

The global wooden decking market in 2020 was valued at $15 billion USD, of which the North American markets made up about 35%, or $5.25 billion USD1. In a 2019 National Association of Homebuilders (NAHB) survey2, 20.3% of all new houses included decks. Although this estimate is lower than historical averages, the global pandemic has led to an increasing interest in outdoor living spaces, which will likely cause this market to increase. Wood is an excellent building material for outdoor decking because of its  reasonable cost and low maintenance requirements; if properly installed and maintained, a wood deck can provide a long-lasting benefit to the homeowner.

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Warm, Wet Wood Means Fungi to Follow: Is Your Home a Mushroom Magnet?


Climate index for decay hazard. Higher numbers indicate greater hazard.

Fungi are the principal agents of decomposition in ecological systems, and are an unavoidable fact-of-life. Although particularly prevalent in the south, where temperature and humidity create ideal conditions, wherever you find organic matter, fungi will be close behind.

When it comes to wood and wooden structures however, these pint-sized parasites can create big problems, leaving unsightly stains or even weakening buildings to the point of structural failure.

Two kinds of major decay fungi are recognized: brown rot and white rot. With brown-rot fungi, only the cellulose is extensively removed, the wood takes on a browner color, and it can crack across the grain, shrink, collapse, and be crushed into powder.


Representative samples of four common types of fungal growth on wood: (a) mold discoloration; (b) brown rotted pine (note the dark color and cubical checking in the wood); (c) white rot in maple (note the bleached appearance); (d) soft-rotted preservative-treated pine utility pole (note the shallow depth of decay)

With white-rot fungi, both the lignin and cellulose are usually removed, so the wood may lose color and appear whiter than normal. It does not crack across the grain, and until severely degraded, it retains its outward dimensions, does not shrink or collapse, and often feels spongy.

When combating fungi, the temperature and moisture content of the wood are essential to consider.

Most fungal decay can progress rapidly at temperatures that favor the growth of plant life in general. For the most part, decay is relatively slow at temperatures below 50 degrees Fahrenheit and above 95 degrees Fahrenheit. Decay essentially ceases below 35 degrees Fahrenheit or above 100 degrees Fahrenheit.

Serious decay also only occurs when the moisture content of the wood is above the fiber saturation point (about 30 percent). Fully air-dried wood usually will have a moisture content not exceeding 20 percent, and should provide a reasonable margin of safety against fungal damage.

Brown, crumbly rot, is sometimes called dry rot, but the term is incorrect because wood must be damp to decay, but may become dry later. There are also a few dry-rot fungi that have water-conducting strands; such fungi are capable of carrying water (usually from the soil) into buildings or lumber piles.


The decay cycle (top to bottom). Thousands of spores produced in a fungal fruiting body are distributed by wind or insects. On contacting moist, susceptible wood, spores germinate and create new infections in the wood cells. In time, serious decay develops that may be accompanied by formation of new fruiting bodies.

The early stages of decay are often accompanied by a discoloration of the wood, which can be difficult to recognize but is more evident on freshly exposed surfaces of unseasoned wood than on dry wood. Abnormal mottling of the wood color, with either unnatural brown or bleached areas, is often evidence of decay infection.

Late stages of decay are easily recognized, because the wood has undergone definite changes in color and properties. The character of these changes depends on the organism and the substance it removes.

If you see these tell-tale signs of decay on your wooden structures, and cannot dry the wood or turn down the temperature, researchers at the Forest Products Laboratory (FPL) offer these tips for cleaning outdoor surfaces prone to fungi, like your deck or siding. Fungus will always be among us, but detecting it, managing it, and mitigating its damage is well within our control.

For more information, please see Chapter 14 of FPL’s Wood Handbook: Wood as an Engineering Material

Essential Science: Researchers Patent a New and Natural Frugal Fungicide


A selection of different composite wood products both untreated and treated with essential oil. The samples dipped in essential oil exhibit far less mold growth.

Sitting at the Forest Products Laboratory (FPL), microbiologist Vina Yang recounts a news story that took Madison, Wisconsin by storm. Fourteen years ago, more than 35 students and faculty mysteriously fell ill at Caesar Chavez Elementary School. Although the school had only been open for two months, it quickly and inexplicably turned from Madison’s newest educational institution into a health nightmare. Ailments ranged from sudden onset asthma, to respiratory problems, to severe allergic reactions, and it was only after the school’s inevitable closure that officials found the source of the problem—hidden behind the pristine drywall and gleaming floor tiles of the new building, inadequate ventilation had caused mold to take a firm hold at Caesar Chavez.

Today, Yang and fellow researcher Carol Clausen are working hard on developing new techniques to combat the mold plaguing the world’s wood-containing residences, businesses, and storage facilities. More than seven years of research recently culminated in a patented method of using essential oils derived from plants to inhibit mold on cellulose-containing materials such as paper, lumber, and ceiling tiles.

Yang and Clausen’s Durability and Wood Protection unit traditionally studies preservatives for wood in exterior applications, but it quickly became apparent that there was demand for a less toxic solution for indoor use as well.

“We would always take calls from consumers asking for ways to prevent mold on the inside of houses,” Clausen recalls, adding that essential oils can be as effective as chemical fungicides without the associated health concerns, which are elevated when used in a household environment. Perhaps the biggest, albeit subjective, drawback of using the oils indoors is the odor, as they tend to smell strongly of the parent plant. “One person came by the lab and just loved the smell—others came by and told me to close the door,” added Yang.

From the original arsenal of seven oils that Yang and Clausen began to study in 2007—thyme, ajowan, dill weed, Egyptian geranium, lemongrass, rosemary and tea tree—only the thyme oil compositions received the patent earlier this year. Patent Number 8,986,757 now awaits licensees and industry partners to deliver new products to consumers.

Yang suggests that applying the oil to wood stored in warehouses or lumber yards could prolong its storage life, while Clausen foresees the treatment as an easy way for companies to provide peace of mind to consumers. “I especially foresee a lumber or construction company using our technology as a way to provide inexpensive protection to customers,” Clausen said.

The oil can be dipped, sprayed, or brushed onto wood surfaces, and in some cases, simply exposing the material to oil vapor is enough to inhibit mold growth, making it the ideal process for fumigating large spaces or large volumes of material.

A photograph illustrating the effectiveness of essential oil as a mold inhibitor when applied to wood as an oil vapor. The treated wood stakes (left) experienced far less mold growth than their untreated counterparts (right).

A photograph illustrating the effectiveness of essential oil as a mold inhibitor when applied to wood as an oil vapor. The treated wood stakes (left) fared much better than their untreated counterparts (right).

Essential oils cost roughly $18.00 per pound, and when they are diluted for use, the cost is about two cents per gallon, and fractions of a penny per square foot. Essential oil technology becomes even more affordable when one considers that health problems caused by interior mold accounted for $2.8 billion in 2002 alone.

The research may have come a bit late to rescue the doomed Caesar Chavez Elementary School and prevent the legal action that resulted from the building’s poor construction, but both researchers hope that in the future, new buildings will benefit from these surface treatments.

Clausen cautions that although effective, essential oils are not a replacement for public education or good building practices such as the proper installation of ventilation systems or flashing.

“Prevention is the key, and educating the consumer is huge, especially in flood prone areas or in regions that face seasonal problems with mold,” Clausen said. “If they could just keep the buildings dry, that would solve all of their problems.”

Proper Soil Grading Helps Keep Decay at Bay

According to Build Green: Wood Can Last for Centuries, a common homeowner and contractor mistake following construction projects is to set the finish grade for soil or mulch above the level of wood framing. Soil contact is one of the primary culprits in wood decay.


Soil graded high against the exterior brick veneer will contribute to decay problems in untreated wood members below the grade line. Similar problems will occur with soil graded high against stucco and siding.

Where untreated wood is used in a structure, it should be at least eight inches above the finish grade for framing members and six inches above finish grade for siding. Composite products should never be used in contact with soil.

Preservative treatments are designated for above-ground use or in-ground contact (buried in soil or touching soil). When planning your building, it is important that you specify the right treated wood for your specific need and that you insist that the treatment be of certified quality and be labeled accordingly.


Here’s what not to do. In this photo, the finish grade on the yard is above the level of wood framing inside the wall. To make matters worse, the lawn sprinkling system is providing constant wetting of the stucco siding on this home. (Photo provided by Steve Easley, Steve Easley & Associates, Inc.)

A vast array of treated wood is available for the homeowner. Choosing a preservative approved for ground contact, properly grading soil, and avoiding constant wetting will go a long way in protecting your outside wood structure from decay caused by the moisture in soil.