Wood Decay, Degradation & Stain
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Causes and Control of Wood Decay, Degradation & Stain 1 Contents Moisture .................................................................................3 Wood Degradation: Causes and Control..............................4 Weathering ...................................................................................................... 4 Naturally Decay-resistant Species ........................................................... 5 Wood Decay ...........................................................................5 Dealing with Decay ...................................................................................... 6 Stains ......................................................................................7 General Stain Control ................................................................................... 9 Insects .................................................................................. 10 Insects of Moist Wood ...............................................................................10 Pests of Dry Wood .......................................................................................12 Marine Wood Borers ...................................................................................15 Preservative Treatments .................................................... 17 Oil-type Preservatives ................................................................................17 Waterborne Preservatives ........................................................................18 Treatments for Wood Composites .........................................................20 Precautions with Preservative Treatments ....................... 20 Use-site Precautions ...................................................................................20 Handling Precautions ................................................................................21 Selection of Pressure-treated Wood ................................. 21 Over-the-counter Preservatives .............................................................23 Installation and Inspection Tips ........................................ 23 Conclusions ......................................................................... 25 22 Causes and Control of Wood Decay, Degradation & Stain Our society depends on wood for a variety make heartwood more decay resistant than of uses. As population increases, so does our sapwood). Sapwood is wood near the bark and need for wood. Steel, concrete and aluminum is often lighter in color than heartwood. Nutri- are some alternatives to treated wood in cer- ent translocation occurs only in sapwood. Al- tain applications, but they have higher material though most wood species can be treated with costs, higher energy requirements in the pro- a preservative, certain species are considered duction process, greater air and water pollution difficult to treat because of their permeability or environmental protection costs, and greater and anatomical features. Douglas fir, a western dependency on foreign sources for materials. species, has below-average permeability and Substitute materials may not be appropri- is classified as difficult to treat. Species such ate for some uses. For example, some types of as white oak have inclusions in the vessels steel may corrode; concrete may deteriorate in called tyloses. These inclusions also decrease salt water; and plastic may not have the neces- permeability and make treating more difficult. sary strength, durability and structural integ- In general, lumber that has a high percentage rity. Wood is a renewable natural resource that, of heartwood or is improperly seasoned will if properly treated, maintained and placed in be more difficult to pressure treat. Southern service, will last indefinitely. It is critical for us yellow pine (SYP) characteristics make it use- to use our wood resource efficiently. ful for many applications and easily treatable. Most pressure-treated lumber in the South is This publication is intended to increase Southern yellow pine. your knowledge of the causes and control of wood decay, degradation and stain. A common Moisture cause for replacing wood structures is decay It is commonly believed that wood shrinks or degradation. Wood decay and most insect as it loses moisture and swells as it gains mois- problems can be prevented for years by proper- ture. This is partially true. Actually, wood will ly using and protecting wood. The heartwood change dimension only between two precise of some species, such as black locust and Osage moisture conditions. One condition is when orange, also has a unique chemical composi- the wood is void of moisture. This is termed tion that makes it very durable. the ovendry condition. The second condition Two common terms used to describe is when the wood fibers are saturated with wood features are heartwood and sapwood. moisture. This point usually occurs at about Heartwood is wood in the inner section of 30 percent moisture content for most Louisi- a log and is entirely composed of dead cells. ana species. As wood is dried from an original This region has a higher concentration of green condition, sometimes more than 100 extractives (phenolic-based compounds that percent moisture content, moisture is first lost 3 from the cell cavities. No shrinkage will occur der the performance of finishes and paints, and until the wood reduces to a moisture content induce surface stains. of about 30 percent (fiber saturation point). If drying continues below 30 percent moisture Wood Degradation: content, water is removed from the cell walls Causes and Control and shrinkage occurs. The amount of shrinkage Wood degradation is not the same as wood or swelling depends on the species, density and decay. The three primary sources of wood deg- board direction. radation are fungi (decay or rot), insects and Pressure treatment with waterborne pre- weathering. In addition, fire can also degrade servatives raises the moisture content above wood. The organisms that decay wood have the fiber saturation point, and shrinkage will four basic requirements: moisture (generally 25 occur as the wood dries down to its in-service to 100 percent of dry-wood weight), oxygen, moisture content. In many applications, such temperature (generally between 50 degrees and as deck boards, this shrinkage is not a major 95 degrees Fahrenheit) and food (the wood concern. When dimensional stability is critical, itself ). We can control wood degradation by it is imperative that the lumber be kiln dried altering one or more of these requirements. A after treatment (KDAT). Any KDAT lumber sawmill will often keep its logs moist under a you buy should be kept under a roof or at least sprinkler system to saturate the logs with water under cover and off the ground. and create an anaerobic environment in which there is insufficient oxygen for most wood de- The dimensional stability of different wood cay organisms and insects. Wood can undergo species is affected by width and density differ- slow bacterial degradation in fresh water or be ences between earlywood and latewood in the attacked by marine borers in brackish or salt growth rings. For example, in species having water, however. wide, dense latewood bands and low-density earlywood bands, the differential shrinking It is impractical for consumers to keep and swelling of the bands with changes in lumber under a sprinkler system or buried in moisture content can cause large stresses in mud! Most wood decay can be prevented by the wood that can result in raised grain and simply keeping the wood dry. If lumber is dried a defect known as shelling. Raised grain will to 6-8 percent moisture content for indoor uses tend to be more pronounced on flat grained or 15-18 percent for outdoor uses, it should lumber. Shelling is an extreme case of raised not decay if the moisture content is maintained grain in which the below 20 percent. A common cause of wood latewood bands decay is when untreated wood is alternately ex- separate from the posed to wet and dry conditions, as in ground earlywood bands contact, or when it collects moisture and re- to form a knife- mains moist for an extended period. To prevent like or spearlike this situation, either keep your untreated lum- edge. This is one ber dry or use treated wood if you suspect it reason why deck will get wet in service. Be sure to use pressure- lumber is often treated lumber approved for ground contact if recommended to you are building a structure that requires one be placed pith- end of some boards to be in contact with soil side down (or and moisture. bark-side up). If Weathering the two sides of a particular board The surface of wood can be degraded if Figure 1. Severe weathering oc- are of equal qual- the wood repeatedly becomes wet and dry, is curred on the surface of these treated exposed to high and low temperatures and is southern yellow pine boards used to ity, it’s better to place the board bark-side up. If, construct a boat dock. Most docks are however, the pith side is clearly the better side, exposed to direct sunlight. This degradation continually exposed to wet and dry place this side up. causes roughening of the surface, checking, conditions and direct sunlight. It is rec- splitting and wood cell erosion. Erosion, caused ommended to brush apply each year a Moisture greatly affects lumber in use and water repellent that also protects from by the loss of wood cells from the lumber sur- ultraviolet (UV) degradation of the can quickly lead to deterioration. Moisture can face,